search

SlashDevin / NeoGPS

NMEA and ublox GPS parser for Arduino, configurable to use as few as 10 bytes of RAM
GNU General Public License v3.0
719 stars 197 forks source link

Altitude--MSL or wrt ellipsoid? #91

Closed CF20852 closed 6 years ago

CF20852 commented 6 years ago

First, let me thank you (SlashDevin) for this library. I'm adapting it for a project to measure hike elevation gain using various configurations of Microduinos and Arduinos with GPS shields and barometric pressure sensors (e.g., BMP180/280 devices). I find GPS altitude is usually noisy enough to cause apparent elevation gain and loss where there is none, and Google Maps elevation data (SRTM, I think) does not have good enough resolution to accurately reflect actual elevation when one is hiking on a steep slope. Moreover, MapMyHike was providing radically different elevation gain data than the elevation profiles generated by Google Earth and Google Maps, so I wanted to try to figure out which was more accurate. (I have to correct the barometric altimetry data for the local atmospheric pressure, which I can get from NOAA's Aviation Weather Center for a nearby airport.)

When I look at your Data Model document, I see the following: "an altitude (above ellipsoid, not Mean Sea Level), accessed with"

But when I look at the code in NMEAGPS.cpp, I see it is pulling altitude data out of field 9 of the GxGGA message, which according to the u-blox NEO-6M Receiver Description and Protocol Specification, is MSL altitude. Can you explain, please?

SlashDevin commented 6 years ago

LOL, I just noticed that last week. I'll have to investigate...

SlashDevin commented 6 years ago

After some research, I see that all GPS devices output altitude WRT the ellipsoid, aka MSL. This is a reference surface that is a good approximation of the sea level surface, especially at the coasts.

A different reference surface, the "geoid", describes the land surface. To get your height above ground, you would have to subtract the fix.geoidHeight() from the fix.altitude() (these functions return meters).