203 
Figure 3. Photographic mosaic illustrating the ten 
essentially cloud-free, summertime Landsat 
images which were used to create the digital, 
map-registered data base of Pennsylvania. 
Longitude 78°W 
Figure 4. Digitized county boundaries for the Pennsylvania 
data base derived from 1:250,000 scale U.S.G.S. 
topographic maps.