[gdal-dev] Datum transformation method when projecting from a sphere to NAD83 shapefile

[Limei Ran]

Hi Frank:

Thank you for your information.  Is any reason that PROJ4.6. or newer 
does not do any datum transformation when going from LCC Sphere to NAD83 
Albers?
We are using many shapefiles generated in arcGIS, GDAL or by other 
tools.  We want to have the consistent shapefiles.  Could you tell me 
which approach is correct or proper?

You are right about the difference.  We have GDAL1.5.0 compiled with 
PROJ4.5 and GDAL1.5.2  compiled with PROJ4.6.  The projected shapefiles 
using ogr2ogr from the two versions of GDAL do have quite big position 
shiftings.

Thank you,

Limei


Frank Warmerdam wrote:

> Limei Ran wrote:
>
>> Hi:
>>
>> I am using ogr2ogr to project a shapefile defined in a sphere to 
>> NAD83.  My command is like the following:
>>
>> ogr2ogr -t_srs "+proj=aea +lat_1=29.5 +lat_2=45.5 +lat_0=23 
>> +lon_0=-96 +x_0=0 +y_0=0 +ellps=GRS80 +datum=NAD83 +units=m +no_defs 
>> " -s_srs "+proj=lcc +a=6370000.0 +b=6370000.0 +lat_1=33 +lat_2=45 
>> +lat_0=40 +lon_0=-97" wrf12km_nc_g152.shp wrf12km_nc.shp
>>
>
> Limei,
>
> You have not provided a datum definition for the source coordinate
> system, only an ellipsoid definition.
>
> If you are using PROJ 4.6.0 or newer no datum transformation at all
> will be applied.  The LCC coordinates will be reprojected to lat/long
> on the indicated ellipsoid, and then those lat/long values will
> projected into AEA on the NAD83 datum with no attempt to do any
> "change of ellipsoid" or datum shift between the LCC and AEA geographic
> coordinate system.
>
> If you are using anything older than PROJ 4.6.0, there will be a
> change of ellipsoid applied (which will be quite substantial since the
> LCC is on a sphere) implemented by converting from lat/long on the
> LCC sphere, to geocentric coordinates, and then from geocentric 
> coordinates
> to lat/long on the AEA ellipsoid.  This will result in large changes in
> latitude.
>
> So, basically, the approach used depends on the software version.
>
> Best regards,