[gdal-dev] Telling lies with geotiif metadata
wendell at enflight.com
Tue Aug 26 09:41:04 EDT 2008
I am trying to do something I probably shouldn't ...
I have about 30 geotiffs that tile the continental US (aviation sectional
charts). Each of them uses a slightly different projection -- LCC with
different standard parallels and origins for each chart.
Using gdalwarp, I converted everything to a single projection -- epsg 3395
-- but the quality suffered (I was never able to get any resampling method
other than "near" to work properly; the colormaps kept getting screwed up.
There doesn't seem to be any way to tell gdalwarp "stop trying to make a
new colormap, just use the old one!")
The result is OK for display on a screen (I have a nifty OpenLayers
interface to it), but I also need to print good-quality versions. Sometimes
the prints will need to cross the boundaries between the geotiffs. These
prints also need to be at a (nearly) constant scale, which is inconsistent
with using 3395 anyway, since it is a Mercator projection.
I could warp everything to some other projection (LCC with a single set of
standard parallels, say) but I am willing to accept a small amount of
mis-registration at the boundaries between charts if it would save me the
loss of quality in the warp. (In fact, people do this with the paper
charts all the time -- at many airports you'll see multiple charts glued to
the wall, abutting. The misregistration is typically less than a few
millimeters across a four-foot chart.)
[Of course, the misregistration is probably cumulative in some sense; with
the paper charts one can simply "eyeball" the error and slide the chart so
as to minimize it. I'm not sure how to express this mathematically ... ]
So, what I am thinking of doing is to rewrite the geotiff metadata so as to
simply lie about the projection. gdal_translate has options called -a_srs
and -a_ullr that appear to do at least part of what I want.
Is there a "null projection" that I could give to gdal_translate so as to
not to munge any of the actual pixels?
Is there some better way of re-writing the geotiff metadata than by passing
it through gdal_translate?
Is this a silly idea that I should receive the cartographic equivalent of
being disbarred for even suggesting?
Many, many thanks for any insight that you can provide!
[ I'm sending this to the list since Nick's email is inop a
the moment. Wendell ]
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