[Proj] Problem overlaying georeferenced images in Google,
jorge.arevalo at gmail.com
Fri May 30 05:45:25 EDT 2008
>/ Eric Miller wrote:
/>/ > Yes, Google maps uses a spherical model rather than
/>/ > ellipsoidal (hence, +a == +b in the destination coordinate
/>/ > system). So, it's not really WGS84. However, my
/>/ > understanding is that GMaps projection assumes there
/>/ > isn't any difference. So, source coordinates should be
/>/ > converted to Geodetic on the WGS84 ellipsoid/datum and then
/>/ > projected onto the Google Mercator "WGS84" sphere
/>/ > ignoring the fact that the models are really different. So,
/>/ > the +towgs84 parameter on the destination should probably be
/>/ > defined as +towgs84=0,0,0.
/> Frank wrote:
>/ > I haven't followed this issue closely, but please be
/>/ > sure you are aware of:
/>/ > http://trac.osgeo.org/proj/wiki/FAQ#ChangingEllipsoidWhycantIconvertfromWGS84toVirtualEarthMercator
> ah, ok. I happened to be looking at this problem yesterday WRT OpenStreetMap's PostgreSQL exporter -m flag (->Mapnik):
> One thing I don't understand in that is what the "+over" term is for.
> It is new to me.
> Also, Jorge- is that 3785 EPSG term for Google's merc a new official EPSG one or a custom thing? if a custom thing maybe it is better to stick with
> <900913> "Chris' funny epsgish code for the google mercator"
> (from /usr/share/proj/esri.extra)
The EPSG code 3785 is a custom one, but the same of 900913 (I didn't know it thanks), except for the "+wktext" term. What does it mean?
OK, I'm lost with this issue. Could my calcs be wrong? I have, for example, this image:
And is georeferenced by this world file:
The image is obtained for a MIF/MID couple of files with UTM projection, zone 30, ED50 datum (Mapinfo said... I don't know what
are the dX, dY, dZ parameters, buth they must be -84, -107, -120). Really, the raster file obtained from MIF/MID files isn't a
PNG file, but I did the transform to "see" what I was doing.
OK, I need to reproject the image, and the output image must be used to overlay it in a GMaps' frame. This is my last attempt:
-s_srs "+init=epsg:23030 +towgs84=-84,-107,-120,0,0,0,0"
-t_srs "+init=epsg:3785 +towgs84=0,0,0,0,0,0,0"
-tr 250 250
Then, I transform the TIFF file in a PNG (transformation OK) and I have this image:
But the image doesn't fit exactly in the map... Is there any error in this reprojection? If the projection is OK, I need to review
the rest of the code...
Thanks a lot!
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