Coordinate rounding is different from Mapnik
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- Dominant language
- C++
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Description
WGS84 coordinates are usually not exact multiples of tile coordinates. When they are not, Tippecanoe rounds toward the top left, by doing the projection math as precisely as possible and then masking off low-order bits. It looks like Mapnik rounds toward the equator and prime meridian instead.
The symptom is that (at z0) in Tippecanoe [ -1, -1 ] will appear as [ -1.054688, -0.966751 ], but [ 1, 1 ] will appear as [ 0.966797, 1.054628 ]. In Mapnik, they are [ -0.966797, -0.966751 ] and [ 0.966797, 0.966751 ].
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
No file, test, or entry point is named. Locate the coordinate projection and tile-coordinate rounding path, then compare its behavior with the z0 examples in the issue and Mapnik's results. Done means the rounding behavior matches the stated Mapnik outputs without regressing other coordinate conversions.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- cpp
- Domain
- cli, data
- Issue type
- Bug
- Difficulty
- 3/5
- Estimated time
- 1-2 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 32/100