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 <[email protected]>: > 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. > > _______________________________________________ > dev mailing list > [email protected] > http://lists.openstreetmap.org/listinfo/dev > -- Regards, Thomas Wood (Edgemaster) _______________________________________________ dev mailing list [email protected] http://lists.openstreetmap.org/listinfo/dev
