On Mon, Nov 07, 2011 at 08:14:44AM -0800, Paul Ramsey wrote: > EPSG:4326 was chosen as the "only mandatory projection" for OGC web > services around 2000, five years before Google decided that > not-quite-Mercator should be the new standard. > > It is also "unprojected", that is the units are latitude/longitude, so > projecting it only requires a forward projection, while projecting > not-quite-Mercator requires both an inverse projection into > latitude/longitude and then a forward projection into your desired > destination. > > As the units are latitude/longitude on WGS84, folks can easily match > them to their handy GPS unit outputs. > > Whether 4326 is more or less f***ed up at the poles is a matter of > debate, I suppose, but at a minimum it wastes fewer pixels on the > north and can be used at slightly higher latitudes.
This is the one that caused me to go with it. Storing Greenland in EPSG:900913 is a big waste -- and more importantly, projecting backwards from EPSG:900913 introduces more weird projection effects than going the other way, in my experience working with it. (Going from EPSG:4326 to a local projection generally worked fine; going from 900913 to a local projection seemed, in a few limited cases, to be more noisy). Since these images aren't designed to be consumed directly, really -- that is, OAM archive images aren't meant to be read as tiles by web clients -- using the format which is preferred by web clients seemed unneccesary. Most local projections -- especially at northern latitudes -- seem to be closer to EPSG:4326 than 900913, and you can get rid of weird 'edge cases' with projecting near the poles: you can just make requests for what is commonly thought of as the 'whole world' rather than only being able to practically work up to some arbitrary limit without getting really ridiculous. -- Chris > (Though storing > northerly imagery in *either* 4326 or 3857 seems like a bad idea, IMO, > given the distortions introduced and damage the resampling process > therefore will do to the data.) > > So, I'd guess an answer would be: history; simplicity. > > All arbitrary decisions are arbitrary, but some are more arbitrary than > others. > > P. > > On Mon, Nov 7, 2011 at 4:40 AM, Josh Doe <[email protected]> wrote: > > I've been trying to find the discussion or rationale for why EPSG:4326 > > is the projection of choice (or only supported projection?), but > > haven't had any luck. It seems to me that most imagery will eventually > > be consumed in EPSG:3857/900913, though of course not exclusively, so > > it seems that would be a better choice. Is there any technical reasons > > why EPSG:4326 is preferred, or any statistics on it being more > > prevalent? I think the answer to this belongs in the docs. > > > > Thanks, > > -Josh > > > > _______________________________________________ > > talk mailing list > > [email protected] > > http://host134.hostmonster.com/mailman/listinfo/talk_openaerialmap.org > > > > _______________________________________________ > talk mailing list > [email protected] > http://host134.hostmonster.com/mailman/listinfo/talk_openaerialmap.org -- Christopher Schmidt Web Developer _______________________________________________ talk mailing list [email protected] http://host134.hostmonster.com/mailman/listinfo/talk_openaerialmap.org
