The Impact of Conformal Map Projections on Direct Georeferencing

If an aerial-triangulation (AT) is performed in the National Projection System (NPS), then the conformal map projection (which is the base for the NPS) effects the results of the AT by raising three problems: P1) the effect of the Earth curvature, P2) the different scales in plane and height, and P3) the variation of the scale in plane across large areas of interest. Problem P1 may be solved by the so-called Earth curvature correction. Whereas P2 and P3 have negligible effects in plane and height (at least for non-mountainous areas) when performing a conventional AT using controland tie-points (so-called indirect georeferencing), their effect in height is not negligible (even for plane surfaces) when performing direct georeferencing (using GPS/INS). Simple solutions for solving these problems are presented – by considering the varying planar scale with the point heights or the principal distance.