Descrepency between planned height and actual height
I planned a mapping flight with an elevation of 150ft from the takeoff point.
After getting my images back and trying to import them into ArcGis, they were only ~23m off the ground according to the metadata associated with the images, roughly half of the planned height.
Does anyone know the possible reasons of why this might have happened?
Comments
6 comments
Were you planning in ATL (above takeoff location) or AGL (above ground level)?
ATL
If you are intending to use AGL you need to select that as the altitude system.
Because that is the system arcgis is using?
I don't know which part of acrgis you are using, but I know it has the ability to report AGL altitudes because it is the same underlying system we are using to enable AGL in Dronelink. The critical piece is understanding that the drone uses a barometric altimeter, which means all altitudes are measured above takeoff location by default.
I've had a bit of practice at dealing with DJI drones for mapping. One of the tricks is they do relative heights really well. But their absolute height information is really crap. (apart from the RTK ones.. and even then its not that flash)
In practice this means if you are standing at say 60feet above sea level then ask it to fly at 100feet above take off level they do that just fine. But.. the numbers they record for the take off height may be nothing like 60feet. It might be 5 feet or 200 feet that it decided to record as at the takeoff level, (or anything in between. I just had a flight on the weekend. that started at 300 feet below sea level according the the drone.
This means that your photos come in with OKish XY coordinates in the exiif data, but total crap for the Z ones. My bet is in your example your drone flew at the correct height, but stamped the photos with the wrong one because it guessed the wrong height at the start.
If you need real world Z values, the way to fix this is with ground control points. If you only need it to be roughly correct you could try retrofitting ground control from objects you can see in existing data you have (eg google earth) then retrofitting it, but much better is if you can get some survey accurate positioning of your own GCPs.
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