#GIS from a .NET developer's perspective

Transformation problems with NetTopologySuite and GeoTools.NET

The last couple of days I've been working on doing on-the-fly transformations in SharpMap. I started out by implementing all the interfaces in the OGC "Coordinate Transformation Services" specification. These specifications are just great and makes my work of designing APIs so much eaier. Anyway when I started at the actual implementation I also looked at NetTopologySuite (which is a .NET port of Java Topology Suite and GeoTools.NET for a little help on the more complex transformation formulas.

It didn't really work out. All my results was either wrong, resulted in Overflow and DivisionByZero exceptions and NaN values. All I could then do was working through the EPSG transformation formulas, which was I was trying to avoid in the first place (oh by the way, EPSG recently released v7.10 of their coordinate system database).

Well I finally hacked it, and my conclusion is that there are several problems in the NTS code that causes very inaccurate (if not completely wrong) transformations. Problems like an incorrect constant for converting between radians and degrees, transformation formulas that are wrong (at least the Mercator projection - haven't checked the others yet), transformation-factories that doesn't respect the order of source and target coordinate systems (it will always transform from geographic to projected coordsys), and using the TransformList(array-of-points) method in many cases gives a different result than Transform(point).

I emailed my findings to Diego and Andrew who's doing a great job on NTS, so that they can benefit from this work well, but this is just to warn you people out there that NTS is probably giving your incorrect or inaccurate transformation results, until the bug is corrected. This is by no means Diego's fault; I guess he just copied it from GeoTools.NET (since the code is more or less the same), but this reminds me why we always use the national cadastras transformation libraries for transforming data...

Oh well as you can probably understand from this post, SharpMap now supports on-the-fly transforrmation! How cool is that? :-) If you can't wait for the next release, grap the source from the GotDotNet workspace. Warning though: I haven't checked the rest of the formulas thoroughly yet, but Mercator projection is checked and corrected and UTM seems to be working...

Comments (1) -

  • NotSoSharpGIS

    5/9/2006 2:12:43 AM |

    I haven't checked this out yet, but it is very exciting! I saw the reserved space in the old source for transformations and thought it would be a great feature to have. Give us the breakdown. How would I go about transforming say a point in decimal degrees to UTM 10, WGS84.
    Or, have I missed the boat on this.

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