Wider scale geoid spheroid file needed

[mggr]

A geoid-spheroid file is required for processing with a DEM based on the geoid. For UK flights, we use osgb02.cgrf, which is a high accuracy description as recommended by the OS.

The other files we have in ~arsf/dems/geoid-spheroid are: igg2005.grd (Portuguese grid?) and sphseplx.grd (European region). Both are apparently a 7.5min grid.

To process Ethiopia (and IPY and other) data, using SRTM DEMs, we need a larger grid.

This ticket is somewhat belated - the issue was raised during Bill's visit to PML in March, but pushed recently as we started to work on Ethiopia data.

[mggr]

Work order no: 200805-2.

Bill suggested several options, including a permanent fix (#144) and has just sent a file with the following accompanying text:

I have sorted a g-s file that will cover Ethiopia in fact it covers the whole world on a 15 min grid for 90s-90n and 180w to 180e. The original grid was 7.5 mins but as mentioned I can no longer find that. This one is from the official GPS info site:

http://earth-info.nga.mil/GandG/wgs84/gravitymod/index.html

You will see that there are new coeffs for 2008 due and I will incorporate them into azgcorr when they finally release them.

To keep the file small I have stuck to their original format of I2 in cm so I had to make some minor changes to my grid handling routines which then required mods to azgcorr......  I have attached a new version of azgcorr (4811) and the g-s grid (sphsep15lx.grd).

I have partially tested the new grid using a UK project. Could you try it out using an existing overseas project and see what the difference is. You only have to replace the g-s file, the rest is transparent, although you could do an old and new run with the paras -vl and -ve 3  to get debug listing from the g-s correction part.

[mggr]

Ben testing with Latera.

[benj]

New file looks fine. Causes an almost imperceptible shift (<1m) as against the previous file. Marking ticket fixed.

[mggr]

Work orders renumbered to make it obvious when one is missing. This work order is now wo2008004.