to keep the altitude absolute, draw an extrude down to terrain, and show direction on path.
The conversion work well, but the resulting file has some trouble:
1) the resulting KML is as expected and XML, with an open tag and close tag for each Placemark. This generate a very big file, it this example is 3.8 MB.
2) there are many #track-N (with N up to 12) composed of many placemark named PRESALTTRK-NNN (up to 1819) with barometric altitude there are many #track-N (with N up to 12) composed of many placemark named GNSSALTTRK-NNN (up to 1819) with GPS altitude The two paths are overlapping and redundant, often is preferable choose only one, say filter one of the two.
Using and online service like: http://cunimb.net/igc2kml.php I got the "2016-10-29_6ATXETK1_online.kml" version, that has: 1) Waypoint saved as many "#line" of 500 coordinates, so no open/close tag in between. 2) only GPS altitude (or barometric altitude) is kept. This save lot of space and resulting file is 42 KB.
Removing some of gpsbabel conversion output options I can reduce a little the output, but result is anyway an hundred size the online conversion. I tried many options and filters combinations but I can't generate a small file.
Is there a way to output the waypoint as a single #line (or many 500 coordinate #lines)? Is there a way to select GPS or barometric altitude?
Another less important question: is there a way to move up-down the track to compensate GPS elevation error (supposing costant)?