However for detection of features in good 
radiometric contrast condition it is sufficient to 
image the object in 2-3 resolution elements. In such 
cases a pixel resolution of 5-6 m is sufficient. Thus 
IRS-1C resolution is sufficient for identification of 
features required for 1: 50,000 scale mapping. 
Pixel size from Photogrammetric criteria The 
measurement of reliable terrain height from digital 
stereo data recorded by tilting camera are heavily 
influenced by geometric principles involving B/H 
ratio , sensor attitude , pixel size and correlation 
accuracy . The photogrammetric approach assumes 
sufficient transformation capability for bringing 
corresponding images in approximate congruence. 
The error px expected in image parallax 
removal when employing digital correlation 
technique is given by 
px < K* (pixel size), 
where values 02 < K < 15 shows the 
degree of correlation. 
The parallax error may be converted into the 
height error (h) through the use of base to height 
ratio inherent in the parallax equation, when the Y- 
parallax is O. 
H H © Px 
AS m 
TB 
and analogous to digital sensor 
APSF 
ATL -T——— 
2 tana 
where 
o px or Apisthe total error in parallax 
measurement in the image plane, 
SF is the scale factor = HA, 
f is the focal length, 
H is the flying height of sensor above the 
average terrain elevation, 
B is the base distance between exposure 
stations, 
œ is the half angle between 
intersecting rays, referenced to local vertical (H/B 
= 1/2*tan a ). 
Scale factor (H/F) multiplied by parallax (o px) in 
the image plane yields the pixel size on the ground: 
H 
-—*op=c Px = K*(Pixel size)(m on ground) 
F 
Therefore, Pixel size - 1/K * B/H* c h 
The relationship between the closest relative 
contour interval meeting the 90 percent criteria and 
the precision of instruments ( o ) for relative spot 
heights 
33oh 
03*C1 
Ci 
or sh 
Pixel size Ps 1/K * B/H.* 0.3* C.I. 
Assuming B/H =0.6 and the desired contour interval 
for 1:50,000 mapping is 20 m we get 
Ps=083X06X20m 
= 10 m for 1:50,000 scale maps. 
The accuracy of spot heights referenced to a 
recognised datum is largely determined by factors 
which create geometric displacements in the 
sensor data and produce correlation or 
measurement errors in the along track direction. 
These displacements or errors (AX) may be 
expressed in fractions of pixel at the ground or in 
arc seconds. 
From a comparison of the mapping standards and 
the standards of IRS-1C Data Products it can be 
concluded that 
- It is possible to obtain point height and 
contour information compatible with 1:50,000 and 
1:25,000 scale topo maps from IRS-1C PAN stereo 
pairs. 
-Most of the spatial land cover information 
required for updating of topographic maps at 
1:50,000 scale can be obtained from IRS-1C PAN 
and LISS-3 data. Certain features may require field 
verification. At 1:25,000 scale a larger component of 
field verification and complementation may be 
required. 
825 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996