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A PC-based analytical stereoplotter for wetland inventories: An efficient and economical photogrammetric instrument for field offices
Forest Ecology and Management, 33/34 (1990) 571-581
571
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
A PC-based analytical stereoplotter for wetland inventories: An efficient and economical photogrammetric instrument for field offices William S. Warner Institute for Georesourcesand Pollution Research, The Agricultural Research Council of Norway, Boks 9 NLH 1432-~s (Norway.)
ABSTRACT Warner, W.S., 1990. A PC-based analytical stereoplotter for wetland inventories: An efficient and economical photogrammetric instrument for field offices. For. Ecol. Manage., 33/34:571-581.
Highly flexible photogrammetric systems, controlled entirely by PC technology, can collect primary data from aerial photographs and process these data in a variety of cartographic systems and statistical programs. This paper focuses on a PC-based analytical plotter used by the USDA Forest Service and several organizations in Norway. Its accuracy (20 microns), flexible application (from making simple 3-D measurements to GIS data entry), and cost (US$30 000) make it suitable for field-office purposes. Discussion includes other affordable, efficient and portable PC-based instruments that now enable the field office to make 3-dimensional measurements directly from aerial photographs.
INTRODUCTION
Aerial photography is one of the most valued sources of information for wetland identification and inventory. Originally, photogrammetry was developed to derive data from inaccessible or large areas to produce maps. Today, with the aid of computers, micro-electronics and software, it is one of the most sophisticated measuring techniques. Moreover, photogrammetry is capable of obtaining results just as accurate, if not more so, than conventional surveying methods (Westerhuis, 1988). Although aerial photographs are a primary source of information for forest inventory, methods of using aerial photographs in the field offices have remained nearly unchanged for several decades. And despite proven application of computer-based photogrammetry to forestry, it still appears far removed from the field forester. Where we have seen significant advancements in the application of photogrammetry is not in forestry field offices, but in the regional offices. The organizational trend towards decentralization is bringing many activ0378-1127/90/$03.50
© 1990 Elsevier Science Publishers B.V.
572
W.S. WARNER
TABLE 1 PC-based analytical plotters Priced under US$40 000. Manufacturer
Model
Description
MPS
Basic instrument of novel design. 4-micron accuracy at photo scale. Limited to smallformat photos only.
AP190
Optical mechanical unit that handles small- and large-format photos. 20-micron accuracy.
Stereobit
New, simple optical mechanical instrument. Resolution (not accuracy) l0 microns.
Adams
Carto
Galileo
ities that were once confined to a central office now to the field. Not long ago, decision-makers depended upon - and to a certain degree were hobbled by large mainframes or expensive minicomputers for data analysis and management. The introduction of the personal computer (PC) changed that by making computing more accessible, efficient, and economical. In field offices today, the PC is nearly as common as the pocket stereoscope. And now we are just starting to see the PC make photogrammetric equipment suitable for the field offices. Photogrammetry is a three-dimensional-measurement science, and the fundamental problems of the analytical approach have essentially all been solved. One of the most important products resulting from the solution of these problems has been the analytical stereoplotter, which has finally found almost universal acceptance in the commercial mapping community (Kleinn, 1987). An analytical stereoplotter (sometimes called an 'analytical plotter') is a stereocomparator, encoded so that coordinate measurements on photographs can be passed to a computer and converted into digital form (Konency, 1980; see Fig. 1 ). The three-dimensional (X, Y and Z) measurements are digitized by mounting the measuring mark on a suitable cross-slide system and encoding each of these axes individually, using linear or rotary encoders. The PC determines the relative position of the stereomodel and drives the photographs to their positions in real-time. Currently there are several PC-based analytical plotters on the market, ranging in price from US$28 000 upwards. Whereas a few PC-based sell for less than US$40 000 (Table 1 ), the majority of systems are priced between $50 000 and $150 000. Whereas the Japanese manufacturers have a large market share in surveying equipment, the photogrammetric market is domi-
A PC-BASEDANALYTICALSTEREOPLOTTERFOR WETLANDINVENTORIES
573 1.0~Ol0 1.0000"20 1.0000~ 1.000~60
I 000L20
1.0~0122
l.O00lO
1,o0o244 1.ooo4~ 1.000977 1.00t955 1.003913 1.007841 1:013744 1,031735 1.064,178 1.133113 1.283944 1,64~$13 2.717595 7.383325 54.543020
1.ooo2o 1.oo04o 1.00081 1.00163 1,00328 1.00657 1.01319
i .000980 1.001950 1.003910
1007840 1.01574.0 1031740
(x,y)L
1064480 1.133110
(x,y)R
1.0GO00 1.00001 1.0GO02 1.00003
I.OG02~ 100o49o
from photographs
1,0o~(~ 1.00~15 1.00003] 1.000061
1.02656 1.05382
I.II033 1.23332 1,32109 2.31372 5,35334 28.65829 2974.94000O 2974.941000 821.29804 8850270.~ 8850273.0®000 ,574530.4706] 1,283940 1.648510 2.717600 7383320 54 543000
•
D A T A SEPARATION BY F E A T U R E C O O E S
aft data
°3 1 API90
MAIN
MENU
Current
JOBFILE
>) E X A M P L E . J O B
A. B. C. D. E. F. G. I. J. K. L.
- NEWJOB READJOB INTCOL CAMDATA INTORI RELCOL RELORI CPPFIL ABSORI PTCOL
- M a n a g e the J O B F I L E a r c h i v e list• -Initialize a new JOBFILE. -Examine the c o n t e n t s of a J O B F I L E . -Register l o c a t i o n of p h o t o s . -Establish Camera-constants file. -Compute Interior Orientation parameters. -Register parallax-free, photo-data. -Compute Relative Orientation parameters. =Build a Control {Pass) p o i n t s file. -Compute Absolute Orientation parameters. -Digitise p o i n t s , lines, a r e a s , ...
