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A concept for topographic map updating using digital orthophotos
Photogrammetria, 40 (1985) 87--94 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
87
A C O N C E P T F O R T O P O G R A P H I C MAP U P D A T I N G U S I N G D I G I T A L ORTHOPHOTOS
J~IRGEN PETERLE Institute for Photogrammetry and Topography, University o f Karlsruhe, Englerstrasse 7, D-7500 Karlsruhe 1, F.R. Germany
(Received March 29, 1.984)
ABSTRACT Peterle, J., 1985. A concept for topographic map updating using digital orthophotos. Photogrammetria, 40: 87--94. A method for topographic map updating is based on two digital orthophotos from digital aerial photographs. The first orthophoto corresponds to the old level of the map, the second contains, among others, the updating information. The working-steps begin with orthophoto computation and end with the generation of a screened groundplan of the map; they are done semi-automatically by computer. The concept of this method is introduced; problems of extracting cartographically relevant information from digital images are discussed and results are shown.
INTRODUCTION U p d a t i n g t o p o g r a p h i c maps always includes an e n o r m o u s a m o u n t o f w o r k . It is o b v i o u s t h a t e f f o r t s t o speed up this process have been made. T h e s e include t h e use o f rub-on-plastics f o r line-elements, s y m b o l s etc., and t h e use o f o r t h o p h o t o s . Including o r t h o p h o t o s in the updating-process, red u c e d t h e r e q u i r e d w o r k - l o a d o f the L a n d e s v e r m e s s u n g s a m t (National Survey Office) o f B a d e n - W u e r t t e m b e r g b y u p t o 30%. F o r the series T K 50 ( T o p o g r a p h i c Map 1 : 5 0 0 0 0 ) t h e revision p e r i o d is n o w 5 years, b u t still a b o u t 10 years f o r the series T K 2 5 . Meanwhile o r t h o p h o t o g e n e r a t i o n with m i n i - c o m p u t e r s is possible at m o d e r a t e cost (Wiesel, 1 9 8 3 ) . This is o n e reason, w h y a m e t h o d f o r t o p o g r a p h i c m a p u p d a t i n g using digital o r t h o p h o t o s is being d e v e l o p e d at the I n s t i t u t e for P h o t o g r a m m e t r y and T o p o g r a p h y (IPT) at t h e T e c h n i c a l University o f Karlsruhe ( W - G e r m a n y ) . SHORT REVIEW OF A CONVENTIONAL TECHNIQUE The updating technique of the Landesvermessungsamt Baden-Wuerttemberg (Ewig a n d Riedinger, 1 9 7 6 ) is divided into t h r e e parts: (a) p r e p a r a t o r y w o r k (data acquisition, o r t h o p h o t o g e n e r a t i o n b y Zeiss O r t h o c o m p Z 2
0031-8663/85/$03,30
© 1985 Elsevier Science Publishers B.V.
etc.); (b) map revision; (c) reproduction work. The central part of this method is the comparison between orthophoto and old ground plan. A blue copy of the old ground plan is superposed with the orthophoto on a light-table. The cartographer traces the differences from the orthophoto to the blue copy, generalization and displacement follow immediately by freehanddrawing. In addition a mask for deleting the old parts of the ground plan is made. Hereby the material for the reproduction work is provided. CONCEPT FOR A "DIGITAL METHOD"
The digital method retains the three parts of the conventional method. But a comparison between orthophoto and old ground plan by computer is not possible, since the ground plan is generalized. Therefore, the central element of the digital method is a comparison of two orthophotos: the first corresponds to the old level of the map, the second contains, among others, the updating-information. This comparison of the edge-detected orthophotos pixel by pixel will facilitate the location of areas with differences in the orthophotos. These areas are separated and treated with special techniques in order to extract cartographically relevant information (roads, buildings) and to process it interactively. The result consists of a line,skeleton of the new elements of the ground plan. According to desired map scale, the lines must be generalized and adapted to the map. An essential difference to the con, ventional methods is the separation between extraction of planimetric data and its map-conform presentation (generalization). The conventional method calls for feature extraction, generalization, and displacement in one step; the advantage of the new digital method is the possibility to use the extracted features for updating maps of a different scale. Digital map updating (concept)
A. Preparatory activities a. b. c. d.
Revision program Photographic flight (planning and mission)
Data acquisition (field-exploration etc.) Orthophoto generation (working scale (ws) 1:10 000)
B.
