Your drone relies on its built-in barometric altimeter to measure heights and to control its altitude. While the accuracy of this altimeter usually is quite high (+/- 0.5 m or +/- 1.6 ft), in difficult conditions (e.g. strong wind gusts, pressure fluctuations, etc.) it can drop considerably to about +/- 2 m (+/- 6.6 ft) or worse. For example, if you were to measure tower height with your drone several times in succession, you might observe discrepancies of up to 2 m (6.6 ft).
This must be considered when planning flights and when entering obstacles (see previous section). Also, while flying the mission, continually check on your display that the drone is flying at the intended altitudes, that all antennas and the tower top are fully captured, and that the capture scans the tower as planned. If your drone deviates too much from the intended flight path, consider cancelling the mission and adjusting the flight parameters to compensate for the observed error.
Also, make sure that you always keep your drone within visual line-of-sight and observe it closely when it approaches obstacles. Be ready to pause or cancel a mission if your drone gets too close to an obstacle. How to do so is explained in the respective capture guides for Scanlink and Mission Planner.
It is important to understand that altimeter errors are cumulative. Therefore, launching flights midair to safe time (e.g. after completion of mission planning) can result in increased inaccuracy. If you do experience inaccuracies (e.g. due to challenging weather conditions), we recommend that you land your drone before launching a mission.
Are you experiencing height inaccuracies and incorrect altitudes that cannot be explained with altimeter inaccuracy? If so, remember to always launch your drone from the take-off position you have marked when planning the flights. It is important that this is the same position as the one from where you launched your drone to determine the tower dimensions.