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Altitude

Hi There,

 

I have run a grid of approx 300x200 on a vacant site at an altitude of 50m.

 

When I import this to process the altitude is defined at approx 85m. Am I computing something incorrectly?

Hi Jarrod,

The images that are saved on the drone’s SD card are geotagged by DJI. Regarding the Lat./Log. coordinates, the drone’s GPS saves reliable information in the image EXIF. There is no problem from that side.

However regarding the altitude there might be some inaccuracies depending on the location when you are mapping. DJI now measures the absolute elevation above sea level using EGM 96 as the reference. It used to record the elevation above ground level but this is no longer the case since the firmware released in March 2016. However the vertical coordinate is still not fully reliable. Indeed, we made testing in our office here and we found that the vertical coordinate is off by several meters that can reach an error of 100 meters. Some users noticed the same.

Note that this is just an offset meaning that the within the model, the accuracy is not affected, only the absolute location. DJI released more recent drone firmware, but as far as we know, the issue remains. Therefore our developers suspect something wrong in the DJI EXIF regarding the vertical coordinates. As a consequence, we always recommend to process with ground control points (GCPs) in order to fix these uncertainties. About using GCPs: https://support.pix4d.com/hc/en-us/articles/202558699

If you do have GCPs, you could get for instance 5 points from a Web Map Service server over your area (e.g. Daft Logic):
https://support.pix4d.com/hc/en-us/articles/202560149

  1. Make sure to define the appropriate GCP coordinate system. The Daft Logic website takes the coordinates from Google Maps which refers to the mean sea level (MSL) egm96 to estimate the vertical coordinate.
  2. On the menu bar, click Project > GCP/MTP Manager… > GCP Coordinate System > Edit… > choose WGS 84.
  3. Then click Advanced Coordinate System > select MSL egm96.
  4. Then insert/import the points according: https://support.pix4d.com/hc/en-us/articles/202560349
  5. In the field Type make sure to set Ground Control Point, and manually enter the coordinates you took from the website.
  6. Then on the menu bar, click Process > Reoptimize.

Hope this will help :).

Regards,

i am using phantom 4 rtk , when i use just a check point my rmse value almost 1 meter … what should i do to improve my accuracy of data … below is my result when i use just a chekpoint no gcp 

 

@Nurul,

Do you by any chance also see big differences between the initial and optimized camera parameters in the Camera Optimization and Internal Camera Parameters sections of the quality report?

Are you using vertical datums that are different than egm84, egm96, egm2008 geoids or the ellipsoid of the selected horizontal system, e.g.  WGS 84 ellipsoid?

 this is my summary for this project summary … i think the datum use in height fitting in gps that we take the gcp value quit difference from the pix4dmapper… i will check the datum .

@ Nurul,

have you used DRTK-2 paired with Phantom 4 RTK?