M. N. P.
PTOUT RVPTIN PLOTP
-Display
R. Q.
-Create an ASCII output -Read Revised ASCII file
file. into
JOBFILE.
r e s u l t s of d i g i t i s i n g . -HELP-descrlption o f M e n u selections. -QUIT-exit f r o m A P 1 9 0 M A I N M E N U to DOS. Choice? (Enter letter) : - -
v
Fig. 1. The PC converts photo measurements into digital form. The AP 190 analytical plotter creates ASCII output files which can be used for spreadsheets or computer-aided drawing programs.
nated by the Europeans: Swiss (Kern/Wild); German (Zeiss); Italian (O.M.I. and Galileo); and, for a minor part, some English and Australian companies. THE I N S T R U M E N T
One particular PC-based analytical plotter, the Carto AP 190, has demonstrated its usefulness as a field-office instrument. In 1983, the Royal Norwegian Council for Scientific and Industrial Research initiated a project to develop an inexpensive instrument that was simple to use, low in cost, compatible with mass-marketing software packages and portable for installa-
574
w.s. WARNER
Fig. 2. Carlo Instruments' AP 190 analytical plotter.
tion in field offices (Carson, 1987a). The desktop instrument, as shown in Fig. 2, is based upon a mirror stereoscope, and its mechanics are relatively simple. Essentially, it is a high-quality, mirror stereoscope - with interchangable optics ( 1-8 × magnification ) - set over a photo-carrier. The photo-carrier handles small- and large-format photographs, ranging from 35-mm transparencies to satellite imagery in print form. The functions and specifications of the AP 190 are discussed by Carson ( 1987 ). The instrument was rigorously tested, and it proved to provide the accuracy necessary for digital mapping from aerial photographs (Carson, 1985 ). The optics and measurement system provide photo coordinate accuracy to better than 20 lan (Carson, 1987b ), which corresponds to a ground distance of 2 ft ( ~0.65 m) on a 1:32 000scale photo. Software is sophisticated yet minimal; it provides digital output to implement the proven mathematics of photogrammetry. Specifically, it performs the standard photogrammetric orientation routines and removes y-parallax in real-time. Presented in a user-friendly menu format, management of operations is arranged in a logical sequence (i.e., the programs are ordered as they would be used for digitizing a stereomodel; Carson, 1987a). Of particular interest is the rather simple measurement operation. Since the photo-carrier, under computer control, continuously removes y-parallax through the stereomodel, the user can concentrate on the task at hand, examining features on the 3-dimensional terrain. When he looks through the optics, a small dot of light appears to float above the terrain. The dot can be lowered to the ground by turning a control knob on the photo-carrier. When the dot is at the point to be measured, a record button is pressed and the coordinates (X, Y, and Z) of the point are sent to the microcomputer and stored in a data file. Functionally, the instrument measures landscape features directly from a
A PC-BASED ANALYTICAL STEREOPLOTTER FOR WETLAND INVENTORIES
57 5
3-dimensional stereomodel. It can be used as a mensuration tool: counting objects; measuring line-length, area, and height; and computing special values, such as slope and azimuth. These data can then be entered into a geographical information system, a cartographic model, a statistical program, or transferred to a plotter for generating maps. To illustrate the AP 190's flexibility, three applications are presented. Application 1 - Mensuration
Although 80-85% of photogrammetry's application still consists of mapping, it is gaining prevalence in mensuration. For example, in 1987 the Norwegian Forest Research Institute used an AP 190 with large-format aerial photos to measure 24 landscape characteristics (e.g. the height of trees, the area of riparian buffers, the point location of residual trees, the length of logging roads .... ; Warner, 1988a). Using 1:15 000-scale blank-and-white prints, the AP 190 operator measured ground coordinates with a planimetric accuracy of 76 cm and a spot-height accuracy of 46 cm. The same analytical plotter was used by the Norwegian Institute for Soil and Forest Mapping, Department of Forest Mensuration, to measure tree height from 70-mm color slides. The photos were exposed simultaneously at 500 ft ( .~ 150 m ) from two Hasselblad cameras m o u n t e d on a light-weight aircraft, one camera m o u n t e d on each wing-tip. Even though the B: H ratio was small (i.e., the ratio of the distance between the two cameras to the flying height), the AP 190 was able to measure tree height to within 50 cm. Considering that the trees were narrow-topped conifers, this is an acceptable estimate for predicting the volume of trees. This accuracy with 70-mm photography is not an isolated case. A study using another PC-based analytical plotter, an Adam Technology MPS-2, with 70-mm photos, also measured tree heights to within 50 cm (Anonymous, 1987 ). The functions and specifications of the MPS-2 system are described by Chamard ( 1987 ). During the same period, 1987, the AP 190 was tested in two field offices in Alaska's Tongass National Forest. Using both large- and small-format photographs, Reutebuch ( 1987 ) conducted tests ofphotogrammetric approaches to data measurement, such as determining the area of a thinning unit and measuring debris in stream channels using 70-mm photography; he concluded that the analytical plotter offered a vastly improved way to directly measure features from aerial photos in a forestry field-office setting. Application 2 - CAM
In addition to providing mensuration data, aerial photographs provided data for computer-aided mapping (CAM). With an AP 190, photogrammetric data can be captured from photographs, linked with a variety of carto-
576
W.S+WARNER
234800
234700
234802~000
+
~
24100
24200
24300
i
24400
Fig. 3. Block diagram and corresponding topographic map created from 35-mm aerial photos ( l : 10 000 photo scale).
