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Description:
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I think there is a bug in uDig rendering subsystem that occurs when drawing UTM projections (see attached picture). I encountered this bug when implementing a new graticule graphic (UTM coordinate grid) which I soon will contribute to the uDig community.
Steps to reproduce
- Create a new map
- Set CRS to EPSG:32633 (or any other UTM projection if you have test data that represents a proven UTM graticule)
- Add Graticule graphic (pull request with source code coming soon)
- Add 1km graticule (WMS, or any other graticule data source)
- Goto scale 1:50 000 (2cm on map equals 1km on ground)
which should produce something similar to bottom half attached screen dump of GIMP (which produce correct screen to world mapping, which is another assumption I made).
The problem
Looking at the picture, is is evident (I hope), that something is wrong. The graticule graphic (top half) and the graticule proven correctly by extensive use (WMS, bottom half) produce exactly the same grid (I have implemented the graticule using gt-grid). But the scalebar which shows 4km, which should match the 03 line, does not because both graticules are too wide (+3.4cm | 1.7km, measured with GIMP).
The conclusion
Since the scalebar and GIMP agrees, I'm lead to believe that something is wrong with the mapping of UTM projection to screen coordinates. And because I use gt-grid and compares this output with a proven WMS version of it, I'm also lead to believe that something is wrong with the uDig rendering system. I hope that this is not the case, but I can't see any other likely cause.
Critique
Just in case, I made some counter-claims:
- It is a coincidence that the graticule graphic and the "proven" graticule (WMS) agrees
The results are validated against the 1km square graticule maintained by the Norwegian Mapping Authority. Because it is heavily used, I don't have any reason not to trust this source.
- Does GIMP actually produce correct world to screen projection?
Yes, I believe so. Just capture something from the screen, measure it using GIMP. Print it, and measure it again with a tape measure. It should match (it did when I tested it).
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