Drawing Building

dave_zh · 2009-09-28T19:47:22.000Z

I draw some building using josm (WSG84 projection). If i look at them on openstreetmap.org page a day later they look very bad (not right-angled).
If i do it with Merkator projection they look good. Should one always use Merkator projection in OSM?
I know about the different projections used, but I think that a building that is right-angled in the editor should also be right-angled in a rendered map.
Please give some advice
Thanks
Dave

JRA · 2009-09-29T10:19:07.000Z

Hi,

If you draw something in one projection (in your case epsg:4326) and show it in another (epsg:900913) the drawing will be reprojected and it does not visually look the same. Due to the nature of 4326 projection objects which are right-angled in the nature do look distorted on an orthophoto in northern and southern parts of the world. If you are working by placing rectangular boxes to present buildings instead of tracing them visually from an image, use Merkator (Google projection, or epsg:900913, or epsg:3857) in JOSM because it is the same projection that is used in OSM slippy maps. Make a test by downloading some building outlines which look good to you on a slippy map and show them in epsg:4326 in JOSM. They will look distorted but it is not an error.

chris66 · 2009-09-29T14:39:14.000Z

dave_zh:

Should one always use Merkator projection in OSM?

Normaly yes. Only if you draw from SAT images the distortion might be smaller,
if you use lat/lon.

Chris

dave_zh · 2009-09-29T18:39:46.000Z

Thanks a lot.
Dave