Editing Geographic coordinate conversion (section)

=== Grid-based method ===

[[File:Datum Shift Between NAD27 and NAD83.png|thumb|Magnitude of shift in position between NAD27 and NAD83 datum as a function of location.]]

Grid-based transformations directly convert map coordinates from one (map-projection, geodetic datum) pair to map coordinates of another (map-projection, geodetic datum) pair. An example is the NADCON method for transforming from the North American Datum (NAD) 1927 to the NAD 1983 datum.<ref name=ESRI_grid>{{cite web|title=ArcGIS Help 10.1: Grid-based methods|url=http://resources.arcgis.com/en/help/main/10.1/index.html#//003r00000013000000|publisher=ESRI|access-date=5 March 2014|archive-date=4 December 2019|archive-url=https://web.archive.org/web/20191204151744/http://resources.arcgis.com/en/help/main/10.1/index.html#//003r00000013000000|url-status=live}}</ref> The High Accuracy Reference Network (HARN), a high accuracy version of the NADCON transforms, have an accuracy of approximately 5 centimeters. The National Transformation version 2 ([[NTv2]]) is a Canadian version of NADCON for transforming between NAD 1927 and NAD 1983. HARNs are also known as NAD 83/91 and High Precision Grid Networks (HPGN).<ref name=nadcon_harn>{{cite web|title=NADCON/HARN Datum ShiftMethod|url=http://www.bluemarblegeo.com/knowledgebase/geocalc/classdef/datumshift/datumshifts/nadcon.html|publisher=bluemarblegeo.com|access-date=5 March 2014|archive-date=6 March 2014|archive-url=https://web.archive.org/web/20140306000427/http://www.bluemarblegeo.com/knowledgebase/geocalc/classdef/datumshift/datumshifts/nadcon.html|url-status=live}}</ref> Subsequently, Australia and New Zealand adopted the NTv2 format to create grid-based methods for transforming among their own local datums.

Like the multiple regression equation transform, grid-based methods use a low-order interpolation method for converting map coordinates, but in two dimensions instead of three. The [[NOAA]] provides a software tool (as part of the NGS Geodetic Toolkit) for performing NADCON transformations.<ref name=NOAA_NADCON>{{cite web|title=NADCON - Version 4.2|url=http://www.ngs.noaa.gov/PC_PROD/NADCON/|publisher=NOAA|access-date=5 March 2014|archive-date=6 May 2021|archive-url=https://web.archive.org/web/20210506162736/https://www.ngs.noaa.gov/PC_PROD/NADCON/|url-status=live}}</ref><ref name=Mulcare>{{cite web|last=Mulcare |first=Donald M. |title=NGS Toolkit, Part 8: The National Geodetic Survey NADCON Tool |url=http://www.profsurv.com/magazine/article.aspx?i=1193 |publisher=Professional Surveyor Magazine |access-date=5 March 2014 |url-status=dead |archive-url=https://web.archive.org/web/20140306001134/http://www.profsurv.com/magazine/article.aspx?i=1193 |archive-date=6 March 2014 }}</ref>