graphic programs, then illustrated on a plotter - all at one work-station. In this connection, the Norwegian Institute for Georesources and Pollution Research (GEFO) uses an AP 190 with small-format (35-mm) aerial photographs to collect data for digital terrain models (DTMs). As an example, Fig. 3 illustrates a block diagram and corresponding topographic map produced by digitizing random points on 1 : 10 000-scale stereomodel. The DTM has a spot height accuracy of 70 cm. GEFO's primary use of the AP 190 is to capture 3-dimensional data for describing a topographic profile, such as aspect or slope length. These quantifiable data are integrated with qualifying data, such as soil series, to develop a soil-erosion model, which will be capable of transforming the spatial data (collected with the AP 190) in order to determine high-risk areas of soil loss. Application 3 - GIS
A common field office application for a CAM program is to digitize maps for a geographical information system (GIS). Two GISs widely used in North
A PC-BASED ANALYTICAL STEREOPLOTTER FOR WETLAND INVENTORIES
577
America and Scandinavian forestry are Terrasofi and ARC/INFO. Like most GIS programs, they are designed to collect data from a digitizing board via the RS232 serial communication port (COM1), a standard feature on the IBM-PC and most compatibles. When entering GIS data directly from aerial photographs, the AP 190 serves as a digitizing tablet and its mounted stereomodel functions as a 3-dimensional map. The GIS programs accept the AP 190's string of ASCII characters as they would any digitizing tablet; depending upon the contents, the system either logs the coordinate as data or executes a menu-related function. For example, when objects on the photos are digitized, these photo-coordinates can be transformed into map-coordinates for map registration. In short, by digitizing polygon data directly from aerial photos, field offices can produce map products, manage spatial data, yield spatial statistics and facilitate cartographic modeling. The AP 190 has proven an efficient tool for GIS data input. The Alaska Region of the USDA Forest Service demonstrated that the AP 190 could input polygon information directly to a GIS system, and thus eliminate the need for error-prone and tedious 'eyeball' transfer of ploygons from photos to maps (Reutebuch, 1987). Currently, the Norwegian Institute for Soil and Forest Mapmaking (NIJOS) uses two AP 190s for Terrasoft data entry. This autumn ( 1989 ) the Institute will interface the AP 190s with ARC/INFO. DISCUSSION
A major issue facing the application of PC-based analytical plotters in field offices is staff capability to use the instruments. There are two questions: First, can users with little or no training in photogrammetry make acceptable measurements? Carson ( 1985 ) demonstrated that n e a , y everyone, from the experienced to the novice, can measure photo-coordinates with an AP 190 well enough to locate ground coordinates to better than 50 cm from 1 : 15 000 aerial photography. Later, Reutebuch ( 1987 ) reported that, in a workshop to instruct users, 24 of 25 forestry staff were able to measure the distance between two points on 1:12 000 paper prints to within 8 ft (~2.5 m) of the ground distance of 4550 ft ( ~ 1400 m). In both studies, the users had no more than 10 minutes training. The second question is more difficut to quantify: Are PC-based analytical plotters easy to use? One of the strongest features of any PC-based analytical plotter is its 'user-friendliness' - or so we are told by the manufacturers. But the word 'friendly' has as many different meanings as there are users; 'userfriendly' to a photogrammetrist developing the analytical routines, or a computer programmer preparing the software, does not necessarily mean 'userfriendly' to the field forester using the instrument. Most of the manufacturers report that their instruments are designed for
578
w.s.WARNER
those with limited training in photogrammetry; but, in general, their user manuals don't reflect this notion. Admittedly, after a few minutes of handson instruction, almost anyone can make simple measurements and perform basic functions with a PC-based analytical plotter. Of course, the training needed to use the entire system requires several days - if not weeks - of instruction, depending on the operator's knowledge of data handling and photogrammetry. Reutebuch (1987) reported that USDA Forest Service field staff can be trained to use the AP 190 system in less than a week. At the Norwegian Forest Research Institute, however, it took a m o n t h before the operator became confident. Some concluding words of advice: Field foresters will find their introduction to PC-based analytical plotters considerably easier if they are familiar with PC (DOS) systems and the basic concepts of aerial photography and photogrammetry. (Regarding the former, I recommend MSDOS (Norton, 1985) a n d , for the latter, Manual of Aerial Photography (Graham and Read, 1986 ) and Elements of Photogrammetry (Wolf, 1974 ). ) In addition to accuracy and ease of use, one of the greatest attractions to PC-based analytical plotters is that they are well-designed for Forest Service field offices. Most units easily fit on a desk-top and they're portable enough to transport in an automobile. Other than providing a PC, no specialized furnishings are needed. However, they interface with other equipment commonly found in the field office, such as a plotter and digitizing tablet. For example, when aerial photos lack control points, the AP 190 can be used with a digitizing tablet to bridge ground control from topographic maps (Reutebuch and Shea, 1988; Warner, 1988) ~ The final word of praise for PC-based analytical plotters is 'economy'. In the past, resource managers shied away from photogrammetry not only because training requirements were extensive, but also because the equipment was expensive. The PC changed that. Although there are few units priced under US$40 000, we are seeing new PC-based analytical plotters enter the market. For example, Zeiss now offers the P-3 Planicomp, and Galileo recently introduced its Stereobit - both new inexpensive desktop systems - and TOPCON will market its PD-1000 from spring 1989. Three years ago, only a few PC-based analytical plotters were commercially available; today there are several. It is to be expected that, within the next few years, more will enter the market. Since most systems are accurate enough to meet measurement and mapping standards, the competitive edge will go to manufacturers offering units that (i) are easier to use and more affordable than instruments currently on the market, and (ii) will not become quickly obsolete. The trend, I suspect, will be towards systems of modular design: i.e., qt is possible to obtain control points through photogrammetriccontrol extension (aerotriangulation), or 'bridging' as it is often called, by supplementingphoto control from a sparse network of ground-surveyedphoto control (Wolf, 1974).
