**WebODM using wrong altitude field for DJI drones → incorrect GSD calculation**

EvilThug

Hey team, I'm seeing a GSD discrepancy with DJI Mavic 3 Pro imagery and wanted to confirm if this is expected behavior.

The Issue:

  • Mission flown at 80m altitude (confirmed in mission KMZ: executeHeight=80)
  • DJI EXIF contains two altitude fields:
  • drone-dji:RelativeAltitude = +79.900 (correct - height above ground)
  • drone-dji:AbsoluteAltitude = +63.154 (GPS MSL altitude)
  • GPSAltitude = 63.154 (same as Absolute)

WebODM's Output:

  • Reported GSD: 5.5 cm/px
  • This matches calculation using AbsoluteAltitude (63m)

Expected Output:

  • GSD should be 2.18 cm/px (using RelativeAltitude of 80m)
  • Formula: (17.3mm / 12mm) × 80m / 5280px × 100 = 2.18 cm/px

Question:
Does WebODM prioritize GPSAltitude over DJI's RelativeAltitude field? For drone photogrammetry, RelativeAltitude (AGL) is the correct value since missions are flown at constant height above terrain, not constant MSL altitude.

Sample EXIF from affected image:

drone-dji:AbsoluteAltitude="+63.154"
drone-dji:RelativeAltitude="+79.900"
drone-dji:AltitudeType="GpsFusionAlt"

Thanks for any insight! Let me know if you need sample images to test.

Grand-Bahamian-Way-4-27-2026-report.pdf — view on Discord

14 Replies

DowAJS

Did you specify your GSD before running WebODM? If not, I personally find setting --orthophoto-resolution to your expected GSD (2.18 in your case), fixes this for me.

EvilThug OP
Screenshot posted by EvilThug
EvilThug OP

Where would I change that?

EvilThug OP

So it is not auto-detected?

DowAJS

It's in the custom options, same place as you find rolling-shutter, fast-orthophoto etc. (as a side note I highly advise going over the options to get best results for your specific use case, find them here https://share.google/B2h1RCQWQ7tT6QpD9 )

What catches me though is that your average GSD is 2.18, but in inches rather than centimetres, which results in around 5.5cm. Not sure why it's displaying in inches rather than cm, but it looks like your issue could be related. @pierotofy might be able to give more insight here

EvilThug OP

oh wait

EvilThug OP

lol

DowAJS

Found your solution?

EvilThug OP

2.1in is 5.5 cm

EvilThug OP

lol

EvilThug OP

im a dummy

EvilThug OP

I thought the report was in Inches

EvilThug OP

since that is the settings I have for Webodm

DowAJS Best answer

May be worth switching over to keep everything metric if that's what your drone image metadata is in, but hopefully you're all sorted now!

👍 2

Need more help?

This discussion happened in the WebODM community Discord. Join and ask your own question.

Join View Original Thread

Related questions

This page mirrors a public discussion from the WebODM Discord. How to request a removal.