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I'm working with datasets that are spread out across the northern part of the Gulf of Mexico, stored in NAD 27. I've been told the polygons shown above were obtained from BOEMRE (formerly known as MMS).

I'm projecting it into Web Mercator (WGS 84), and would like to choose the most suitable geotransformation. Arcmap presents a long list of choices, but I'm not sure which one would be the overall best fit. Is there a map somewhere showing how the error varies for various geotransformations?

Better yet, is there an industry standard geotransformation for GoM whose parameters I could plug into a custom geotransformation?

  • 2
    The NAD27 to NAD83/WGS84 datum transformation is not given by any (short) mathematical formula: it's represented by various grids of displacements (constructed so that bilinear interpolation is sufficiently accurate). Most of the Gulf is covered by the main North American grid. Its accuracy is typically within one meter of the subsequent HGPN coordinates (available on land).
    – whuber
    Aug 4, 2011 at 17:58
  • Thanks whuber! I'm going to assume grids cover the full extent of my study area, as shown in the updated question. Aug 4, 2011 at 19:25

3 Answers 3


The documentation you are looking for is originally in NOAA Technical Memorandum NOS NGS-50: see p. 10 for the definitions of the NADCON regions in the Gulf/Caribbean region and pp. 28-30 for error estimates.

You can also download the original grid files from the NADCON pages and inspect them to check actual errors within any region covered by those grids.


Esri has a support page and a pdf file that lists the general geographic extents of the different transformations. Somewhere there's a page that contains pdf maps of the NAD transformations, but I haven't found that one yet. This document show projections and transformations by specific areas of interest, but doesn't list a transformation for the Gulf of Mexico


For individual offshore oil leases, EPSG lists several possibilities that are NAD27/WGS84.

EPSG online registry

Almost all have comments that say replaced by 15851 (NAD 1927 To WGS 1984 (79)) which uses NADCON conus grid file). US Bureau of Land Management (BLM) states that NADCON (conus) should be used for NAD27/NAD83 conversions, but I can't turn up a reference for it.

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