Applying Geographic Transformations When Reprojecting

Jul 24, 2012 at 12:33 AM

Hi, I have been coding in the ESRI ArcObjects world for awhile now and just found DotSpatial which I am really excited about, so thank you much!  Have a question about applying a geographic transformation if I may...

There are tons of KnownCoordinateSystems, so think we are well covered there, but what about applying geographic transformations?  I see that DotSpatial.Projections.Reproject.ReprojectPoints has an option for an IDatumTransform, is that the same?  And I see KnownTransform, but not a lot of them in there...

For example I want to convert something from WGS1984 to NAD1927StatePlaneCaliforniaVFIPS0405, but in ESRI world I need to apply the transformation called NAD_1927_To_WGS_1984_79_CONUS otherwise my conversion is off by several hundred feet.  Can I do this with DotSpatial? And if so, is there a way to derive the list of valid transformations given two coordinate systems?

Thanks a lot for any help!

Jul 24, 2012 at 7:35 PM
Edited Jul 24, 2012 at 9:53 PM

There are more projections in existence than those provided by default, but you can use your own as follows...

You have to create your own transformation to make that kind of reprojection. Usually the parameters of the projection string contain the transformation parameter. You can create a ProjectionInfo.FromESRIString(...) to pull this information out of a properly crafted string. Then, pass this ProjectionInfo object to the ReprojectPoints method.

Jul 25, 2012 at 4:06 PM

Ok, thank you much for that information mudnug!

Guess I'll need to learn the format of those strings.  All I could find from ESRI is a pdf (geographic_transformations.pdf) that contains the dx, dy, dz, etc.  But not all transformations seem to be listed with this info, for example the one I mentioned before (NAD_1927_To_WGS_1984_79_CONUS) only is listed with a code (15851).

I unfortunately cannot find where they actually store these additional transformation strings... I can find all their .prj projection files, but not these additional transformation files.  Custom transformation files have an extension of .gtf, but cannot find any of those on my hard disk either.  I'll keep researching and hunting. 

Thanks again!