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The Canada Center of Remote Sensing (CCRS), Ottawa, is not discussed 
here at length, due to the fact that it does not collect and 
distribute Landsat MSS over Western Europe and the developing dountries 
of the World, the major areas of interest to the International Institute 
for Aerial Survey and Earth Sciences (ITC). 
Figure 3 illustrates the seven ground recieving stations already established 
(1978) and the six more which are anticipated in the period 1978/80. 
At this stage only Brasil and Italy do receive data in real time over 
parts of South America and North Africa. 
Computer outprints, cloud cover and image quality 
Although it is more time consuming, best results may be obtained by 
putting in a request to EDC and/or the local distribution centre for 
an up-to-date outprint of available imagery indicating maximum cloud 
cover and minimum image quality rating. In fact this is an iterative 
process. Depending upon the available data base of the areas of interest, 
one is able to threshold imagery of inferior quality and high cloud 
cover as illustrated in Figures 4 and 5. 
With regard to the first choice, useful general guidelines on maximum 
cloud cover of well covered areas (more than 20 images per year) are as 
follows: arid and semi-arid zone 10%, tropical monsoon, savanna, 
subtropical and marine zones 30%, tropical rainforest and humid 
continental zones 50%. 
Though an initial threshold per channel of a minimum quality rating of 
3-4 is sufficient, the choice for a crop survey should aim at a 
potential colour composite of either bands 4,5 and 6 or 4,5 and T in a 
quality rating of 8, indicated as 888x or 88x8 on the outprint, 
where the quality of x is immaterial. 
Telespazio, Italy has an improved system of rating cloud cover and image 
quality. The cloud cover ig indicated separately for each quadrant of 
a particular scene. Image quality is rated for the electronics 
(missed signals) and for the interpretability (presence of haze).