DIGMAP: a computer program for accurate acquisition by digitizer of geographical coordinates from conformal projections

A computer program is described for the acquisition of geographical coordinates from maps in widely used cylindric and conic conformal projections: Direct Mercator, Lambert conformal, UTM, and Polar Stereographic. It uses the geometrical affine transformation of homogeneous coordinates and the direct and inverse analytical mapping functions of conformal projections. Selection between different reference ellipsoids and datum shift procedures is provided. Tests of accuracy were performed for direct and inverse mapping algorithms and for conversion of tablet coordinates to geographical coordinates. The results indicate that the uncertainty of capturing geographical coordinates is, in general, twice the graphic precision of 0.2 mm at any map scale and projection. Accuracy is increased by using good quality maps. The acquisition of calibration points is the most critical step. Mathematical expressions and source listing of the FORTRAN program are provided. © 1986.