Altitude not accurate - very low over resevoir

I sent my SkyEye over our local reservoir at what should have been 25m over the water. As you can see from the footage below, the plane was down to about 4 or 5 meters or less - I was lucky not to lose it.

What is the accuracy of the altitude? I have an APM2.0 in the pane running firmware 3.4

Is there a way to calibrate it?

Have a look

The Baro in the APM2 (or other flight controllers) just isn’t super accurate and depending on a whole bunch of things i.e. if its exposed to sunlight, your APM is in the flight breeze of the plane, localised atmospheric conditions etc. it can drift over the course of the flight. In your flight its possible that the atmospheric conditions over the lake were different to that on the land where you started.

Its recommended to use a LIDAR for accurate low altitude flying but you will need a Pixhawk for that.

Remember when you power on to leave the plane still and wait for GPS lock as that will then calibrate the

Thanks, Grant.