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I have some data that is in NAD27, but I am having a hard time displaying it in QGIS. I think that the problem is with the CRS. I cannot seem to find the correct NAD27 option. The closest one seems to be EPSG:2027 (NAD27(76)/UTM zone 15N), but my data points are still about 25 meters off.

This data plots up just fine in ArcGIS, and I have transformed it to NAD83, which I can use with no problem in QGIS, but this is an awkward workaround, and I would like to migrate this project into QGIS entirely. Does anyone have an idea on this?

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Just to clarify - your x and y values are in decimal degrees? spatialreference.org/ref/epsg/4267 –  L_Holcombe Jan 24 '13 at 2:12
    
They are not...they're given to me in meters. Does that mean that they've already been transformed? I'm actually a bit confused on this point. Also, I notice that there are no NAD27 systems here (spatialreference.org/ref/?search=NAD27) that cover my study area, which is in South Louisiana. –  CRE Jan 24 '13 at 5:24
    
NAD27 is actually a datum, not a projection. If you look at the info for EPSG:4267, it only defines a spheroid and how many radians are in a degree. If your data is in feet or meters, it has been projected. South Louisiana State Plane is a possibility, although it uses feet, not meters. (That sounds like something an oil company would do). But you could easily change the units. spatialreference.org/ref/epsg/26782 –  L_Holcombe Jan 24 '13 at 5:33
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1 Answer 1

The offset of 25 metres is an accuracy matter. The NAD27 CRS uses a different ellipsoid position than NAD83 or WGS84. The difference is normally given by +towgs84 values in QGIS. In early days, a rough 3-parameter or 7-parameter Helmert transformation was made to shift the data correctly.

Your first guess, EPSG:2027 is defined in QGIS with

+proj=utm +zone=15 +ellps=clrk66 +units=m +no_defs

In this definition, no shift (+towgs84) parameters are used at all. Try as a better solution:

EPSG:26715 +proj=utm +zone=15 +datum=NAD27 +units=m +no_defs

The +datum=NAD27 option uses a ntv2-grid for high accuracy.

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Gosh, I didn't know this. When did QGIS start using grid-shift files by default? Is it only NAD27? N. –  nhopton Jan 24 '13 at 13:11
    
Actually, proj had it nearly from the start, 13 years ago: trac.osgeo.org/proj/browser/trunk/proj/src/pj_datums.c?rev=848 and it is the only one incorporated by default: trac.osgeo.org/proj/browser/trunk/proj/src/pj_datums.c . You might guess why... –  Andre Joost Jan 24 '13 at 16:07
    
Thanks Andre, that solved it. –  CRE Jan 24 '13 at 20:59
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