Map revision a. Location of updating-areas in the orthophotos (ws) b. Object-extraction and interactive w ~ r k ~ o u p (ws) c. Generalization and displa~ment (map scale (ms)) d. Adaption of symbols (ms)
C.
Reproductive activities a. Adaption of updating,areas to the digital map; deleting of unwanted elements in the old ground p ~ (ms) b. Checking of the u p d a ~ map at the display; data saving c. Screening d. Output of the screened map to folio
89 PROBLEMS OF REALIZATION
The realization of the concept depends on the answer to all problems, concerning extraction of cartographically relevant information from digital images, and digital techniques of generalization and displacement of groundplan-elements. Especially the extraction of small objects in built-up areas is very difficult. The different methods of pattern recognition are not able to satisfy the high demands of cartographic applications; located objects must be worked up interactively. Nevertheless, interactive processing is considerably shortened by automatic object-extraction. For purposes of map design the problems of generalization and displacement are not yet solved. ACTUAL STATE OF THE METHOD The actual state of the m e t h o d is shown by Figs. 1 to 8. Fig. 1 shows the two digital orthophotos as computed from digital aerial photographs. The first o r t h o p h o t o (Fig. 1 left) corresponds to the old level of the map, the second (Fig. 1 right) contains, among others, the updating-information. Edge-detection restricts the orthophotos to cartographically relevant information (e.g. boundary-lines of roads and buildings, etc.), so that they get a
Fig. 1.
90
planimetric presentation. Fig. 2 (left) shows the result of edge-detection oi the first o r t h o p h o t o , Fig. 2 (right) the superposition of the result of both edge
Fig. 2.
For locating small objects, the updating-areas in both o r t h o p h o t o s are treated with a location-procedure, which analyses by statistical m e t h o d s whether a pixel belongs to an object of specific extension, and, if so, flags it {Figs. 3 and 4). The flags in both images are checked for a radius of tolerance in the counter-image, and are cancelled if they violate a position criterion. The result of this location means, that there is -- with high probability - - a small object at the marked position (Fig. 5). The second part of this location,procedure is n o t y e t programmed; Hereby small areas around the marks shall be analysed to find contour-lines a n d the position of a building using conditions like rectangularity, side.ratios, a n d s o on.
Figure 6 shows an updating-area in both o r t h o p h o t o s with linear elements. After preprocessing and edge~letection, the updating-area is worked up inter-
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93
Fig. 7.
Fig. 8.
94 actively on a display station: line-smoothing, detetmg of undesired pixets and line-elements, adaption of line-connections. Then the new elements {Fig. 7 above), extracted from the second o r t h o p h o t o , are copied into the old map (Fig. 7 below), some parts of the old map -- such as those being crossed by the new elements -- must b e deleted; the final result is the line.skeleton of the new map (Fig. 8 above). The superposition of the extracted map with the second o r t h o p h o t o is shown in Fig. 8 (below). CONCLUSIONS The introduced Concept is n o t y e t complete and robust. It will be modified in accordance w i t h the progress in digital image processing and com, puter technology. The developments in pattern recognition and a u t o m a t e d p h o t o interpretation, as well as of methods for generalization and displacem e n t are of great interest.
REFERENCES Ewig, K. :and Riedinger, G., 1976. Kartenfortfiihrung mit Orthophotos. Presented Paper, XIII. Int. Congr. der internationalen GeseUschaft fiir Photogrammetrie, Helsinki 1976. Wiesel, J., 1983. Digital Image Processing for Orthophoto Generation. Presented Paper, Pattern Recognition in Photogrammetry, Specialist Work-Shop, Graz 1983.