A PC-BASED ANALYTICALSTEREOPLOTTER FOR WETLAND INVENTORIES
579
units which are tailored to the needs of a specialized field as well as the user's skill level in data handling and photogrammetry. This trend is apparently underway. We are just starting to see innovative alternatives to traditional stereo-analytical plotters. Although stereo-photogrammetry is the preferred technique for primary data acquisition for topographic mapping, there are negative aspects (Bethel, 1987 ). Consequently, a few manufacturers have developed 3-dimensional digitizing systems based on monocular viewing. Bethel (1987) explains how the Kern DMP- 1 digital monoplotter uses single photographs with stored DTM data to generate 3dimensional data. Another innovative approach is the Rolleimetric MR system. The photographs are taken with either a 35-mm or 70-mm metric camera. The grid plate in front of the film plane incorporates accurately spaced reference crosses, which appear in the photograph. Paper enlargements of these survey photographs are measured on a digitizing tablet with the aid of a magnified cursor. With these data, the Rolleimetric PC can provide accurate 3-dimensional reconstruction of the photographed object. In addition, the plotter connected to the computer can automatically produce ground-plans or elevations to scale as required. Although the hardware investments involved in photogrammetry are higher than traditional inventory equipment, the prices appear to be coming down. That is, the buyer gets more value for money than a few years ago. Moreover, innovations in software and graphics now enable photogrammetrically gathered data to be entered in a variety of mass-marketed and relatively inexpensive programs: map products, spatial data management, spatial statistics and cartographic modelling. Currently, the USDI's National Cartographic Information Center offers a listing of more than 800 sources for software for computer mapping and related disciplines (Anonymous, 1988 ). Looking towards the future, there is little doubt that computer-assisted inventory will rely upon PC-based photogrammetric equipment. Combined with recently improved small-format photography, light or microlight aircraft that can be employed at low cost, and a plethora of software packages, PC-based plotters will undoubtedly take an active role in wetland inventories. To illustrate just how close the future is, Graham ( 1988 ) conducted a map-revision survey for Britain's Ordnance Survey using a microlight aircraft, a 70-mm metric camera, and a PC-based stereocomparator. Now with the emergence of commercially available small-format 'metric' cameras and 2-seater microlights fitted with pontoons, one can foresee the advancements to come in wetland inventory and mapping. CONCLUSION With the PC an established tool in the field office, processing data is no longer the major challenge for the resource manager: the emphasis has shifted
580
W.S. WARNER
to acquiring reliable data for processing. For decades, photogrammetry has proven an accurate and efficient procedure for acquiring primary data for professional mapmakers. It was not that long ago that analogue plotters, using mechanical measurements systems, dominated photogrammetry. Today analytical plotters using sophisticated software are the industry standard. Over the past decade, several analytical plotters have been introduced by major manufacturers. Most use a host minicomputer, are designed for professional mapmakers, and are too expensive for field offices. There are, however, a few PC-supported units designed for those with limited photogrammetric training, and which fit the resource manager's budget. When we consider the investments involved in inventory systems, we should note the basic advantages of PC-based photogrammetry: (i) field offices have easy access to current and historical aerial photos, taken at different scales; (ii) data acquisition using photogrammetric methodology is faster than data collected directly in the field; (iii) the portable, desktop equipment does not require a special office setting (e.g., air conditioning); (iv) data is directly obtained in referred coordinate systems; (v) photo-measurement data interacts with a variety of cartographic systems and statistical programs; and (vi) the instrument is compatible with other PC-based office equipment (plotters, printers, digitizing tablets ). Studies in Norway, England, Australia, and the U.S.A. have demonstrated that PC-based photogrammetry offers the field office ultimate measurement accuracy, speed and economy. But despite the immediate appeal, managers should be acutely aware that it is not sufficient for an organization to purchase an analytical plotter with some attractive graphics software, hire or retrain one or two enthusiastic individuals and then expect instant success. For any service organization dealing with complex products, as in the manufacturing industry, new tools can only be used effectively if they are properly integrated into the whole work process and not tacked on as an afterthought.
REFERENCES Anonymous, 1987. Land Management Applications Using the Adam Technology MPS2 to Digitizc Small Format Photography. Western Australian Department of Conservation and Land Management, Como; pp. 2-13. Anonymous, 1988. Sources for Software for Computer Mapping and Related Disciplines. National Cartographic Information Center, USDI Geological Survey, Reston, VA, 429 pp. Bethel, J., 1987. DMP-I, the Kern digital monoplottcr.In: Proc. Int. Conf. and Workshop on Analytical, Instrumentation, International Society for Photogrammctry and Remotc Sen-
APC-BASEDANALYTICALSTEREOPLOTTERFORWETLANDINVENTORIES
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sing, 2-6 November 1987, Phoenix, AZ. American Society for Photogrammetry and Remote Sensing, Falls Church, VA, pp. 278-281. Carson, W.W., 1985. An accuracy test of a new stereoscopic instrument. Kartog. Plan., 2 (85 ): 227-233. Carson, W.W., 1987a. An analytical plotter based upon a personal computer. In: Proc. Int. Conf. and Workshop on Analytical Instrumentation, International Society for Photogrammetry and Remote Sensing, 2-6 November 1987, Phoenix, AZ. American Society for Photogrammetry & Remote Sensing, Falls Church, VA, pp. 251-259. Carson, W.W., 1987b. Development of an inexpensive analytical plotter, Photogramm. Rec., 12 (69): 303-306. Chamard, R.R., 1987. The MPS-2 microanalytical photgrammetric system: a small-format system for close-range photogrammetry. In: Proc. Int. Conf. and Workshop on Analytical Instrumentation, International Society for Photogrammetry and Remote Sensing, 2-6 November 1987, Phoenix, AZ. American Society for Photogrammetry & Remote Sensing, Falls Church, VA, pp. 233-238. Graham, R.W., 1988. Small format aerial surveys from light and microlight aircraft. Photogramm. Rec., 12 (71): 561-573. Graham, R.W. and Read, R.E., 1986. Manual of Aerial Photography. Butterworth, London, 346 pp. Kleinn, K.E., 1987. PC power in analytical photogrammetry. In: Proc. Int. Conf. Workshop on Analytical Instrumentation Society for Photogrammetry and Remote Sensing, 2-6.November 1987, Phoenix, AZ. American Society for Photogrammetry & Remote Sensing, Falls Church, VA, PP. 260-267. Koency, G., 1980 How the analytical plotter works and differs from an analog plotter. In: Proc. Analytical Plotter Symp. and Workshop, 20-25 April. American Society for Photogrammetry, Falls Church, VA, pp. 31-75. Norton, P., 1985. PC-DOS: Introduction to High Performance Computing. Prentice Hall, New York, 324 pp. Reutebuch, S.E., 1987. PC-based anaytical stereoplotter for use in Forest Service field offices. 1987 ASPRS-ACSM Fall Convention. American Society Photogrammetry & Remote Sensing, Falls Church, VA Tech. Pap., pp. 223-236. Reutebuch, S.E. and Shea, R.D., 1988. A method to control large-scale aerial photos when surveyed ground control is unavailable. USDA Forest Service, Pacific Northwest Experiment Station, Seattle, WA, 6 pp. (Unpubl). Warner, W.S., 1988a. Multiple-use characteristicsof Norwegian clearcuts: using aerial photrographs to digitize in three dimnesions. Scand. J. For. Res., 3: 401-416. Warner, W.S., 1988b. Bridging control for 35mm aerial photos: Problems when surveyed ground control is unavailable. Kartog. Plan., 88: 655-661. Westerhuis, K., 1988. Photogrammetry - an underated measuring technique. Geod. Info. Mag., 2: II. Wolf, P.R., 1974. Elements of Photogrammetry. McGraw-Hill, New York, 562 pp.
