[Gdal-dev] gdalwarp, expedia, and GPSDrive

S Clark smc+gdal at dogphilosophy.net
Fri Sep 10 22:54:07 EDT 2004

On Friday 10 September 2004 08:17 pm, you wrote:
> As for the Expedia projection, it is NOT a Miller projection - at least
> not at small scales.  My bet is that it is a simple orthographic
> projection, with the central meridian and the latitude of origin both
> set to the coordinates of the center point on the current map view.
> So if you're going to work with Expedia maps - and if my guess is
> correct - then you will need to know the latitude and longitude of the
> center point of each map.  You might want to see if GPSDrive works
> correctly with small-scale Expedia maps, where assumptions that might
> work for large scales will begin to break down.

Last time I checked the expedia maps in GPSDrive (some time ago - I 
haven't been using them for a while due to copyright restrictions 
imposed by the Expedia terms of use) they seemed to work at scales
down around 1:5000 or so, at least.  I don't THINK that's changed at
all.  (I'm not actually planning to work with expedia maps themselves, 
I just want to be able to make "expedia-compatible" maps in the same
format, so that they work with GPSDrive...)

The latitude and longitude of the center point on the map is one of 
the few facts that I *DO* have readily available for every image I've
been downloading.

Would it be possible then, just with the information mentioned thus
far, to give me an example "convert a UTM map with a known center latitude and 
longitude and scale to an orthographic map with the same center 
latitude/longitude and the same scale" command line for gdalwarp?  Or
do I still need to calculate further bits of information to feed gdalwarp
before it can work?

I've just tried:

/usr/bin/gdalwarp -s_srs '+proj=utm +zone=12 +datum=WGS84 +k=0.9996 
+lat_0=0.000 +lon_0=-113.2167' -t_srs '+proj=ortho +lat_0=42.94584 
+lon_0=-113.2167 +datum=WGS84' map_42.94584_-113.2167_5512.jpeg 

And still get the "no affine transformation and no GCPs".  

Presumably, to give it "Ground Control Points" I need to somehow specify at
least either the upper-left pixel's lat/lon, or perhaps there's a way I can
feed gdalwarp the "scale" and have it calculate that, in addition to the
lat/lon of the center point?


More information about the Gdal-dev mailing list