87
A C O N C E P T F O R T O P O G R A P H I C MAP U P D A T I N G U S I N G D I G I T A L ORTHOPHOTOS
J~IRGEN PETERLE Institute for Photogrammetry and Topography, University o f Karlsruhe, Englerstrasse 7, D-7500 Karlsruhe 1, F.R. Germany
(Received March 29, 1.984)
ABSTRACT Peterle, J., 1985. A concept for topographic map updating using digital orthophotos. Photogrammetria, 40: 87--94. A method for topographic map updating is based on two digital orthophotos from digital aerial photographs. The first orthophoto corresponds to the old level of the map, the second contains, among others, the updating information. The working-steps begin with orthophoto computation and end with the generation of a screened groundplan of the map; they are done semi-automatically by computer. The concept of this method is introduced; problems of extracting cartographically relevant information from digital images are discussed and results are shown.
INTRODUCTION U p d a t i n g t o p o g r a p h i c maps always includes an e n o r m o u s a m o u n t o f w o r k . It is o b v i o u s t h a t e f f o r t s t o speed up this process have been made. T h e s e include t h e use o f rub-on-plastics f o r line-elements, s y m b o l s etc., and t h e use o f o r t h o p h o t o s . Including o r t h o p h o t o s in the updating-process, red u c e d t h e r e q u i r e d w o r k - l o a d o f the L a n d e s v e r m e s s u n g s a m t (National Survey Office) o f B a d e n - W u e r t t e m b e r g b y u p t o 30%. F o r the series T K 50 ( T o p o g r a p h i c Map 1 : 5 0 0 0 0 ) t h e revision p e r i o d is n o w 5 years, b u t still a b o u t 10 years f o r the series T K 2 5 . Meanwhile o r t h o p h o t o g e n e r a t i o n with m i n i - c o m p u t e r s is possible at m o d e r a t e cost (Wiesel, 1 9 8 3 ) . This is o n e reason, w h y a m e t h o d f o r t o p o g r a p h i c m a p u p d a t i n g using digital o r t h o p h o t o s is being d e v e l o p e d at the I n s t i t u t e for P h o t o g r a m m e t r y and T o p o g r a p h y (IPT) at t h e T e c h n i c a l University o f Karlsruhe ( W - G e r m a n y ) . SHORT REVIEW OF A CONVENTIONAL TECHNIQUE The updating technique of the Landesvermessungsamt Baden-Wuerttemberg (Ewig a n d Riedinger, 1 9 7 6 ) is divided into t h r e e parts: (a) p r e p a r a t o r y w o r k (data acquisition, o r t h o p h o t o g e n e r a t i o n b y Zeiss O r t h o c o m p Z 2
0031-8663/85/$03,30
© 1985 Elsevier Science Publishers B.V.
etc.); (b) map revision; (c) reproduction work. The central part of this method is the comparison between orthophoto and old ground plan. A blue copy of the old ground plan is superposed with the orthophoto on a light-table. The cartographer traces the differences from the orthophoto to the blue copy, generalization and displacement follow immediately by freehanddrawing. In addition a mask for deleting the old parts of the ground plan is made. Hereby the material for the reproduction work is provided. CONCEPT FOR A "DIGITAL METHOD"
The digital method retains the three parts of the conventional method. But a comparison between orthophoto and old ground plan by computer is not possible, since the ground plan is generalized. Therefore, the central element of the digital method is a comparison of two orthophotos: the first corresponds to the old level of the map, the second contains, among others, the updating-information. This comparison of the edge-detected orthophotos pixel by pixel will facilitate the location of areas with differences in the orthophotos. These areas are separated and treated with special techniques in order to extract cartographically relevant information (roads, buildings) and to process it interactively. The result consists of a line,skeleton of the new elements of the ground plan. According to desired map scale, the lines must be generalized and adapted to the map. An essential difference to the con, ventional methods is the separation between extraction of planimetric data and its map-conform presentation (generalization). The conventional method calls for feature extraction, generalization, and displacement in one step; the advantage of the new digital method is the possibility to use the extracted features for updating maps of a different scale. Digital map updating (concept)
A. Preparatory activities a. b. c. d.
Revision program Photographic flight (planning and mission)
Data acquisition (field-exploration etc.) Orthophoto generation (working scale (ws) 1:10 000)
B.
