[Talk-se] Orebro releases geodata under an ODbL license

[Ture Pålsson]

2011/1/22 Hanno Hecker <vetinari+osm at ankh-morp.org>:

> Hmm, either I've an error, or the coordinates from these are ~130m
> off from what we have in OSM...
> I've used EPSG:3021 (== RT90 2.5 gon V) to convert these into useable

It's been a while since I looked into this, but I remember that
conversion WGS84 <-> RT90 is tricky because of the different geodetic
datums. I believe that the method actually recommended by
Lantmäteriverket is to fake the calculation by fudging the parameters
in the Gauss-Krüger formula
(http://www.lantmateriet.se/templates/LMV_Page.aspx?id=5197).

Somewhere on their web site (I can't find it now; I can never find
anything on their web when I need it...) I also found parameters for
an approximate "six-parameter transform" which I translated into proj4
syntax as follows (this is from WGS84 to RT90):

cs2cs +proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs +to +proj=tmerc
+lat_0=0 +lon_0=15.80827777777778 +k=1 +x_0=1500000 +y_0=0
+ellps=bessel +units=m +no_defs
+towgs84=414.1055,41.3265,603.0582,-.8551,2.1413,-7.0227,0

I believe it worked decently when I tried it, but I have forgotten all
about how I came up with those numbers at the end...

  -- Ture