215 
PHOTOGRAMMETRIC COMPILATION 
The ability to extract positional and elevation data from photogrammet- 
ric stereo models is given by the standard formulas 
For 
position 
= a v = ^ x a 
1 f X 
a x 
(5) 
For 
elevation 
= « x H x a p 
B f p 
°Z 
(6) 
in which H is the flight altitude, B is the base distance between the 
two images making up the stereo pair, f is the sensor focal length, 
o x is the coordinate measurement error on the image, and Op is the 
parallax measurement error in the stereo model. The object space values 
H and B are expressed in identical units - usually meters; the image 
space values o x , Op, and f are expressed in common units - usually milli 
meters, For modern photogrammetric instrumentation - particularly 
analytical stereoplotters - working with continuous tone photographs, 
°x ~ °p ~ 10 pm is a reasonable expectation. Proponents of electro- 
optical systems sometimes claim that measurements can be made to sub 
pixel accuracy, but no one has yet made a Landsat planimetric image map 
with a documented standard error of less than 60 m in position 
(Colvocoresses, 1979). The ability to determine parallaxes in electro- 
optical images will be critical in future systems which will provide 
useful B/H ratios. Proponents have claimed 0.1 pixel is possible, but 
0.5 pixel seems an optimistic projection. 
Formulas (5) and (6) apply only to a stereomodel which has been properly 
oriented to ground control points. There is little economic justifi 
cation for flying a space system to map limited areas. One needs to be 
concerned with areas approaching sub-continental size. Areas of this 
size which do not already have reasonably accurate maps are mainly to be 
found in the developing nations. Most of these areas do not have a 
basic survey so that ground control points are not available in order to 
provide position and elevation data for photogrammetric mapping. How 
ever if one is able to precisely locate the exposure stations and to 
precisely determine the attitude of the sensors at the moment of expo 
sure, it is possible to eliminate the need for ground control points. 
SPACECRAFT POSITION AND ATTITUDE 
Spacecraft .position is given fundamentally by tracking data, orbit compu 
tation, and time. Accuracies expected for the several systems in current 
and planned use are given in Table 2. 
Table 2 
SPACECRAFT POSITION ACCURACY 
(la) 
along 
track 
across 
track 
altitude 
Spacecraft Tracking and Data 
Network - STDN (current) 
110 
m 
130 
m 
130 m 
Propagated 1 rev 
260 
130 
150 
Tracking and Data Relay 
Satellite System-TDRSS (1984) 
430 
460 
90 
Propagated 1 rev 
610 
460 
90 
Shuttle inertial measurement IMU 
1000 
100 
100 
Post flight analysis 
300 
30 
30 
NAVSTAR GPS (1987) 
7- 
■15 
7- 
-15 
10-20