Ground_distance in frame meta data reports 3.5m while drone is on the ground

The ground_distance value in the frame meta (yuv_frame.vmeta()) data is reporting 3.5m when the drone is on the ground. (This happens both when Anafi drone as well as with Sphinx.) As soon as the drone takes off I seem to get correct readings of this value. In what situations can this value be trusted?

{'drone_quat': {'w': 0.5787353515625, 'x': 0.0377197265625, 'y': 0.02825927734375, 'z': -0.8140869140625}, 'location': {'latitude': 55.68080139160156, 'longitude': 13.059118747711182, 'altitude': 2.07421875, 'sv_count': 21}, 'ground_distance': 3.5, 'speed': {'north': 0, 'east': 0, 'down': 0}, 'air_speed': -1, 'frame_base_quat': {'w': 0.57916259765625, 'x': 0, 'y': 0, 'z': -0.81512451171875}, 'frame_quat': {'w': 0.5081787109375, 'x': -0.73956298828125, 'y': -0.32171630859375, 'z': -0.302001953125}, 'exposure_time': 30.0390625, 'gain': 1600, 'awb_r_gain': 1.128173828125, 'awb_b_gain': 0.272216796875, 'picture_hfov': 69, 'picture_vfov': 42.26953125, 'link_goodput': 16236, 'link_quality': 5, 'wifi_rssi': -70, 'battery_percentage': 91, 'animation': 0, 'state': 'LANDED', 'mode': 'MANUAL', 'frame_timestamp': 113782753, 'automation': {'framing_target': {}, 'flight_destination': {}, 'followme_enabled': 0, 'lookatme_enabled': 0, 'angle_locked': 0, 'animation': 'NONE'}}

The only documentation of this format that I have found is:
https://developer.parrot.com/docs/pdraw/metadata.html#version-2-and-version-3-binary-formats

Is there a more detailed documentation available somewhere?