Investigations of aerial cameras from high towers

29 Investigations of Aerial Cameras from High Towers 1) by Prof. B. H A L L E R T Stockholm. It should f i r s t be noted t h a t the i n v e s t i g a t i o n s in the tower p r i m a r i l y were s u g g e s t e d for i n v e s t i g a t i o n s of the specific infuence upon the g e o m e t r y of the c a m e r a f r o m d i f f e r e n t outer factors for i n s t a n c e t e m p e r a t u r e , vibrations, filters, film or plates etc. T h i s is evident f r o m t h e paper which w a s distributed at the conference and which w a s published in P h o t o g r a m m e t r i c Engineering, September 1957 u n d e r the title: Determ i n a t i o n and Correction of Systematic E r r o r s in the F u n d a m e n t a l Operations of Aerial T r i a n g u l a t i o n . F r o m t h i s paper, page 780, line 24, I quote: . . . " F i r s t of all it is n e c e s s a r y to investigate which f a c t o r s m a y cause systematic errors, p a r t i c u l a r l y the t e m p e r a t u r e v a r i a t i o n s but also other factors, for instance filters, etc. Such investigations m u s t be p e r f o r m e d u n d e r conditions which a r e as similar to the actual p h o t o g r a p h y conditions as is possible, and which allow the influence of the actual f a c t o r s to be d i s t i n g u i s h e d as well as possible from other f a c t o r s t h a t m a y have similar effects upon t h e bundles of r a y s . . . . For such i n v e s t i g a t i o n s a h i g h radio tower h a s been used." F r o m these lines it is quite evident t h a t the p r i m a r y purpose of the tests f r o m the tower is to d e t e r m i n e the variations of the g e o m e t r y of the c a m e r a with the variations of one specific factor a t a time, for instance the t e m p e r a t u r e or d i f f e r e n t filters. If p h o t o g r a p h s ave taken w i t h the c a m e r a in t e m p e r a t u r e s v a r y i n g between + 4 0 ° C and - - 4 0 ° C keeping all other f a c t o r s constant, the specific influence of the t e m p e r a t u r e v a r i a tions upon the g e o m e t r y of the p h o t o g r a p h s evidently can be uniquely determined t o g e t h e r with the correct a c c u r a c y from the simple m e a s u r e m e n t s and computations which characterize the grid method (the method of the least s q u a r e s ) . Similarly the influence of v a r y i n g filters or port g l a s s e s etc. can be investigated keeping all other factors constant. F r o m special resolving power t a r g e t s on the g r o u n d the differences in the resolving power can be determined with v a r i a t i o n s of one factor a f t e r another. The considerable a d v a n t ages of the tower method for such purposes are quite evident. A g r e a t n u m b e r of exper i m e n t s h a v e been carried out, the r e s u l t s of which will be published in a n e a r f u t u r e . For m a n y of the mentioned purposes the used method seems to be the only possible one. The objections by the three mentioned gentlemen are really s t r a n g e . Experience h a s indicated t h a t the tower method also can be used for a calibration of aerial c a m e r a s with good results. As indicated in m y paper (page 783) the c a m e r a c o n s t a n t (calibrated focal length) c a n be corrected with respect to the limited distance 125 meters. The position of the principal point should not be afflicted with corresponding s y s t e m a t i c e r r o r s when the lens is s y m m e t r i c a l a r o u n d the optical axis. F o r a p p r o x i m a t e l y vertical p h o t o g r a p h s the a c c u r a c y of these two elements of the interior orientation are of little importance in normal p h o t o g r a m m e t r i c work. The u s u a l r e q u i r e m e n t s are doubtless m u c h e x a g g e r a t e d . The "projective e r r o r s of the elements of. the interior orientation get compensated by the elements of the external orientation. B u t the non-projective 1) Some r e m a r k s with respect to some critical s t a t e m e n t s by I)r. Schmid, Dr. Brown and Mr. Blachut a t the I n t e r n a t i o n a l P h o t o g r a m m e t r i c Conference on Aerial T r i a n g u l a t i o n , A u g u s t 28-31, 1957, Ottawa. The s t a t e m e n t s were published in P h o t o g r a m m e t r i a , XIV, 2, 1957/58, pages 8 7 - 89.

30

Investigations of Aerial Cameras from High Towers, Hallert

errors are a l w a y s very d a n g e r o u s (radial a n d t a n g e n t i a l distortion, affinities, a s y m m e tries etc.) These e r r o r s can be determined m o s t conveniently with the tower method a n d t o g e t h e r with t h e correct accuracy ( s t a n d a r d error in accordance with the method of the least s q u a r e s ) . U p to now the determination of non-projective s y s t e m a t i c e r r o r s with t h e tower method can be compared with the r e s u l t s of other methods only concerning the radial distortion. In the f i g u r e s 3c a n d 3d in the above mentioned publication a comparison with the radial distortion f r o m laboratory m e a s u r e m e n t s w a s d e m o n s t r a t e d for two c a m e r a s A v i o t a r and Aviogon respectively. The m a x i m u m differences are for A v i o t a r about 4 microns and for Aviogon about 2 microns. These discrepancies can probably be explained by t h e accidental errors of the m e a s u r e m e n t s with both methods. Concerning the other error~ no comparison h a s been p e r f o r m e d since no i n f o r m a t i o n is available concerning such errors f r o m n o r m a l l a b o r a t o r y tests. Finally, it would be m o s t i n t e r e s t i n g to see t h e r e s u l t s of a d e t e r m i n a t i o n of the s t a n d a r d errors of the observations f r o m a multicollimator calibration, as computed f r o m the condition of the central projection in accordance with the method of the least squares. The a r r a n g e m e n t of t h e collimators is well suited fro' such computations and the general solutions of the n o r m a l equations according to the g r i d method can be used. How accurate is the a d j u s t m e n t of the angles of the collimators and how often m u s t this a d j u s t m e n t be p e r f o r m e d ? The time consumption for such a d j u s t m e n t s a n d t e s t s should be considered in i n f o r m a t i o n about t h e n e c e s s a r y time for c a m e r a calibration.