571
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
A PC-based analytical stereoplotter for wetland inventories: An efficient and economical photogrammetric instrument for field offices William S. Warner Institute for Georesourcesand Pollution Research, The Agricultural Research Council of Norway, Boks 9 NLH 1432-~s (Norway.)
ABSTRACT Warner, W.S., 1990. A PC-based analytical stereoplotter for wetland inventories: An efficient and economical photogrammetric instrument for field offices. For. Ecol. Manage., 33/34:571-581.
Highly flexible photogrammetric systems, controlled entirely by PC technology, can collect primary data from aerial photographs and process these data in a variety of cartographic systems and statistical programs. This paper focuses on a PC-based analytical plotter used by the USDA Forest Service and several organizations in Norway. Its accuracy (20 microns), flexible application (from making simple 3-D measurements to GIS data entry), and cost (US$30 000) make it suitable for field-office purposes. Discussion includes other affordable, efficient and portable PC-based instruments that now enable the field office to make 3-dimensional measurements directly from aerial photographs.
INTRODUCTION
Aerial photography is one of the most valued sources of information for wetland identification and inventory. Originally, photogrammetry was developed to derive data from inaccessible or large areas to produce maps. Today, with the aid of computers, micro-electronics and software, it is one of the most sophisticated measuring techniques. Moreover, photogrammetry is capable of obtaining results just as accurate, if not more so, than conventional surveying methods (Westerhuis, 1988). Although aerial photographs are a primary source of information for forest inventory, methods of using aerial photographs in the field offices have remained nearly unchanged for several decades. And despite proven application of computer-based photogrammetry to forestry, it still appears far removed from the field forester. Where we have seen significant advancements in the application of photogrammetry is not in forestry field offices, but in the regional offices. The organizational trend towards decentralization is bringing many activ0378-1127/90/$03.50
© 1990 Elsevier Science Publishers B.V.
572
W.S. WARNER
TABLE 1 PC-based analytical plotters Priced under US$40 000. Manufacturer
Model
Description
MPS
Basic instrument of novel design. 4-micron accuracy at photo scale. Limited to smallformat photos only.
AP190
Optical mechanical unit that handles small- and large-format photos. 20-micron accuracy.
Stereobit
New, simple optical mechanical instrument. Resolution (not accuracy) l0 microns.
Adams
Carto
Galileo
ities that were once confined to a central office now to the field. Not long ago, decision-makers depended upon - and to a certain degree were hobbled by large mainframes or expensive minicomputers for data analysis and management. The introduction of the personal computer (PC) changed that by making computing more accessible, efficient, and economical. In field offices today, the PC is nearly as common as the pocket stereoscope. And now we are just starting to see the PC make photogrammetric equipment suitable for the field offices. Photogrammetry is a three-dimensional-measurement science, and the fundamental problems of the analytical approach have essentially all been solved. One of the most important products resulting from the solution of these problems has been the analytical stereoplotter, which has finally found almost universal acceptance in the commercial mapping community (Kleinn, 1987). An analytical stereoplotter (sometimes called an 'analytical plotter') is a stereocomparator, encoded so that coordinate measurements on photographs can be passed to a computer and converted into digital form (Konency, 1980; see Fig. 1 ). The three-dimensional (X, Y and Z) measurements are digitized by mounting the measuring mark on a suitable cross-slide system and encoding each of these axes individually, using linear or rotary encoders. The PC determines the relative position of the stereomodel and drives the photographs to their positions in real-time. Currently there are several PC-based analytical plotters on the market, ranging in price from US$28 000 upwards. Whereas a few PC-based sell for less than US$40 000 (Table 1 ), the majority of systems are priced between $50 000 and $150 000. Whereas the Japanese manufacturers have a large market share in surveying equipment, the photogrammetric market is domi-
A PC-BASEDANALYTICALSTEREOPLOTTERFOR WETLANDINVENTORIES
573 1.0~Ol0 1.0000"20 1.0000~ 1.000~60
I 000L20
1.0~0122
l.O00lO
1,o0o244 1.ooo4~ 1.000977 1.00t955 1.003913 1.007841 1:013744 1,031735 1.064,178 1.133113 1.283944 1,64~$13 2.717595 7.383325 54.543020
1.ooo2o 1.oo04o 1.00081 1.00163 1,00328 1.00657 1.01319
i .000980 1.001950 1.003910
1007840 1.01574.0 1031740
(x,y)L
1064480 1.133110
(x,y)R
1.0GO00 1.00001 1.0GO02 1.00003
I.OG02~ 100o49o
from photographs
1,0o~(~ 1.00~15 1.00003] 1.000061
1.02656 1.05382
I.II033 1.23332 1,32109 2.31372 5,35334 28.65829 2974.94000O 2974.941000 821.29804 8850270.~ 8850273.0®000 ,574530.4706] 1,283940 1.648510 2.717600 7383320 54 543000
•
D A T A SEPARATION BY F E A T U R E C O O E S
aft data
°3 1 API90
MAIN
MENU
Current
JOBFILE
>) E X A M P L E . J O B
A. B. C. D. E. F. G. I. J. K. L.
- NEWJOB READJOB INTCOL CAMDATA INTORI RELCOL RELORI CPPFIL ABSORI PTCOL
- M a n a g e the J O B F I L E a r c h i v e list• -Initialize a new JOBFILE. -Examine the c o n t e n t s of a J O B F I L E . -Register l o c a t i o n of p h o t o s . -Establish Camera-constants file. -Compute Interior Orientation parameters. -Register parallax-free, photo-data. -Compute Relative Orientation parameters. =Build a Control {Pass) p o i n t s file. -Compute Absolute Orientation parameters. -Digitise p o i n t s , lines, a r e a s , ...
