I have GPS data that has been recorded in Datum WGS84. The client has a GIS built in Datum NAD27. Having looked into the conversions online I would like to know how to do this rigorously with minimum loss of accuracy. Ordinarily I would build a GIS in WGS84 that uses GPS data and project it to the desired UTM (16N in this case). I know I can transform in ArcGIS 10 but is this rigorous enough?

  • What kind of GPS do you have ?
    Commented Aug 19, 2013 at 16:55
  • 7 different ones that are all holding data. All Garmins. Models 62s, 62, etrex 20, etrex vista HCx. I'm using DNRGPS to take the data off the GPS units, using no projection. I'm then defining the projection in Arc Catalog to World>WGS84 and then I was wondering if I should transform at that point and project to NAD27 (but which transformation to use?) Commented Aug 19, 2013 at 17:09
  • According to this help.arcgis.com/en/arcgisdesktop/10.0/help/003r/pdf/… transformation 18 should be the one. Commented Aug 19, 2013 at 17:12
  • You could reproject in the DNRGPS without having to go to striaght to ArcGIS..I dont know why your client has a different built but he or she should be able to fix that by reset to Datum WG84. I am not sure which GPS he or she is using.
    Commented Aug 19, 2013 at 17:58
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    Commented Aug 19, 2013 at 18:06

1 Answer 1


Some information that would have been helpful if you provided: which ArcGIS version do you have? What accuracy is the customer looking for? What location does the data represent?

You might try a few different paths of transformations. I'm going to assume the data is within the contiguous 48 states of the US and a relatively recent version of ArcGIS 10.1.

You can try a single step, file-based transformation, NAD_1927_To_WGS_1984_79_CONUS. This uses the same conus grid file as the NADCON software.

Because the GPS units are all recreational (or mapping) grade receivers, this more complicated workflow may not really improve results. Use a three step transformation: WGS_1984_(ITRF00)_To_NAD_1983_HARN, NAD_1983_To_HARN_(insert appropriate state/region abbreviation here) and finally, NAD_1927_To_NAD_1983_NADCON.

  • 1
    the data is in Calakmul, Mexico. And the GIS version (as stated in the original question ;) ) is ArcGIS10. Using transformation 18 (NADtoWGS) seems to have done the trick. Thanks for the in depth answer mkennedy. Much appreciated Commented Aug 19, 2013 at 19:27

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