Image position below map

PIX4Dmapper PIX4Dmapper Questions/Troubleshooting
1

Hi,

Ive been using pix for years and all of a sudden the last few surveys have produced the following;

p4d
p4d976Ă—765 278 KB

Is this correct? 9 GCP (Aeropoints) but the images seem to be displaying lower then the surface of the model?

DJI M200 V2, X5S, PIX V4.5.3, flown at 60m using DJI Pilot App.

2

Hi @operations1,

Could you post here the Quality Report of your project so we can check the accuracy?

Thanks in advance!

3

report
report919Ă—11196 922 KB

Hi,

Hopefully this works.

4

Hi,
It seems to me that the geotagging of the height of the EXIF information in the image data retrieved from your drone is probably wrong.
I’ve had a similar experience before.

Maybe the value of “GPSAltitudeRef” in EXIF is set to “Below Sea Level”? (Even though the location you surveyed is actually “Above Sea Level.”)
There are two types of altitude that are recorded in the EXIF of the drone image: “GPS Altitude” and “Relative Altitude”.
“GPS Altitude” is an ellipsoid height measured by GPS and the accuracy is not very good.(not RTK)
“Relative Altitude” is the altitude against the ground with a barometric pressure sensor, and the accuracy seems to be better than “GPS Altitude”.

The “GPS Altitude” is not very accurate as a value, but the relative variation during the flight may be small (Somehow I just feel so. I don’t know how I can explain this, though.).

In Pix4D, the height value is referred to the “GPS Altitude”, so the resulting height will be out of sync.
Therefore, a workaround is to correct the EXIF information (height) directly with exiftool, etc.

The work flow is ( with exiftool, etc.):

  • Checking the altitude of the captured image.
  • GPS Altitude is changed to Relative Altitude.
  • Add an offset to the GPS Altitude (which contains the Relative Altitude value). It adds the elevation value (or ellipsoid height, depending on the use case) of the takeoff point to convert it to an elevation value (or ellipsoid height, depending on the use case).

Quick:

  • Change the height to what it should be, directly in the image properties in Pix4D.
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