M. N. P.
PTOUT RVPTIN PLOTP
-Display
R. Q.
-Create an ASCII output -Read Revised ASCII file
file. into
JOBFILE.
r e s u l t s of d i g i t i s i n g . -HELP-descrlption o f M e n u selections. -QUIT-exit f r o m A P 1 9 0 M A I N M E N U to DOS. Choice? (Enter letter) : - -
v
Fig. 1. The PC converts photo measurements into digital form. The AP 190 analytical plotter creates ASCII output files which can be used for spreadsheets or computer-aided drawing programs.
nated by the Europeans: Swiss (Kern/Wild); German (Zeiss); Italian (O.M.I. and Galileo); and, for a minor part, some English and Australian companies. THE I N S T R U M E N T
One particular PC-based analytical plotter, the Carto AP 190, has demonstrated its usefulness as a field-office instrument. In 1983, the Royal Norwegian Council for Scientific and Industrial Research initiated a project to develop an inexpensive instrument that was simple to use, low in cost, compatible with mass-marketing software packages and portable for installa-
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Fig. 2. Carlo Instruments' AP 190 analytical plotter.
tion in field offices (Carson, 1987a). The desktop instrument, as shown in Fig. 2, is based upon a mirror stereoscope, and its mechanics are relatively simple. Essentially, it is a high-quality, mirror stereoscope - with interchangable optics ( 1-8 × magnification ) - set over a photo-carrier. The photo-carrier handles small- and large-format photographs, ranging from 35-mm transparencies to satellite imagery in print form. The functions and specifications of the AP 190 are discussed by Carson ( 1987 ). The instrument was rigorously tested, and it proved to provide the accuracy necessary for digital mapping from aerial photographs (Carson, 1985 ). The optics and measurement system provide photo coordinate accuracy to better than 20 lan (Carson, 1987b ), which corresponds to a ground distance of 2 ft ( ~0.65 m) on a 1:32 000scale photo. Software is sophisticated yet minimal; it provides digital output to implement the proven mathematics of photogrammetry. Specifically, it performs the standard photogrammetric orientation routines and removes y-parallax in real-time. Presented in a user-friendly menu format, management of operations is arranged in a logical sequence (i.e., the programs are ordered as they would be used for digitizing a stereomodel; Carson, 1987a). Of particular interest is the rather simple measurement operation. Since the photo-carrier, under computer control, continuously removes y-parallax through the stereomodel, the user can concentrate on the task at hand, examining features on the 3-dimensional terrain. When he looks through the optics, a small dot of light appears to float above the terrain. The dot can be lowered to the ground by turning a control knob on the photo-carrier. When the dot is at the point to be measured, a record button is pressed and the coordinates (X, Y, and Z) of the point are sent to the microcomputer and stored in a data file. Functionally, the instrument measures landscape features directly from a
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3-dimensional stereomodel. It can be used as a mensuration tool: counting objects; measuring line-length, area, and height; and computing special values, such as slope and azimuth. These data can then be entered into a geographical information system, a cartographic model, a statistical program, or transferred to a plotter for generating maps. To illustrate the AP 190's flexibility, three applications are presented. Application 1 - Mensuration
Although 80-85% of photogrammetry's application still consists of mapping, it is gaining prevalence in mensuration. For example, in 1987 the Norwegian Forest Research Institute used an AP 190 with large-format aerial photos to measure 24 landscape characteristics (e.g. the height of trees, the area of riparian buffers, the point location of residual trees, the length of logging roads .... ; Warner, 1988a). Using 1:15 000-scale blank-and-white prints, the AP 190 operator measured ground coordinates with a planimetric accuracy of 76 cm and a spot-height accuracy of 46 cm. The same analytical plotter was used by the Norwegian Institute for Soil and Forest Mapping, Department of Forest Mensuration, to measure tree height from 70-mm color slides. The photos were exposed simultaneously at 500 ft ( .~ 150 m ) from two Hasselblad cameras m o u n t e d on a light-weight aircraft, one camera m o u n t e d on each wing-tip. Even though the B: H ratio was small (i.e., the ratio of the distance between the two cameras to the flying height), the AP 190 was able to measure tree height to within 50 cm. Considering that the trees were narrow-topped conifers, this is an acceptable estimate for predicting the volume of trees. This accuracy with 70-mm photography is not an isolated case. A study using another PC-based analytical plotter, an Adam Technology MPS-2, with 70-mm photos, also measured tree heights to within 50 cm (Anonymous, 1987 ). The functions and specifications of the MPS-2 system are described by Chamard ( 1987 ). During the same period, 1987, the AP 190 was tested in two field offices in Alaska's Tongass National Forest. Using both large- and small-format photographs, Reutebuch ( 1987 ) conducted tests ofphotogrammetric approaches to data measurement, such as determining the area of a thinning unit and measuring debris in stream channels using 70-mm photography; he concluded that the analytical plotter offered a vastly improved way to directly measure features from aerial photos in a forestry field-office setting. Application 2 - CAM
In addition to providing mensuration data, aerial photographs provided data for computer-aided mapping (CAM). With an AP 190, photogrammetric data can be captured from photographs, linked with a variety of carto-
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234800
234700
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+
~
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24200
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i
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Fig. 3. Block diagram and corresponding topographic map created from 35-mm aerial photos ( l : 10 000 photo scale).
