200 
2.5 Fieldwork 
Two periods of field work totalling 2\ months were completed as mentioned pre 
viously, the first during the months of May and June and the second at the begin- 
2 
ning of October 1973. In total about 7 weeks (70%) were spent in the 1200 km 
Merida region for the 1 : 250, 000 map, and 3 weeks (30%) in the remaining part 
of the 25, 000 km 2 test area for the 1 : 1 , 000 , 000 map. 
These field visits had two aims. The first was the collection of ground truth data 
to supplement the existing information on the area and the second was to investi 
gate the possibilities of using ERTS-1 imagery as a kind of base map for conduc- 3,2 
ting soil surveys. In the collection of ground truth data a series of reflectance 
measurements of the top soil were included. These measurements were made 
using an ERTS groundtruth radiometer that operates at the same frequencies as 
the MSS system of the ERTS -1 satellite. Unfortunately these series of measure 
ments were far from complete due to cloudy weather at the time of field work. 
During the second period of field work in October 1973 a series of approximately 
70 pre-selected points were surveyed in the general survey area, but mainly out 
side the area that had been investigated by ITC in the previous years using ordi 
nary aerial photographs. The points were selected along main roads, on the basis 
of visual interpretation of the satellite imagery, to represent different interpre 
tation units. It was found during field work that most of the points correspond in 
reality with a different land form unit and this we felt was a step towards proving 
both the validity and the accuracy of the interpretation lines. 
The problem of orientation in the field and locating particular points on the 
imagery was solved by superimposing a series of transparent maps at the same 
scale on the imagery. These maps were produced in the Cartography Department 
of ITC by photographically reducing or enlarging an existing 1 : 200, 000 topo 
graphical map of the region. 
3. PRELIMINARY RESULTS AND DISCUSSION 
3.1 General 
The imagery of the MSS system of the ERTS -1 satellite is influenced by the sur- . 
1 
face conditions of the earth and secondly by factors such as atmospheric absorp 
tion, system irregularities, etc. The surface conditions include plant coverage A 
both in kind and degree, surface moisture, texture of top soil, colour of top soil, 
surface stones, rock outcrops, slope in both shape and degree, etc. Most of 
these conditions, with the exception of plant coverage, relate to the top horizon 
of a soil profile. Because in most cases the top of the soil reflects conditions 
occurring deeper in the profile, there are strong reasons to examine the use of