Hello and thank you for opening the code to the modbus server component of the victron ecosystem.
There seems to be a mismatch between the values documented and actually returned for the dbus path: /Altitude
The XLSX states:

and attributes.csv contains:
'com.victronenergy.gps,/Altitude,d,m,2808,int32,10,R'
The value currently returned by 1052 should be in meters.
However, the XLSX states m/s as the correct unit of measurement.
I believe the XLSX should be updated to reflect the altitude as meters but I don't want to diverge from the XLSX without confirming it is wrong.
For reference the issue on my integrations issue tracker:
sfstar/hass-victron#28