graphic programs, then illustrated on a plotter - all at one work-station. In this connection, the Norwegian Institute for Georesources and Pollution Research (GEFO) uses an AP 190 with small-format (35-mm) aerial photographs to collect data for digital terrain models (DTMs). As an example, Fig. 3 illustrates a block diagram and corresponding topographic map produced by digitizing random points on 1 : 10 000-scale stereomodel. The DTM has a spot height accuracy of 70 cm. GEFO's primary use of the AP 190 is to capture 3-dimensional data for describing a topographic profile, such as aspect or slope length. These quantifiable data are integrated with qualifying data, such as soil series, to develop a soil-erosion model, which will be capable of transforming the spatial data (collected with the AP 190) in order to determine high-risk areas of soil loss. Application 3 - GIS
A common field office application for a CAM program is to digitize maps for a geographical information system (GIS). Two GISs widely used in North
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America and Scandinavian forestry are Terrasofi and ARC/INFO. Like most GIS programs, they are designed to collect data from a digitizing board via the RS232 serial communication port (COM1), a standard feature on the IBM-PC and most compatibles. When entering GIS data directly from aerial photographs, the AP 190 serves as a digitizing tablet and its mounted stereomodel functions as a 3-dimensional map. The GIS programs accept the AP 190's string of ASCII characters as they would any digitizing tablet; depending upon the contents, the system either logs the coordinate as data or executes a menu-related function. For example, when objects on the photos are digitized, these photo-coordinates can be transformed into map-coordinates for map registration. In short, by digitizing polygon data directly from aerial photos, field offices can produce map products, manage spatial data, yield spatial statistics and facilitate cartographic modeling. The AP 190 has proven an efficient tool for GIS data input. The Alaska Region of the USDA Forest Service demonstrated that the AP 190 could input polygon information directly to a GIS system, and thus eliminate the need for error-prone and tedious 'eyeball' transfer of ploygons from photos to maps (Reutebuch, 1987). Currently, the Norwegian Institute for Soil and Forest Mapmaking (NIJOS) uses two AP 190s for Terrasoft data entry. This autumn ( 1989 ) the Institute will interface the AP 190s with ARC/INFO. DISCUSSION
A major issue facing the application of PC-based analytical plotters in field offices is staff capability to use the instruments. There are two questions: First, can users with little or no training in photogrammetry make acceptable measurements? Carson ( 1985 ) demonstrated that n e a , y everyone, from the experienced to the novice, can measure photo-coordinates with an AP 190 well enough to locate ground coordinates to better than 50 cm from 1 : 15 000 aerial photography. Later, Reutebuch ( 1987 ) reported that, in a workshop to instruct users, 24 of 25 forestry staff were able to measure the distance between two points on 1:12 000 paper prints to within 8 ft (~2.5 m) of the ground distance of 4550 ft ( ~ 1400 m). In both studies, the users had no more than 10 minutes training. The second question is more difficut to quantify: Are PC-based analytical plotters easy to use? One of the strongest features of any PC-based analytical plotter is its 'user-friendliness' - or so we are told by the manufacturers. But the word 'friendly' has as many different meanings as there are users; 'userfriendly' to a photogrammetrist developing the analytical routines, or a computer programmer preparing the software, does not necessarily mean 'userfriendly' to the field forester using the instrument. Most of the manufacturers report that their instruments are designed for
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those with limited training in photogrammetry; but, in general, their user manuals don't reflect this notion. Admittedly, after a few minutes of handson instruction, almost anyone can make simple measurements and perform basic functions with a PC-based analytical plotter. Of course, the training needed to use the entire system requires several days - if not weeks - of instruction, depending on the operator's knowledge of data handling and photogrammetry. Reutebuch (1987) reported that USDA Forest Service field staff can be trained to use the AP 190 system in less than a week. At the Norwegian Forest Research Institute, however, it took a m o n t h before the operator became confident. Some concluding words of advice: Field foresters will find their introduction to PC-based analytical plotters considerably easier if they are familiar with PC (DOS) systems and the basic concepts of aerial photography and photogrammetry. (Regarding the former, I recommend MSDOS (Norton, 1985) a n d , for the latter, Manual of Aerial Photography (Graham and Read, 1986 ) and Elements of Photogrammetry (Wolf, 1974 ). ) In addition to accuracy and ease of use, one of the greatest attractions to PC-based analytical plotters is that they are well-designed for Forest Service field offices. Most units easily fit on a desk-top and they're portable enough to transport in an automobile. Other than providing a PC, no specialized furnishings are needed. However, they interface with other equipment commonly found in the field office, such as a plotter and digitizing tablet. For example, when aerial photos lack control points, the AP 190 can be used with a digitizing tablet to bridge ground control from topographic maps (Reutebuch and Shea, 1988; Warner, 1988) ~ The final word of praise for PC-based analytical plotters is 'economy'. In the past, resource managers shied away from photogrammetry not only because training requirements were extensive, but also because the equipment was expensive. The PC changed that. Although there are few units priced under US$40 000, we are seeing new PC-based analytical plotters enter the market. For example, Zeiss now offers the P-3 Planicomp, and Galileo recently introduced its Stereobit - both new inexpensive desktop systems - and TOPCON will market its PD-1000 from spring 1989. Three years ago, only a few PC-based analytical plotters were commercially available; today there are several. It is to be expected that, within the next few years, more will enter the market. Since most systems are accurate enough to meet measurement and mapping standards, the competitive edge will go to manufacturers offering units that (i) are easier to use and more affordable than instruments currently on the market, and (ii) will not become quickly obsolete. The trend, I suspect, will be towards systems of modular design: i.e., qt is possible to obtain control points through photogrammetriccontrol extension (aerotriangulation), or 'bridging' as it is often called, by supplementingphoto control from a sparse network of ground-surveyedphoto control (Wolf, 1974).
