[Gdal-dev] gdal/ogr/proj and NAD83(CSRS)

[Robert Coup]

On Wed, Jul 29, 2009 at 5:25 AM, Matt Wilkie <maphew at gmail.com> wrote:

>
> Let's see if I have this straight then: if I specify -t_srs epsg:4617 and
> not
> +nadgrid, no datum transformation occurs, even though the output .prj file
> says it has?


If your projection definition in Proj4/OGR has a datum defined it will go
through one of the following (in order of preference) if the datum's are
different:

 - a gridshift (the first matching one if there are >1)
 - or a 7 parameter transform (+towgs84=#,#,#,#,#,#,# if specified)
 - or a 3 parameter transform (+towgs84=#,#,# if specified)
 - or do nothing

It does that from source-datum to WGS84, then from WGS84 to the dest-datum.
A datum in Proj is defined as an ellipse and a towgs84 or gridshift
combination, +datum= just pre-populates this info. For Proj's purposes,
GRS80 and WGS84 are the same ellipse.

cs2cs -v is quite clear about what it's doing, OGR not so much.

Using the "straight" Proj EPSG defs on my system:

$ cs2cs -v +init=epsg:3579 +to +init=epsg:4617
# ---- From Coordinate System ----
#Albers Equal Area
#       Conic Sph&Ell
#       lat_1= lat_2=
# +init=epsg:3579 +proj=aea +lat_1=61.66666666666666 +lat_2=68 +lat_0=59
# +lon_0=-132.5 +x_0=500000 +y_0=500000 +ellps=GRS80 +units=m +no_defs
# ---- To Coordinate System ----
#Lat/long (Geodetic alias)
#
# +init=epsg:4617 +proj=longlat +ellps=GRS80 +no_defs

No datum transform will happen (ellipses match, and +towgs84 is ommitted (ie
==0)).

HTH,

Rob :)
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