NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Antoine Couëtte
Date: 2024 Apr 4, 07:24 -0700
Dear Paul,
Thank you very much for your reply and for the very interesting reference to https://earth-info.nga.mil/index.php?dir=wgs84&action=egm96-geoid-calc which gives invaluable data on the difference between the WGS84 Ellipsoid and the EGM96 and EGM08 Geoids.
As least, we have here some quite reliable data about part of our early problem which was to obtain reliable heights above the WGS84 Ellipsoid.
If we can obtain reliable heights above Mean Sea level and consider them as actual orthometric heights (which seems a quite valid assumption since it looks like EGM08 approximates the true Earth Geoid by +/- 2 meters vertically) then we can derive reliable heights above WGS84.
And how do the Google Earth heights perform here ?
Not quite well actually, except [maybe] for differences in altitude between 2 nearby points.
As an example, the attached document indicates how the Google Earth Pro Heights compare to either orthometric or WGS84 altitudes.
I have yet to find any consistent rule between all of them. Google Earth Pro altitudes seem totally different from one point to another.
For my MonViso, Lady Moon again with a Basilica Nav Puzzle post, Google Earth Pro seems to indicate actual AMSL level in the vicinity of the Superga esplanade (Attachment Line 17) while on the other point of interest MonViso (Attachment line 16) it shows an altitude which seems in no way related to either AMSL or height Above WGS84.
This the kind of predicament I ran into when attempting to accurately solve for the Photographer's position. I had to make reasonable assumptions through assuming that the Google Earth Pro relative heights adequately reflect the real world in the vicinity of Superga, or in other words that the heights differences are adequately reported in the vicinity of Superga... .
I am to publish fine-tuned end results here and very soon, altogether with the actual position the Photographer very kindly indicated to me meanwhile, and which comes as a nice confirmation of the result I derived.
Back to Google Earth [Pro] Altitudes : from the factual examples attached here and to who-ever needs consistently reliable altitudes, they should be taken with a huge pinch of salt.
Anybody any explanation here ?
Kermit
