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:
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?