Assuming you're only wanting the elevation of ground level above or below sea level at a particular geographic location, the Google Maps Elevation API seems like it would fit your purposes: https://developers.google.com/maps/documentation/elevation/start
Obviously, this isn't the elevation of whatever device is recording data, as a device in a tower is not at ground level, but it might be what you're looking for.
Now, I don't know of a Splunk app that wraps this API in Splunkbase yet, but that doesn't mean you couldn't make one! What you'll need to do in Splunk is create an external lookup python script that would take the lat/long coordinates, pass these to the elevation API, and then return them to Splunk. There are a few examples of doing this in the docs, around Splunk answers, and the Internet:
