Gateway Altitude parameter - value and purpose

Gateways
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  1. Is there any spec that describes what should be the exact value of a Gateway’s altitude parameter?
  2. What is the use of the altitude parameter and is it used in practice anyhow?

My assumption is that altitude should be an absolute altitude, not a relative altitude (relative to local ground).
Absolute in the sense of the altitude of the geolocation where a gateway is positioned, incremented with the relative altitude of the gateway compared to the local ground level (‘0m’). This is at least what I would a GPS based gateway to produce as altitude.
To be exact: altitude of the gateway’s antenna.
Is that correct?

Example:
Altitude of geolocation: -3m below sea level.
Height of gateway antenna to ground: 7m.
Absolute altitude: -3m + 7m = 4m.
So 4m would be the altitude to configure for a non-GPS based gateway (eg single channel packet forwarder).

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You’re correct.

  1. It’s the GPS altitude of the antenna
  2. We don’t use it in TTN yet
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Quite a long time ago but maybe as an imaginative reference for others:

ALTITUDE: vertical distance from M.S.L. to an object (like a GPS antenna, aircraft, weather station’s baro sensor, …).

ELEVATION: vertical distance from M.S.L. to a geolocation/ground (like a hilltop, a reference point, …).

HEIGHT: vertical distance from a geolocation to an object (i.e. between an aircraft and a hilltop).

RELIEF: is the vertical difference in elevation between geographic features (i.e. two hilltops).

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