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units which are tailored to the needs of a specialized field as well as the user's skill level in data handling and photogrammetry. This trend is apparently underway. We are just starting to see innovative alternatives to traditional stereo-analytical plotters. Although stereo-photogrammetry is the preferred technique for primary data acquisition for topographic mapping, there are negative aspects (Bethel, 1987 ). Consequently, a few manufacturers have developed 3-dimensional digitizing systems based on monocular viewing. Bethel (1987) explains how the Kern DMP- 1 digital monoplotter uses single photographs with stored DTM data to generate 3dimensional data. Another innovative approach is the Rolleimetric MR system. The photographs are taken with either a 35-mm or 70-mm metric camera. The grid plate in front of the film plane incorporates accurately spaced reference crosses, which appear in the photograph. Paper enlargements of these survey photographs are measured on a digitizing tablet with the aid of a magnified cursor. With these data, the Rolleimetric PC can provide accurate 3-dimensional reconstruction of the photographed object. In addition, the plotter connected to the computer can automatically produce ground-plans or elevations to scale as required. Although the hardware investments involved in photogrammetry are higher than traditional inventory equipment, the prices appear to be coming down. That is, the buyer gets more value for money than a few years ago. Moreover, innovations in software and graphics now enable photogrammetrically gathered data to be entered in a variety of mass-marketed and relatively inexpensive programs: map products, spatial data management, spatial statistics and cartographic modelling. Currently, the USDI's National Cartographic Information Center offers a listing of more than 800 sources for software for computer mapping and related disciplines (Anonymous, 1988 ). Looking towards the future, there is little doubt that computer-assisted inventory will rely upon PC-based photogrammetric equipment. Combined with recently improved small-format photography, light or microlight aircraft that can be employed at low cost, and a plethora of software packages, PC-based plotters will undoubtedly take an active role in wetland inventories. To illustrate just how close the future is, Graham ( 1988 ) conducted a map-revision survey for Britain's Ordnance Survey using a microlight aircraft, a 70-mm metric camera, and a PC-based stereocomparator. Now with the emergence of commercially available small-format 'metric' cameras and 2-seater microlights fitted with pontoons, one can foresee the advancements to come in wetland inventory and mapping. CONCLUSION With the PC an established tool in the field office, processing data is no longer the major challenge for the resource manager: the emphasis has shifted
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to acquiring reliable data for processing. For decades, photogrammetry has proven an accurate and efficient procedure for acquiring primary data for professional mapmakers. It was not that long ago that analogue plotters, using mechanical measurements systems, dominated photogrammetry. Today analytical plotters using sophisticated software are the industry standard. Over the past decade, several analytical plotters have been introduced by major manufacturers. Most use a host minicomputer, are designed for professional mapmakers, and are too expensive for field offices. There are, however, a few PC-supported units designed for those with limited photogrammetric training, and which fit the resource manager's budget. When we consider the investments involved in inventory systems, we should note the basic advantages of PC-based photogrammetry: (i) field offices have easy access to current and historical aerial photos, taken at different scales; (ii) data acquisition using photogrammetric methodology is faster than data collected directly in the field; (iii) the portable, desktop equipment does not require a special office setting (e.g., air conditioning); (iv) data is directly obtained in referred coordinate systems; (v) photo-measurement data interacts with a variety of cartographic systems and statistical programs; and (vi) the instrument is compatible with other PC-based office equipment (plotters, printers, digitizing tablets ). Studies in Norway, England, Australia, and the U.S.A. have demonstrated that PC-based photogrammetry offers the field office ultimate measurement accuracy, speed and economy. But despite the immediate appeal, managers should be acutely aware that it is not sufficient for an organization to purchase an analytical plotter with some attractive graphics software, hire or retrain one or two enthusiastic individuals and then expect instant success. For any service organization dealing with complex products, as in the manufacturing industry, new tools can only be used effectively if they are properly integrated into the whole work process and not tacked on as an afterthought.
REFERENCES Anonymous, 1987. Land Management Applications Using the Adam Technology MPS2 to Digitizc Small Format Photography. Western Australian Department of Conservation and Land Management, Como; pp. 2-13. Anonymous, 1988. Sources for Software for Computer Mapping and Related Disciplines. National Cartographic Information Center, USDI Geological Survey, Reston, VA, 429 pp. Bethel, J., 1987. DMP-I, the Kern digital monoplottcr.In: Proc. Int. Conf. and Workshop on Analytical, Instrumentation, International Society for Photogrammctry and Remotc Sen-
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sing, 2-6 November 1987, Phoenix, AZ. American Society for Photogrammetry and Remote Sensing, Falls Church, VA, pp. 278-281. Carson, W.W., 1985. An accuracy test of a new stereoscopic instrument. Kartog. Plan., 2 (85 ): 227-233. Carson, W.W., 1987a. An analytical plotter based upon a personal computer. In: Proc. Int. Conf. and Workshop on Analytical Instrumentation, International Society for Photogrammetry and Remote Sensing, 2-6 November 1987, Phoenix, AZ. American Society for Photogrammetry & Remote Sensing, Falls Church, VA, pp. 251-259. Carson, W.W., 1987b. Development of an inexpensive analytical plotter, Photogramm. Rec., 12 (69): 303-306. Chamard, R.R., 1987. The MPS-2 microanalytical photgrammetric system: a small-format system for close-range photogrammetry. In: Proc. Int. Conf. and Workshop on Analytical Instrumentation, International Society for Photogrammetry and Remote Sensing, 2-6 November 1987, Phoenix, AZ. American Society for Photogrammetry & Remote Sensing, Falls Church, VA, pp. 233-238. Graham, R.W., 1988. Small format aerial surveys from light and microlight aircraft. Photogramm. Rec., 12 (71): 561-573. Graham, R.W. and Read, R.E., 1986. Manual of Aerial Photography. Butterworth, London, 346 pp. Kleinn, K.E., 1987. PC power in analytical photogrammetry. In: Proc. Int. Conf. Workshop on Analytical Instrumentation Society for Photogrammetry and Remote Sensing, 2-6.November 1987, Phoenix, AZ. American Society for Photogrammetry & Remote Sensing, Falls Church, VA, PP. 260-267. Koency, G., 1980 How the analytical plotter works and differs from an analog plotter. In: Proc. Analytical Plotter Symp. and Workshop, 20-25 April. American Society for Photogrammetry, Falls Church, VA, pp. 31-75. Norton, P., 1985. PC-DOS: Introduction to High Performance Computing. Prentice Hall, New York, 324 pp. Reutebuch, S.E., 1987. PC-based anaytical stereoplotter for use in Forest Service field offices. 1987 ASPRS-ACSM Fall Convention. American Society Photogrammetry & Remote Sensing, Falls Church, VA Tech. Pap., pp. 223-236. Reutebuch, S.E. and Shea, R.D., 1988. A method to control large-scale aerial photos when surveyed ground control is unavailable. USDA Forest Service, Pacific Northwest Experiment Station, Seattle, WA, 6 pp. (Unpubl). Warner, W.S., 1988a. Multiple-use characteristicsof Norwegian clearcuts: using aerial photrographs to digitize in three dimnesions. Scand. J. For. Res., 3: 401-416. Warner, W.S., 1988b. Bridging control for 35mm aerial photos: Problems when surveyed ground control is unavailable. Kartog. Plan., 88: 655-661. Westerhuis, K., 1988. Photogrammetry - an underated measuring technique. Geod. Info. Mag., 2: II. Wolf, P.R., 1974. Elements of Photogrammetry. McGraw-Hill, New York, 562 pp.