2

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?

Improve this question
3
  • Just to clarify - your x and y values are in decimal degrees? spatialreference.org/ref/epsg/4267
    – L_Holcombe
    Commented Jan 24, 2013 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
    Commented Jan 24, 2013 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
    Commented Jan 24, 2013 at 5:33

1 Answer 1

Reset to default
4

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.

Improve this answer
2

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.