[HOT] Mali follow-up: OrbView-3 image of Mopti available

Jean-Guilhem Cailton jgc at arkemie.com
Sun Apr 8 09:47:18 BST 2012

Yes Fred, you are perfectly right, the image is shifted by about 20 
meters to the east.
 From the GPS tracks, it is actually much easier to adjust globally the 
image with JOSM offset function than it would be with the tools I have 
(that would rather work well with a few accurate ground control points 
known with some degree of confidence).
And with the perspective of processing more than a thousand OV-3 images 
available over North-Mali/Azawad alone, it would be completely 
untractable to correct this on the server side, for each image. It is 
more realistic to stick to the standard process used, eg, with Bing 
images, and manually adjust the offset on the client side.
To ensure that a uniform offset is used over the appropriate area, it 
would be easier to coordinate, eg on this list. As a data point to start 
building a consensus, the offset I came up with last night, when trying 
to get a rough estimate for its value (20 m, like you), was -23.07; -1.28 .

A thing I had forgotten to say is that people who find interesting OV-3 
images on earthexplorer, for example because they cover a town, are 
welcome to share this info. (Like Fred did for the Mopti image). An 
online shared spreadsheet is available for this:
(You can also use other means if you don't like its location)
Depending on how things go, I might adjust priorities between handling 
manually a few images, or working on a script for batch processing of 
many images.

Happy Easter to all those who celebrate it.


Le 08/04/2012 00:44, Frédéric Bonifas a écrit :
> Thanks Jean-Guilhem,
> I started tracing with this imagery but comparing with the GPS tracks,
> it seems like the imagery is shifted by about 20 meters to the east.
> Is it possible to easily correct this offset, or do I have to do it
> manually in JOSM ?
> Fred
> 2012/4/7 Jean-Guilhem Cailton<jgc at arkemie.com>:
>> Hi,
>> An OrbView-3 image (1 m resolution, panchromatic) of Mopti, Mali, is
>> available as TMS at this URL for JOSM:
>> tms:http://osm.arkemie.org/cgi-bin/tiles/1.0.0/mali/{zoom}/{x}/{y}
>> (Use http://osm.arkemie.org/cgi-bin/tiles/1.0.0/mali/$z/$x/$y in Potlatch 2
>> syntax)
>> It is from 2005-03-31, so you can use "source=OrbView-3, USGS, 2005-03-31"
>> It was processed at level L1Gst by USGS. The usual warning about checking
>> the offset with GPS tracks applies. (Some are available for Mopti.)
>> The same URL also contains another image, west of the city of Kidal. It
>> could be used to improve the tracing of a "highway=road" imported from
>> vmap0.
>> Best wishes,
>> Jean-Guilhem
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