[Gdal-dev] NAD83 vs. WGS84

[Frank Warmerdam]

Grabowski, Hank wrote:
> The problem as I've determined is that my "NAD27->WGS84" conversion is
> really just doing a "NAD27->NAD83" conversion.  The NAD27->NAD83
> conversion in ESRI identically matches the conversion I'm attempting in
> OGR.  Is there something wrong with the syntax of my command for the
> conversion to WGS84?  
> 
> ogr2ogr -t_srs WGS84 out.shp roads.shp
> 
> My program's internal conversion to WGS84 has the exact same difference,
> so I figure once I determine how to get the conversion working in
> ogr2ogr I should be able to map it over to my internal code as well.  Is
> it such that NAD83 is the best I can do with OGR? If that's the case
> that's fine.  I just need to make that fact known to our users.

Hank,

There is nothing wrong with your OGR syntax, but (as with geotrans) OGR
assumes that WGS84 and NAD83 are essentially the same which they are at
the precisions you are working at.  What I don't understand is what the
ESRI and USGS software packages are doing.  It seems that since they
don't have a grid shift from NAD27 to WGS84 they are just doing no
datum shift and going on blissfully.

Best regards,

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