I have a problem with the way that QGIS handles transformation parameters between coordinate systems based on the Datum NAD27 and those based on the Datum WGS84. In version 3.12.1 I have found that when loading a layer in NAD27 it asks for the transformation to be used for reprojection. For example, I have data from the mid-1990s that is based on NAD27 which is close to Mexico City. Data is EPSG 26714, UTM NAD27 14N. Of all the options available, most do not apply because they are designed for Canada or the United States or use NTV grids. I have two options for Mexico, which are:

Inverse of UTM zone 14N +NAD27 to WGS84 (18) World -N Hemisphere -102°W to 96°, Mexico Onshore

Inverse of UTM zone 14N +NAD27 to WGS84 (83) World -N Hemisphere -102°W to 96°, Mexico Offshore GoM -Tampico Area

I choose transformation 18, which has parameters documented by NIMA and is also used by ArcMap, however , I always get this warning message:

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I am aware of this question, Reprojecting between NAD27 and WGS 84?

So I wonder What I have to do? Do I rely on Geographic transformation 18? Do I define it manually?

  • Hi Gerardo, the warning seems to indicate that the extension of the chosen transformation does not cover the extension of your layer and it seems that it is only warning you of that. If you continue and compare your data with other known ones, are they aligned? – Gabriel De Luca Apr 5 at 1:37
  • I understand, but Mexico City is nearby the central meridian of zone 14N, also is in the middle of Mexico, so.... My reprojected data is where is supposed to be. Main thing is that the warning makes you immediately think that something is wrong with what you are trying to do. I think the warning should come out only in the event that, as you mentioned, the transformation was not the adequate for your data – Gerardo Jimenez Apr 5 at 2:52

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