Control strip at lower altitude results in 10m shift

[mks_gis]

Hi,

I was playing around with our Mavic 3M to test some general workflows. DJI has the option to fly a set of oblique images at the end of a mission to improve accuracy, so I thought I'd check what impact this has. In addition, I thought it'd be a good idea to fly a nadir control strip at different altitude, to test what effect that has on a model. I was flying with a base and PPKd the triggers using REDToolbox. I also have GCP which I only use as check points.
The main model has 243 images at 60m AGL. There are 12 oblique images at 60m AGL and 8 images in the control strip at 50m AGL. I ran 4 models, all with the same setup, reference settings and markers:

• Main images only
• Main + oblique + control strip
• Main + oblique
• Main + control strip

I use a python script to run each model, so the process is exactly the same. (https://www.agisoft.com/forum/index.php?topic=9578.msg44253)
When I process the model with the main images and the control strip the whole DEM is shifted down by 10m, and I cannot work out why. I have set the capture distances for each image set (right-click on selected images). Why do these 8 images have such a disproportionate effect on the overall model? Am I missing any other setting to accommodate the lower flying height images? The horizontal accuracy is also a bit worse, but only 0.06 mean compared to 0.02 mean against check points. The model with main+oblique+control strip does not have this issue.
Further questions that may be interesting:

• When you set the capture distance for individual images (right-click) is there a way of checking it has actually set it, because if it is repeated the default value is shown again?
• When we have individual accuracy values for every image from the PPK/RTK process, do we still need to set the camera accuracy in the Reference Settings, i.e. does the per image setting override the Reference Setting value?
• The error (m) given is an amalgamation of x/y/z, is there a way in Metashape to see the error per dimension?
• Adding a control strip at a different altitude is sometimes mentioned here as a method to improve a model, but I cannot find an academic reference or technical article about it. If anyone knows of one please post it here.

Thanks

[Paulo]

Hi Gis,

the only that occurs to me is the accuracy of your different altitud control strip is way off the other images accuracy?

In adjustment are all images enabled?

[Dieter]

• Adding a control strip at a different altitude is sometimes mentioned here as a method to improve a model, but I cannot find an academic reference or technical article about it. If anyone knows of one please post it here.

Thanks

Try this:

https://www.researchgate.net/publication/350556530_Photogrammetry_using_UAV-mounted_GNSS_RTK_Georeferencing_strategies_without_GCPs


I think you have an error in your input data, 10 meters as an error is very unusual.


Dieter

[mks_gis]

I think you have an error in your input data, 10 meters as an error is very unusual.

Dieter

Thanks for the link, that paper and the ones referenced were very interesting. I may have found an error in the PPK of the control strip triggers, although not 10m.

[JMR]

2. Per image accuracy override the general setting.
3. The error (m) given is an amalgamation of x/y/z, is there a way in Metashape to see the error per dimension?
errors per dimension are shown in the ref pane when you are in errors view mode.
There could be something wrong either in the processing method or in your source references. Otherwise it is hard to explain 10m

[mks_gis]

errors per dimension are shown in the ref pane when you are in errors view mode.

Well, after all these years of using MS I'd forgotten about that little button, D!ho

It seems there was an error in the PPK that resulted in a z error of 2m. I've reprocessed the trigger coordinates and the model is much improved.