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INTRODUCTION 
Results of an analysis of ERTS-1 multispectral scanner (MSS) data obtained over 
the Montalban area, Province of Teruel, Spain, are presented (Figure 1). The 
objectives of the study were to evaluate the potential of ERTS-1 data for geolog 
ical mapping and to compare the results with geological maps. 
The test site was chosen because the geology is known in detail. In 1971, a geo 
logical map on 1 : 100, 000 scale had been compiled from photogeological maps 
prepared by students of the ITC course in photogeology (See [l] ). This map was 
used as a basis for evaluation of the ERTS-imagery. It served a dual purpose: 
firstly, it was used as a standard classification scheme, and secondly, since it 
had been prepared by means of conventional photogeological interpretation methods, 
it was considered well suited for comparative studies. 
PROCEDURE 
For the analysis of the ERTS imagery an operational procedure was followed, 
essentially along the lines of a routine visual photo interpretation job. This in 
cluded preliminary analysis and interpretation, ground truth acquisition, and 
final analysis of the imagery. Visual interpretation was carried out by skilled 
photogeologists. The basic technique employed was the standard method of aerial 
photographic analysis. However, due to the special formats of the ERTS imagery, 
modifications for visual inspection had to be made. The kinds of modifications 
made fall into three categories: 
(1) change of format, 
(2) utilization of different bands and band combinations, and 
(3) the utilization of various kinds of viewing equipment. 
This resulted in the following approaches: 
1. Visual examination of 1 : 1, 000, 000 scale prints, without optical aids or 
only using a desk magnifying lens. 
2. Visual examination of enlarged images. Enlargement was obtained in various 
ways: (a) photographically, (b) optically by means of a Zeiss Prado projector 
with Dimaron f = 10 cm, 1:2. 8 lens. Various scales of enlargement were 
tried, and it was found that a scale of 1:250, 000 was near-optimal both for 
interpretation and for annotation purposes. 
3. Stereoscopic examination with overlapping sets of imagery. Although stereo 
vision was possible at a reduced level only, it nevertheless proved to be a 
very important aid in the interpretation.