OpenLayers LonLat transformations are not within correct projection bounds

Question

My map uses EPSG:900013 projection. As a result I get values in meters in the range of -20037508.342789244 to 20037508.342789244 when getting my mouse position.

I used the .transform() method of the LonLat class, using EPSG:900913 as the source projection, and (without thinking) used EPSG:4329 as the destination projection.

My question is, why is the EPSG:4329 giving me ranges from -180, 180, -80.05, 85.05 (which i wanted) instead of -180, 180, -90, 90 (which it should have given me, since those are the correct bounds http://spatialreference.org/ref/epsg/wgs-84/)?

Answer 1

I'm relatively sure your source projection (900913) is setting those constraints, so that when you move your mouse, you're limited to travelling so many meters away from 0,0, which corresponds to the 85.05 and -80.05 in your transformations.

Said differently, EPSG 900913 doesn't cover the complete globe. So when you move your mouse to the furthest north/south, respectively, it would transform not to +/- 90, but to 85.05 and -80.05, as you've discovered.

If you go and check this page in the OpenLayers docs, they explain it as follows:

Specifically, most spherical mercator maps use an extent of the world from -180 to 180 longitude, and from -85.0511 to 85.0511 latitude. Because the mercator projection stretches to infinity as you approach the poles, a cutoff in the north-south direction is required, and this particular cutoff results in a perfect square of projected meters.