The WSPR power field is used to convey altitude. (I’m using a 10 mW output into a dipole). The latest data can be found in the WSPRnet database.
Note – this is out of date, we’re now using more precise 6 character grid square and more precise altitude using Type 1 andf Type 3 WSPR messages.
The WSPR data is transferred to APRS.fi with this format: RX_UTC_day RX_UTC_time Callsign Frequency_MHz RX_SNR_dB RX_drift TX_gridsquare TX_power_dBm RX_callsign RX_gridsquare RX_miles