Spatial Mismatch of Coastline in Land - Sea Segments due to Difference in Projection Systems - A Case Study

A new field of study in seabed map compilation and geological correlation has made its presence felt with the appearence of seabed sediment maps published by the Geological Survey of India (GSI). The maps of the land-sea segments prepared by GSI on Mercator projection are corrected by the Survey of India (SOI) for the coastline on Lambert Conformal Conical System. The SOI-certified coastlines are found to be distinctly shifted longitudinally and differ significantly in space from the same as presented in the National Hydrographic Office (NHO) charts. However, it is seen from a programmatic conversion exercise at the Modern Cartographic Centre (MCC), SOI, Dehra Dun, that projection conversion of published geological maps from Polyconic to Mercator in the Iand segment generates a "third coastline" that is proved to be the closest to the ground truth and, as such, is the prerequisite for incorporation of geological/geophysical correlation in a series of Territorial Waters (TW) maps to be published by the GSI. Till adoption of Global Positioning System (GPS) base maps that present the entire surface of the earth - both Iand and sea - on a uniform projection format, this "third coastline" generated through appropriate conversion is betieved to most effectively ensure accuracy of plotting.