How does Phantom4 maintain altitude?

I’m curious if anyone knows how the Phantom 4 maintains altitude while using the DroneDeploy app to run a mission over greatly varying terrain. From what I’ve read, the only altitude capable sensor on the drone are the downward facing visual and sonic sensors, which if acting alone should cause the drone to lose altitude when flying past the edge of a cliff or down a steep hill. My only other theory is that the IMU senses when the drone drops, rises, etc and won’t let it descend if it received no such command from the controller. But this is just a theory. Does anyone know for sure?

I’m trying to get my company to adopt DroneDeploy to survey crop damage caused by construction. We have a contract to pay out landowners per acre of crop damage and they want to make sure it’s accurate. This is really just one of several questions as to the accuracy of DroneDeploy maps that my higher ups have been asking. Currently it costs between $7k and $15k for surveyors to calculate the area of crop damage. But, of course, DroneDeploy can make this a whole lot cheaper.

The DJI drone goes up to an altitude that you specify and then stays that height above its takeoff location throughout the entire flight. It does not adjust its altitude during the flight.

The Phantom 4 also has a barometric sensor to maintain altitude. It uses pressure differential to sense the change in altitude from the takeoff point.

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Phantom 4:

You can set max altitude to 500 m.