we implemented radar/laser altimeter support for DJI M210/M210V2 drones (in addition to previously supported M600/M600Pro/A3).
Set consists of UgCS SkyHub onboard computer, radar or laser altimeter, set of cables and software.
In general, we recommend using a radar altimeter (finally we selected http://en.nanoradar.cn/Article/detail/id/372.html).
After 2 years of practice and feedback from our customers, we decided that laser altimeters have too many restrictions - they don't work reliable under the bright sun, light fog, over water.
Internally the system is pretty complex (diagram for A3 is below, for M210 is pretty the same), but it works :)
Full description and documentation here - https://industrial.ugcs.com/true-terrain-following