I was really hoping someone could guide me through how to measure tree heights from a forest plantation that I’ve mapped. DD support has suggested using a point cloud (.las export) for this but I get similar readings to the built in DD annotations. From what I understand, the height that is reported on the map is related to the height of the take-off point of the drone. So, for instance, if the drone takes-off at 0m and the terrain slopes downward by 10m with a 15m high tree, the map will report the tree as 25m high. Do I have that correct at least?
The problem I am having is that I can’t get information on how high the actual tree is from the ground. The height of the tree from the point of take-off doesn’t help me much as I don’t know how far down the ground sloped to that measuring point. I don’t believe GCPs will help either as it’s not the accuracy I’m fussed about but just getting the actual tree heights from the ground they’re planted in.
I’ve also tried the point cloud and it gives me similar z-axis results as the DD app. Does anyone have experience that could help a complete noob out as to how I can measure the average heights of an area of mapped trees?
If the drone starts out at 0m then the land slopes down 10m you are at -10m. A 15m tree at this location would register 5m.
If you use GCP’s your elevations will be tied to global elevation as opposed to (local) takeoff elevation. So you would have feet or meters above sea level (depending on spatial reference used)
Lastly, imagery can only get you so accurate when calculating tree height in dense forests because imagery alone has a hard time penetrating the tree canopy to actually get the ground below…at least in “leaf-on” conditions. A better tool would be a more expensive option such as Lidar, which can penetrate tree canopy…giving you elevation for both tree canopy and ground below.
give the line tool a go - it will actually draw a cross section over the tree canopy and you may be able to view the tree height relative to the ground.
use the point tool a lot of times, measure the top of the trees, and compare to a ground point somewhere near by.
Both ways you’ll unfortunately need to do the subtraction.
I gave up with DD. They say it’s only me who have ever asked for tree/crop heights (absolute) and that no one has ever had issues with using the heights at take off. It’s very weird that this is not built in as estimated ground elevation data is freely available via Google APIs (if you want it more accurate use GCPs). I’m extremely dissapointed as no one really wants to help at DD. What’s more practical than clicking on a tree/object and seeing it’s absolute height? Clicking in the ground next to it is not an option as with a forest the canopy is so dense that you can’t click in the ground.
Would love to use a point cloud to do this but don’t know what to do to get the absolute height. With the point cloud I can measure height for the trees where I can also see the ground. But those I can’t? Can’t be that difficult including the ground elevation.
@SoulR3aper
Have you looked at some of the Lidar drone units? In theory the Lidar is “supposed” to be able to penetrate a thick canopy. This doesn’t help you with DD; however, it may be something to look in to.