[gdal-dev] Using GDAL with gridded XYZ that has no 'nodata' values?

[Sam Franklin]

Greetings. This is my first post to this mailing list, so apologies if I
commit a mailing-list faux-pas :)

I wonder if anyone can offer a suggestion, as I'm hoping I've missed
something obvious.

I regularly receive bathymetric DEMs, acquired by acoustic multibeam
echosounders (MBES) from offshore geophysical surveys that take the form of
narrow corridors of survey or irregularly shaped areas.

My goal is to have a end-to-end open source/GDAL workflow that converts the
bathymetric DEM to greyscale GEOTIFF which I can then use for further
processing/analysis using GDAL/OGR.

The convention in the offshore sector is to deliver bathymetric data as
single 'gridded' .XYZ ASCII file, where XY coordinate pairs are evenly
spaced to form a grid and the textfile row order is spatially sequential.

However, crucially 'nodata' values are omitted from the XYZ.  I can only
assume the reasoning was to reduce the file size on disc. This is an
industry convention that will not change anytime soon.

gdal_translate obviously does not read these XYZ files as gdal throws an
error due to the lack of true rectangular grid.  I've read the XYZ info
http://www.gdal.org/frmt_xyz.html which is clear to me, no problem there.

I do not want to use gdal_grid on the XYZ as this will introduce a further
interpolation step. The XYZs I receive are already cleansed and gridded by
the surveyor which I don't wish to edit further.

So, I currently use a proprietary marine geophysical package which can
directly read the gridded XYZ without nodata values.  Using this package, I
can export to ESRI ASCII grid (.ASC) which I then use gdal_translate
without issue for my further processing steps.

I'd like the freedom of a non-proprietary solution and the only thing I can
think of is to write a python script to pre-process my XYZ into a format
that gdal will accept.  I was thinking of calculating the XY bounding box
of the data and output an XYZ that blends the original values and nodata
values in the correct row order.

If anyone is interested, see below link to a zip of three separate
examples, containing:
-- 'gridded' XYZ (without nodata values)
-- processed greyscale GEOTIFF (using proprietary package)
-- processed color-relief RGB GEOTIFF (using proprietary package)
https://dl.dropboxusercontent.com/u/4086367/gdal_translate_xyz_examples.zip
All data in these examples are referenced to SRS EPSG:27700.

If I've missed anything obvious, or if anyone has any thoughts of how I can
achieve my goal using GDAL or other, or comment on my proposed solution,
that would be greatly appreciated.

Thanks in advance.
Sam
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