I have a red Artemis Openlog which I’ve connected to a Sparkfun NEO-M9N GPS board via the QWIIK connector.
I cannot make sense of the altitude information. I did RTFM before asking, and I found a reference that the altitude is in mm but that doesn’t seem to fit what I see in the data. Is there maybe some offset value or nonlinearity?
Jim
Altitude should be in meters but keep in mind that GPS altitude isn’t super accurate because the earth isn’t a perfect sphere and vertical (altitude) accuracy is about 3 times worse than horizontal accuracy. 
Thanks for the reply. It doesn’t look like it is meters because the values I’m getting are in the 160,000 to 200,000 range in an area where the height of the ground is about 250 feet above sea level.
Or maybe I’m just hoping for much better altitude estimates than what I’m seeing? I guess 160,000 could mean 16.0000 or 160.000 meters, both of which are way off but isn’t there a document to tell me which it is?
Just following up to say that after a lot of experimenting I’ve determined the values are in fact in mm. I was thrown off by how inaccurate they are!