[OSM-dev] Projection problems

[Thomas Wood]

osm2pgsql uses a spherical mercator projection which cuts down on the
amount of math required to produce a reasonable estimation of the
earth for our uses.
It's not a 'proper' projection in the GIS sense, but it works for our purposes.

http://wiki.openstreetmap.org/index.php/Mercator#Spherical_Mercator

I'm not sure what scaling factor osm2pgsql uses

Somewhat related implementations are at
http://wiki.openstreetmap.org/wiki/Tilenames#Java

2008/12/16 Bradley Kite <bradley.kite at gmail.com>:
> Hi all,
>
> I wonder if any one could be so kind as to help me out with a problem
> I am having with projections:
>
> I am trying to convert from lat/long coords to the Mercator projection
> as is used in osm2pgsql
>
> I am using this code:
>
> http://wiki.openstreetmap.org/index.php/Mercator#Java_Implementation
>
> The Lat/Long I am using is (bounding box around a street I know):
>
> -0.90362548828125,51.38892310050485 to -0.90087890625,51.38720911696496
>
> However, when converted to Mercator projection as per the above Java
> code, I get the following:
>
> -100591.12922329194,6656970.686752424 to -100285.38111015123,6656665.738991335
>
> The y values (6656970 - 6656665) do not match up with data the
> postgres database:
>
> LINESTRING(-100491.52 6690303.05,-100474.32 6690280.11,-100456.16
> 6690259.1,-100438.02 6690235.21,-100434.02 6690222.73,-100430.37
> 6690211.33,-100433.23 6690176.94,-100434.19 6690146.36,-100432.48
> 6690134,-100430.37 6690118.64,-100417.95 6690093.81,-100398.83
> 6690072.78,-100373.27 6690047.38,-100370.16 6690038.6,-100327.34
> 6689918.14,-100309.98 6689869.27,-100245.01 6689884.55)
>
> Note - all the Y values in the DB are ~6690000 not ~6650000 as
> returned by the mercY() function.
>
> The effect of this is that an SQL query to pull data from the database
> is not fetching the correct data.
>
> Would there happen to be an expert on this list that help out with the
> accuracy of the projection code at the above link? It only affects the
> Y coordinate, X seems fine.
>
> Many thanks in advance.
>
> --
> Brad.
>
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>



-- 
Regards,
Thomas Wood
(Edgemaster)