The sample location data have been digitised from a sample site map, as the original field sample cards and the port-a-punch cards were not retrievable in 1998/99. The sample site map was a paper copy - so, although the UTM co-ordinates for locations are given to the nearest metre, the true location of the original sample sites may vary by as much as 500 m, or in some cases even more (see below for technical information and caveats). However, it should be noted that GENP 1967-71 was a low-density regional exploration survey based on the collection of some 1260 samples at key sites within the 25,000 km² project area - a density of approximately 1 sample per 20 km².
Co-ordinates are from UTM zone 17 and the map datum used is the South American Provisional datum 1956.
The UTM co-ordinates have been calculated by an affine transformation of digitiser co-ordinates, using the calculated UTM co-ordinates of half degree latitude/longitude intersections as control points. Comparison of selected sample locations with visually identified sites on the drainage network, as shown on the 1:100 000 scale topographic maps, issued by the Institute Geogrifico Militar, suggests that most co-ordinates in this database are within 0.5 km of their likely position on the topographic map. In some cases the discrepancy could be up to 1.0 km but not more than 1.5 km. Some sites could have been plotted up-stream of their true position to preserve clarity on the sample site map, which is at 1:400 000 scale. In some cases, the plotting of sample sites on side streams some way above their confluence with the main stream is thought to be true, as the sample sites seem to be a short distance up-stream of tracks or mule-paths.
In the light of the above information it is recommended that further investigations of anomalous sites should take the presence of an anomaly as being within a limited area rather than exactly at the location indicated by the sample site co-ordinates. No information is available on the correlation between the topographic maps and GPS determined co-ordinates, but some translational error is to be expected.