[Gdal-dev] Little experiment: comparison of vector projection OGR and ArcGIS

[Dylan Beaudette]

Hi Matt,


On Wednesday 02 August 2006 10:24, Matt Wilkie wrote:
> Hi Dylan,
>
> > A couple days ago I did another experiment, this time incorporating a
> > datum shift: GCS NAD83 --> UTM NAD27. I have summarized the results
> > here: http://casoilresource.lawr.ucdavis.edu/drupal/node/259
> >
> > of note:
> >
> > - the default tranform in ArcGIS does not produce the correct results.
> > - the 'NADCON' transform in ArcGIS produces the same results as OGR.
>
> I asked ESRI-L about this, and the near unanimous response is: there is
> no default transformation in ArcGIS, but an alphabetised list.
>
> "The 'default' transformation that Dylan refers to,
> NAD_1927_To_NAD_1983_6, is just the first in the alphabetical list. As
> it turns it out, it uses NTv2 and is designed for Quebec. ArcGIS
> generally doesn't set any default transformations but leaves it to you
> to decide which one is most appropriate. Dylan's data is outside the
> area covered by the NTv2 grid file, so essentially no tranformation
> occurs when he uses NAD_1927_To_NAD_1983_6. A better reference for him,
> which does list the methods, parameters and areas of use for the
> transformations is:
> http://support.esri.com/index.cfm?fa=knowledgebase.techarticles.articleShow
>&d=21327 "
> -- Melita Kennedy Product Specialist ESRI, Inc.

Ok. I didn't even notice that there was some order in the list. Then I stand 
corrected, in that this was not a 'default' transform, just the 'first' one 
in the list. Seems reasonable.


> Even if mistaken about the "default", thank you for conducting the
> experiment and telling us about it. Otherwise I would sailed along quite
> happily usiing the wrong selections myself. Also thanks to you we also
> know that provided the appropriate transformation method is chosen
> equivalent results can be achieved with both ArcGIS and GDAL, so we are
> all free to choose the tool most readily at hand.
>
> cheers,

Sure. Thanks for taking time to do some of the detective work. I have added a 
small blub at the beginning of the original post, so that future visitors 
will not have to pour over the comments to get at the final conclusions.

http://casoilresource.lawr.ucdavis.edu/drupal/node/259

Cheers,


-- 
Dylan Beaudette
Soils and Biogeochemistry Graduate Group
University of California at Davis
530.754.7341