Name: True AGL (DEM Altitude)
Author: buffoletti

Simple Datafield that calculates the Above Ground Level Altitude based on a Copernicus 90m DEM online fetched grid lookup in meters. It gives live and actual terrain based altitude directly below you while you move without setting reference point or using the start point. 

The current altitude above sea level used to calculate AGL, is the garmin native altitude and therefor affects the displayed AGL. Barometer assisted absolute altitude requires calibration which usually automatically happens shortly into a recorded acitivty. It may not work as expected in pressurized aircrafts. If device has no barometer built in, the reported altitude is not very accurate (vertical GPS accuracy is worse than horizontal).   

Garmin does not allow 3rd party developers access to the onboard DEM data unfortunately. This is the reason, True AGL fetches terrain data from MixAndFly servers. This fetching process is limited to a very small area around your position so the device can handle it. It automatically refetches DEM data, when your about to leave it. The default fetch radius of 30 points will approximately cover area of 5.7km (N-S) x 3.9km (E-W) in the alps, as the grid is non-uniform. 
Fetching requires internet connection of the watch via Garmin Connect IQ App, so it may not work in remote areas or if you are too high. You can increase the fetch area in Garmin Connect IQ datafield settings. But there is a device specific limit, so if the datafield crashes, you need to reduce that number. For reference on FR965 radius up to 75 worked.    

If you like the datafield consider registering the pairing key at mixandfly.de (free). Registered keys are granted higher rate limits (HTTP 429).

Permissions: 
Location and Internet for terrain elevation lookup
Sensor Data for storing the AGL to the fit file (opt out)

Note: This Datafield should not be used as primary flight instrument and True AGL not relied upon for inflight maneuvers.