[Gdal-dev] gdalwarp, expedia, and GPSDrive

Ed McNierney ed at topozone.com
Fri Sep 10 23:00:51 EDT 2004

Sean -

Read my last message about missing info for the source image.  Stop
adding parameters to the -s_srs object; you're confusing the properties
of the data set (the image) with the properties of the projection.  For
the UTM projection as your s_srs, zone= and datum= should be the only
properties you need.

Your UTM map will not have a center point (or upper-left point) with
coordinates expressed in degrees, as the units for that projection are
meters, not degrees.  That's fine - you'll create that in the world file
and gdalwarp will take care of the rest.

	- Ed 

-----Original Message-----
From: gdal-dev-bounces at xserve.flids.com
[mailto:gdal-dev-bounces at xserve.flids.com] On Behalf Of S Clark
Sent: Friday, September 10, 2004 10:54 PM
To: gdal-dev at xserve.flids.com
Subject: [Gdal-dev] gdalwarp, expedia, and GPSDrive

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

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?

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