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the image is one of its two major characteristics. Unlike the 
Gemini and Apollo pictures, ERTS images have the unique property 
of being readily joined together in mosaics. This greatly 
increases the synoptic character of this imagery and extends the 
assessment of contextual relationships to regional and even 
subcontinental proportions. Proper processing can so minimize 
the join or match line differences between successive scenes in 
a single orbital strip, between adjacent strips on successive 
days, and even between strips or individual scenes obtained 
in different orbital cycles, that only careful, close-up inspec 
tion of a mosaic would detect tonal variations. Previously, 
mosaics of imagery from high-flying aircraft have had abrupt 
tonal changes, distortions resulting from the lens system, 
change in illumination during the time of flight, and, moreover, 
they were very costly. 
The other major characteristic of ERTS imagery is its 
repetitive coverage, enabling us to view one area under the con 
ditions which are experienced at different seasons of the year. 
This property does not mean that ERTS imagery is restricted to 
interpretation of time-dependent features; rather it means that 
relatively permanent features such as geological structures and 
landforms may be viewed on a regional scale with many different 
conditions of vegetation growth, snow cover or soil moisture. 
We may therefore select imagery for that season which gives us 
the clearest view of the features we are investigating. 
ERTS therefore provides us with a regional view of the 
earth’s surface at different seasons of the year, and because 
of the high altitude and imaging system, the image is virtually 
an orthophotograph or photo map. 
INTERPRETATION AND APPLICATIONS OF ERTS-1 IMAGES 
Interpretation is the transposition of remotely-sensed data 
into useful information (Gregory, October, 1973). The quality 
of these translations depends largely on the experience and 
knowledge of the interpreter. Interpretation of ERTS data by an 
experienced interpreter, with due allowance for differences in 
the scale of observation and the size of the features, is as 
valid as his inferences from observations in the field. 
ERTS data does not provide as much detailed information as is 
obtainable by a field study, but, on the other hand, the 
regional significance of a feature will frequently not be 
appreciated without the synoptic overview of ERTS imagery. 
Interpretation is a matter of human judgement; prior experience 
and accumulated wisdom are fundamental to interpretation. 
The quality of interpretations will therefore vary from person 
to person.