Paper 3 · June 2023 · GIS: georeferencing, digitising and measurement
Six ground control points spread across the Esigodini sheet, from easting 699000 to 706000 and northing 7751000 to 7756000, are entered before georeferencing. Why are several well spread points used rather than two close together?
ASeveral spread points allow a transformation to be fitted across the whole sheet and its residual error to be measured, so distortion is corrected everywhere rather than only locally.
BThe software requires exactly six points and will not accept any other number, so the count is fixed by QGIS rather than chosen by the analyst.
CSpreading the control points to the corners increases the spatial resolution of the scanned raster, so that more ground detail becomes visible after the fit than the original scan was able to record.
DEach additional point permanently increases the map's scale, so six points convert a 1:50 000 sheet to a larger scale suitable for measurement.
Explanation
A scan carries distortion from paper stretch, folding and the scanner itself, and that distortion varies across the sheet. Control points spread to the corners let a polynomial transformation be fitted over the whole area and let the residual error at each point be read, which is the only check the analyst has on the quality of the fit. Georeferencing cannot add resolution or change scale.
Derived from ZIMSEC Advanced Level Geography 6037/3 Paper 3 Practical Test, June 2023 session, Q3