69) , 
(1976) 
interest. 
collection. 
Interconnection of all data on aerospace imagery, and documentation of 
the existing and future receiving stations is very desirable 
for the user community. Unfortunately, in the short term a 
potential user is forced to request a computer outprint from both the 
local receiving station and the EDC (taking an average of 3-4 weeks 
for the latter), only then he can proceed to make his final choice 
and order the data. In addition there are considerable variations in 
type, format, quality and price of the Landsat derived products between 
the various distribution agencies. 
Sidelap 
There are two reasons why attention should be given to sidelapping 
scenes: i) for multitemporal studies both for digital as well as visual 
analysis and ii) to take advantage of enhancement techniques in the 
broadest sense, in particular for visual interpretation procedures. 
Image enhancement can be obtained using the same bands in black & white 
(monoband) or false colour composites (4,5 and 7) of different seasons 
for binocular studies. Similarly, visual interpretation may facilitated 
by à combination of different bands (multiband) of one or more seasons 
(multitemporal). Figure 10 shows possibilities for monoscopic and 
25. € . ; : : : 
'stereoscopic' image interpretation using monotemporal and multi- 
temporal enhancement techniques. 
A mere indication of path and row number without the essential plotting 
of the survey area against the sidelapping Landsat overpasses may lead 
to disappointing results. 
Sidelap depends upon the latitude, it varies from 14% at 0° latitude 
to 85% at 80° 1atitude. Plotting of Landsat imagery of a survey area 
in Northern India may illustrate the importance of the above remarks 
(see Figure 11). 
  
  
  
2 Stereoscopic exaggeration depends upon the base/height (B/2) ratio. 
For a pair of stereoscopic scenes of two consecutive orbits (1 day 
inbetween), the overlap for example is 20%. B/Z = 148/915 or 0.16. 
In practice stereoscopic vision can be achieved when height 
differences are exceeding 50-100 meters.