[OSM-dev] Projection Issues

[Andy Robinson]

Of course there is no absolute answer here as any projection of the whole
globe has to be fudged.

Having said that I believe the longer term goal should always be from the
perspective of the view centre. When you zoom in on any "map" you want to
see something that as close as possible resembles the real thing, as if you
were hovering yourself above the globe just as you do with google earth. In
which case you are not interested beyond your field of view, in the same way
as you would not be interested in what's beyond the horizon when physically
viewing from above.

So I agree with Lars, any map that is rendered needs to look like what you
would expect to see in real life and rendering unprojected lat and lon on a
flat sheet does not give you that.

Cheers,

Andy 

Andy Robinson
Andy_J_Robinson at blueyonder.co.uk 
>-----Original Message-----
>From: dev-bounces at openstreetmap.org [mailto:dev-bounces at openstreetmap.org]
>On Behalf Of Christopher Schmidt
>Sent: 01 June 2006 12:54
>To: Lars Aronsson
>Cc: dev at openstreetmap.org
>Subject: Re: [OSM-dev] Projection Issues
>
>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
>
>_______________________________________________
>dev mailing list
>dev at openstreetmap.org
>http://lists.openstreetmap.org/cgi-bin/mailman/listinfo/dev