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GPSBabel / gpsbabel

GPSBabel: convert, manipulate, and transfer data from GPS programs or GPS receivers. Open Source and supported on MacOS, Windows, Linux, and more. Pointy clicky GUI or a command line version...
https://www.gpsbabel.org
GNU General Public License v2.0
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explain humminbird geodetic to geocentric conversions. #1326

Closed tsteven4 closed 2 months ago

tsteven4 commented 2 months ago

Document the humminbird geodetic to geocentric conversions. An ellipse was derived to match the data described in gpsbabel-code 2008-08-14. The related math is explained in terms of this derived parameter.

tsteven4 commented 2 months ago

I have an idea for what the actual humminbird conversions between lat,lon and hwr file data are, as opposed to the approximate conversions from 2008 that we use today. I suspect that the mercator projection formulas for the ellipsoid from "Snyder, J. P. Map projections — A working manual. Professional Paper 1395, U.S. Geological Survey, 1987" may be a match. To investigate this I need a rosetta stone of appropriate known lat lon points and a humminbird created hwr file that contains them. If there is a humminbird user out there who can create such a sample I would like to discuss a set of appropriate points. @bjkowalski?

tsteven4 commented 2 months ago

The mercator projections on an ellipsoid are looking good. We need the previously mentioned set of reference points to wrap this up. If we move to the mercator projections for the ellipsoid then geodetic_to_geocentric_hwr and geocentric_to_geodetic_hwr which were explained in this PR go AWAY.

A more modern reference is:

"The Mercator Projections", Peter Osborne, 2013 https://zenodo.org/records/35392

tsteven4 commented 2 months ago

I'm still interested in a set of crafted set of reference points from a humminbird user, but I am killing this PR and will incorporate any resulting changes in a new PR.