I have tried everything that I have found on this site so far and nothing has worked! I have used ArcGIS for 3 years but had never had to mess with coordinate systems as the 1st company I worked for had it down. However the new company does not and I am getting data from the county GIS which is correct, and trying to match the data form the 2nd company with the counties data. The data from the county is GCS_North_American_1983 the company data is GCS_North_American_1927. Not sure of what information is needed but I can get the projection, limits, what ever is needed. Please help as the is really confusing!

  • 1
    I think the Help to read on this starts at North American datums. You need to transform between the two.
    – PolyGeo
    Commented Dec 22, 2014 at 22:03
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    In what format is the data (shp, FGDB,...)? Does the data align when projected on the fly? Have you tried redefining the coordinate system? If so, does it align then? Please update the question.
    – Vince
    Commented Dec 22, 2014 at 22:08
  • You say that "I have tried everything that I have found on this site" but what precisely have you tried. Can you edit your question to list the steps, please? It should just need 1. Start ArcMap 2. Add first dataset 3. Add second dataset and get warning 4. Apply Transformation.
    – PolyGeo
    Commented Dec 22, 2014 at 22:34
  • If the data is within the lower 48 states, pick NAD_1927_To_NAD_1984_NADCON. If the data is elsewhere, tell us so a more appropriate tfm can be recommended. How far off is the data?
    – mkennedy
    Commented Dec 23, 2014 at 17:08

2 Answers 2


Use Project (Data Management) to set whichever shapefile's coordinate system you prefer, equal to the other's.

  • That's only valid if the data is in NAD27. It could be that the vendor is clueless about coordinate systems.
    – Vince
    Commented Dec 22, 2014 at 22:02

@Maksim's answer is ideal, in terms of converting the data in NAD27 to NAD83 (which is what you're probably going to need). However, as @Vince points out you might find that the data from the company (less reliable) is in the improper coordinate system. If that's correct, the Define Projection (Data Management) tool will re-assign a projection without touching the geometry (that is, without moving the coordinates of any points in the data).

If you want to manually check the coordinate transformation for a single point (NAD27 to NAD83 or vice versa), you can use this tool from the National Geodetic Survey.

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