[Gdal-dev] Newbie question

[Frank Warmerdam]

Charlie Van Dusen wrote:
> Greetings list
> 
> I have several nautical charts in BSB format that I would like to be able to
> browse in my GIS viewer (ArcExplorer). The *best* common format between it
> and what gdal_translate produces is GeoTIFF.
> 
> Unfortunately whe I try to read the produced Geotiff with ArcExplorer, it
> fails to open and crashes the application. 
> 
> Can anyone give me some pointers to what I need to do to produce an
> ArcExplorer-ready geotiff from BSB (using gdal).

Charles,

The problem (I presume) is that ArcExplorer doesn't grok GeoTIFF files with
a set of tiepoints to define the georeferencing transform.  Most GeoTIFF files
use a single reference point and a pixel size which I presume is all that
ArcExplorer supports.

Most (all?) BSB charts uses a set of control points, and often they don't
desribe a perfect north up coordinate system.  I don't know if this is because
they are produced by scaning charts or what.

The solution is to resample the charts into a north up coordinate system.
The simpliest way to do this is something like:

gdalwarp 999501.KAP 999501.tif

This basically warps the chart based on it's control points.  The
transformation in minimal in most cases, but the result is a perfectly
north-up image that ArcExplorer can hopefully process.

The caveat is this, the gdalwarp program (till this afternoon) did not
copy a color table to the output file.  I have corrected this in CVS.
If you are up to building GDAL yourself, you should be able to grab
the CVS snapshot tomorrow, build from source, and then use gdalwarp.
Otherwise you will have to wait for the next binary release.  I have
no fixed timetable for that.

Best regards,

-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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