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Wrong DSM and DTM elevation

VarvaraVedukhina

Hi! I am new in Drone2Map and need help with image processing. I have processed my images and got the wrong elevation in my DSM/DTM. I tried different coordinate settings. For example, the altitude of the processing images is about 98 meters and the value of the DTM is about -5 to -20 meters for these settings:

Another example: DTM elevation from -11 to -61 USft for these coordinate settings.

The rial terrain elevation is about 325-304 US ft or 99-92 meters.

I have checked the camera settings. It looks correct. The ground-controlled points are not applied. Please, help.

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VarvaraVedukhina

Hi, Cody, I don't have the opportunity to use the GCP, because it's a 4H project and we don't have the equipment like Propeller AeroPoints, or GPS station to get the coordinates of the GCP.

VarvaraVedukhina

I am not sure, but it looks like the information in my images contains the altitude above the ground. The orthometric height of the territory is about 96 meters, and 98 meters was in the altitude field of the image table, my drone flew at an altitude of 89 meters according to the waypoint. After adjustment with the parameters below, I've got an altitude of about 180 meters, and the DTM looks more correct: 3 meters different from the base map elevation. What do you think about this?

VarvaraVedukhina

Thanks, Cody! I think extracting points from an Esri image base map is a great solution to what I need. I got a vertical accuracy of about 3 meters, but I'm still not sure if it's not just a coincidence. But in any case, I think the GCP from the base map will give greater accuracy. If you can give me a recommendation on how to do this, I would appreciate it.

CodyBinkelheim

I'm glad you have usable results, but for future reference I recommend you always use ground control.

(Even those of you that have an RTK/PPK drone, ground control is not technically NEEDED, but we still recommend using measured points as Check points - withheld from the aerotriangulation - to verify accuracy).

If you don't have accurately surveyed points, you can extract points from the Esri imagery basemap, and that will include Z values from the ArcGIS Online world terrain to ensure you get the proper Z values. Two caveats with using control from the Esri basemaps:
Our basemaps are compiled from multiple sources, and we cannot provide an accuracy statement for the horizontal positions. The same is true for the DEMs used to compile the world terrain, so it's generally good everywhere but we do sometimes discover errors in the DEMs - sometimes very localized - so multiple points and careful QC are still recommended.

Cody B.

CodyBinkelheim

Varvara

for most drones, the Z values *SHOULD* be the orthometric height of the drone, the height above sea level. [BUT as noted above those values can be VERY wrong]

If RTK/PPK they are almost always ellipsoidal height. I have published a map at http://esriurl.com/Map4GroundElevation that can give you both orthometric and ellipsoidal heights if you are interested. (Must log in with ArcGIS Online account to get the Terrain values)

Almost no drones report height above ground (because they don't have a world DEM to know where the ground is). Internally they usually have data regarding height above launch point (so if the entire project is flat, that WOULD be height above ground) but that Z value is not written to the EXIF tags of the JPG images.

Our learn.arcgis.com site has some good training courses for Drone2Map. Since you're new to this, I'd advise you to review our free training.

Cody B.

VarvaraVedukhina

Okay! I think, that I solved the problem. I have adjusted the image altitude because it was above the ground. After adjustment, the elevation of the DTM looks better (corresponds to the default elevation model at list). This is what I needed.

CodyBinkelheim

Varvara

I agree with Mervyn. If it's not an RTK/PPK drone, the altitude values can be terrible. Until 5 months ago the worst I saw was an error of 100 meters, but I've seen a new "record" for bad values, more than 200 meters off. On the other hand, they can be very close to correct - and this can be true for ONE drone, from day to day. Without ground control points, I can almost guarantee your DSM and DTM heights will always be wrong.

You said "The ground-controlled points are not applied" but you didn't include any details. Since your project coordinate system is State Plane, I am guessing you might have used GCPs but they didn't work properly?

Pål_Herman_Sund also gave good advice regarding the vertical coordinate system - whether RTK/PPK or ground control points, check to see if they might be referenced to ellipsoidal height.

If this doesn't solve your problem, please contact support@esri.com - they are very good and should be able to help

Cody B

VarvaraVedukhina

One more question, the altitude that I have for done images is the altitude of dron above the ground?

VarvaraVedukhina

Thank you for your reply! I don't have an RTK drone or GCP, but how do I understand even if I use only a Drone consumer's GPS the error in vertical position can be about a couple of meters, not 100?

pal.herman.sund@geodata.no_Geodata

Hehe, maybe not terrible @MervynLotter , but:
If you have a drone without RTK or PPK GPS you will have to use the GCP's to arrive at "correct" heights, as the recorded XYZ are "far off" or more correct; with code-GPS precision and z possibly relative WGS84 ellipsoid.

If you do use RTK/PPK it might be a height-transformation you are missing - and/or the drone height reference is mismatching the settings you use in Drone2Map. For many height transformations you might need to install the "ArcGIS Coordinate Systems Data" which you will find in My Esri together with the Drone2Map installation package.

MervynLotter

Drones are terrible at recording elevation. Do use ground control points as they not only record X and Y, but also Z values.

See Ground control—ArcGIS Drone2Map Help | Documentation

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