Map revision a. Location of updating-areas in the orthophotos (ws) b. Object-extraction and interactive w ~ r k ~ o u p (ws) c. Generalization and displa~ment (map scale (ms)) d. Adaption of symbols (ms)
C.
Reproductive activities a. Adaption of updating,areas to the digital map; deleting of unwanted elements in the old ground p ~ (ms) b. Checking of the u p d a ~ map at the display; data saving c. Screening d. Output of the screened map to folio
89 PROBLEMS OF REALIZATION
The realization of the concept depends on the answer to all problems, concerning extraction of cartographically relevant information from digital images, and digital techniques of generalization and displacement of groundplan-elements. Especially the extraction of small objects in built-up areas is very difficult. The different methods of pattern recognition are not able to satisfy the high demands of cartographic applications; located objects must be worked up interactively. Nevertheless, interactive processing is considerably shortened by automatic object-extraction. For purposes of map design the problems of generalization and displacement are not yet solved. ACTUAL STATE OF THE METHOD The actual state of the m e t h o d is shown by Figs. 1 to 8. Fig. 1 shows the two digital orthophotos as computed from digital aerial photographs. The first o r t h o p h o t o (Fig. 1 left) corresponds to the old level of the map, the second (Fig. 1 right) contains, among others, the updating-information. Edge-detection restricts the orthophotos to cartographically relevant information (e.g. boundary-lines of roads and buildings, etc.), so that they get a
Fig. 1.
90
planimetric presentation. Fig. 2 (left) shows the result of edge-detection oi the first o r t h o p h o t o , Fig. 2 (right) the superposition of the result of both edge
Fig. 2.
For locating small objects, the updating-areas in both o r t h o p h o t o s are treated with a location-procedure, which analyses by statistical m e t h o d s whether a pixel belongs to an object of specific extension, and, if so, flags it {Figs. 3 and 4). The flags in both images are checked for a radius of tolerance in the counter-image, and are cancelled if they violate a position criterion. The result of this location means, that there is -- with high probability - - a small object at the marked position (Fig. 5). The second part of this location,procedure is n o t y e t programmed; Hereby small areas around the marks shall be analysed to find contour-lines a n d the position of a building using conditions like rectangularity, side.ratios, a n d s o on.
Figure 6 shows an updating-area in both o r t h o p h o t o s with linear elements. After preprocessing and edge~letection, the updating-area is worked up inter-
¢,D
,rl
93
Fig. 7.
Fig. 8.
94 actively on a display station: line-smoothing, detetmg of undesired pixets and line-elements, adaption of line-connections. Then the new elements {Fig. 7 above), extracted from the second o r t h o p h o t o , are copied into the old map (Fig. 7 below), some parts of the old map -- such as those being crossed by the new elements -- must b e deleted; the final result is the line.skeleton of the new map (Fig. 8 above). The superposition of the extracted map with the second o r t h o p h o t o is shown in Fig. 8 (below). CONCLUSIONS The introduced Concept is n o t y e t complete and robust. It will be modified in accordance w i t h the progress in digital image processing and com, puter technology. The developments in pattern recognition and a u t o m a t e d p h o t o interpretation, as well as of methods for generalization and displacem e n t are of great interest.
REFERENCES Ewig, K. :and Riedinger, G., 1976. Kartenfortfiihrung mit Orthophotos. Presented Paper, XIII. Int. Congr. der internationalen GeseUschaft fiir Photogrammetrie, Helsinki 1976. Wiesel, J., 1983. Digital Image Processing for Orthophoto Generation. Presented Paper, Pattern Recognition in Photogrammetry, Specialist Work-Shop, Graz 1983.