[postgis-users] Error in transforming between NAD83 and NAD27 datums

[Glancy, Tom (MNIT)]

PostGIS used to be fussy about where the datum grid shift files were located. Here is a post from 2012:
http://lasmux.blogspot.com/2012/08/postgis-and-nad27-coordinate.html

Not sure if it's still the case, but may be worth considering.

Tom

TOM GLANCY  |  IT PROGRAM MANAGER, ECOLOGICAL AND WATER RESOURCES DIVISION
MN.IT Services @ DEPARTMENT OF NATURAL RESOURCES
651-259-5097 (w)  |  Tom.Glancy at state.mn.us
    Information Technology for Minnesota Government | mn.gov/mnit

________________________________________
From: postgis-users [postgis-users-bounces at lists.osgeo.org] on behalf of Lee Hachadoorian [Lee.Hachadoorian+L at gmail.com]
Sent: Wednesday, February 17, 2016 4:08 PM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] Error in transforming between NAD83 and NAD27      datums

If you are trying to go from dd NAD83 to dd NAD27, you have gotten the
SRIDs backwards in your statement. You are using ST_SetSRID to tell
PostGIS that the original coordinates are NAD27 and then transforming
to NAD83.

When I switch the SRIDs in your statement, I get very close to the
answer you are looking for:

"POINT(-107.073884255496 42.9399531062308)"

Best,
--Lee

On Wed, Feb 17, 2016 at 4:24 PM, Mark D. Andersen <mda at uwyo.edu> wrote:
> I am experiencing errors when I try to use ST_Transform to convert points
> from projections using NAD27 to NAD83, and vice versa.  I don’t receive any
> errors back when I run ST_Transform, but the coordinates that are returned
> are incorrect.  The amount by which they are off (about 50 meters) is the
> amount of error I would expect with an incorrectly specified datum, so I am
> fairly certain that PostGIS is not transforming between NAD27 and NAD83, and
> vice versa.
>
>
>
> ----go from dd NAD83 to dd NAD27
>
> SELECT
> ST_X(ST_TRANSFORM(ST_SETSRID(ST_MAKEPOINT(-107.074497916,42.9399159501),4267),4152)),ST_Y((ST_TRANSFORM(ST_SETSRID(ST_MAKEPOINT(-107.074497916,42.9399159501),4267),4152)));
>
> --should return -107.073884255, 42.9399531071 (or so), but actually returns
> -107.074497916,42.939915951
>
>
>
> I get the same scale of error when trying to go between UTM Zone 13 NAD83
> and UTM Zone 13 NAD27 – it’s basically just giving me back the same
> coordinates that go in, without doing the datum transformation.
>
>
>
> SELECT Postgis_full_version() returns:
>
>
>
> postgis_full_version
>
> POSTGIS="2.2.1 r14555" GEOS="3.4.2-CAPI-1.8.2 r3921" PROJ="Rel. 4.8.0, 6
> March 2012" GDAL="GDAL 1.9.2, released 2012/10/08" LIBXML="2.9.1" RASTER
>
>
>
> What am I missing?
>
>
>
>
>
>
>
>
>
> Mark D. Andersen
>
> Spatial Ecologist // GIS Analyst
>
>
>
> Wyoming Natural Diversity Database
>
> University of Wyoming
>
> Department 3381 // 1000 E. University Ave.
>
> Laramie, Wyoming   82071
>
> PHONE: 307 766 3036
>
> FAX: 307 766 3026
>
> EMAIL: mda at uwyo.edu
>
>
>
>
> _______________________________________________
> postgis-users mailing list
> postgis-users at lists.osgeo.org
> http://lists.osgeo.org/mailman/listinfo/postgis-users



--
Lee Hachadoorian
Asst Professor of Geography, Dartmouth College
http://freecity.commons.gc.cuny.edu/
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