[OSM-dev] Projection Issues

[Christopher Schmidt]

On Thu, Jun 01, 2006 at 05:55:03AM +0200, Lars Aronsson wrote:
> Christopher Schmidt wrote:
> 
> > The problem here is that OSM's setup right now depends on two
> > contradictory ways of dealing with data: 
> >  * Mercator projected
> >  * Unprojected/EPSG:4326
> > 
> > There are several possible ways to fix this.
> 
> Many million years ago, a meteorite landed in central Sweden, 
> causing this perfectly circular ring of lakes,
> http://www.openstreetmap.org/index.html?lat=61.033&lon=14.928&zoom=8
> 
> On OSM's map (and Google Maps) that actually looks like a circle,
> http://maps.google.com/maps?t=h&ll=61.033,14.928&spn=.5,.5&om=1
> 
> On your own map, http://crschmidt.net/mapping/openlayers/osm
> that oval looks twice as wide as it is high.  (Sorry, I don't know 
> how to link to a specific coordinate there.)

That specific map doesn't have the support, but that's okay, you
already convinced me :)

> It's your projection that is broken, not OSM's.  (Well, OSM's and 
> Google's projection is broken too, as it erroneously presents 
> latitudes as straight lines and longitudes as parallels, but this 
> only matters in really small scales.)

I'll state that the projection in use is not *broken* -- that is, there
is nothing geographers would have to complain about -- but that it is
not ideal for higher latitudes. I'll admit that my failing to see this
up until this point was as a result of my US-centric (really, more
equator-centric) upbringing, where I've never had to suffer at the
higher latitudes. 

However, the fact that OSM is using a Mercator projection, but using
units in degrees, is not geographically sound. The whole point of having
a projection is that you're dealing with units in terms of distance on
the surface -- not degrees. Right now, there's no way to request a tile
from OSM in a way that's geographically sound, because it only supports
one way of requesting tiles, and it doesn't match the way the tiles are
rendered.

> >  * Stop with the Mercator Madness :) And start using Unprojected
> >    coordinates. This would probably simplify a lot of math all over,
> 
> Duh!  Google Maps did that mistake in its first two weeks, but 
> they were pretty fast to fix it.  People looking at the satellite 
> image of a well-known sports arena want it to be oval in the right 
> direction, not in the perpendicular one.  If a roundabout is 
> circular in reality, it should be so on the map.  More than half 
> of Sweden, most of Norway, and virtually all of Finland, is north 
> of 60 degrees and cosine(60) = 0.5

Agreed. My lower-latitude recommendations are not sound. I'll be working
to improve all of my rendering such that it takes this into account.

That said, the new dev renderer seems to be using WGS-84... ;)

-- 
Christopher Schmidt
Web Developer