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DSM - DTM showing negative values

My DSM and DTM have about 11 cm of GSD. DTM = 5 x GSD (2.24 …).
To find plant height I do DSM-DTM but the resulting raster containing plants height has negative values in various places.
How can I correct this problem and increase the accuracy of pixel height values?
I have used the parameters below … Should I change them?

Hi,

Are you using any type of RTK/PPK drone? I am having some doubts about the accuracy of the calculation DSM-DTM, because you also need to account for the XYZ uncertainties, especially when processing images captured with consumer drones (the GPS accuracy can reach a few meters). One important replacement for RTK/PPK accuracy would be the homogeneous distribution of GCPs across the image dataset. This can increase the XYZ accuracy and provide more reliable values. Please take a look at the following article about the accuracy that users should expect in Pix4Dmapper: Accuracy of Pix4D outputs.

Please also note that the DSM and DTM are generated from the filtered point cloud. The filtered point cloud is generated from the densified point cloud that is generated based on the Automatic Tie Points. I would closely check the quality of the DSM and DTM, because problematic areas where the DSM and DTM have big triangles or are badly reconstructed are areas where the densified and filtered point cloud have bad, few or no information at all. These reconstruction problems can also affect the DSM-DTM calculation.

Cheers,
Teodora

Thank you Teodora,
I used a GNSS RTK to mark GCPs in the area of interest…
See the Quality Report attached, please. What’s wrong ?

CY_27032019_Multi_report.pdf (2.4 MB)

Dear Teodora and Support members,
Interestingly, according to field measurements, plants have an average height of 80 cm, but the Pix4D volume tool is also providing negative volume values …
What should I do to repair this? Should I remove the filters from DSM? Which are ?
I ask for help to you.
Is there any more accurate tool or software on this issue?

Hello again,
From the quality report, it seems that the DSM has a resolution of 2.25 cm/pixel, while the DTM has a resolution of 11 cm/pixel.

According to the rules of digital image processing, operations between images can only be performed if they have the same pixel size. The easiest way would be to resample the DSM to the spatial resolution of the DTM (11 cm) and then perform the computation.

Cheers,
Teodora

Ok Teodora,

I had already resampled the DSM pixel to 11 cm in ArcGIS and honestly I could not understand the reasons for the negative values in the DSM-DTM operation. I have been able to solve this issue by using plant height calculations with Global Mapper 20, but individually. I think we should wait for future adjustments to the DTM calculation in the next versions of Pix4D.

Thanks,

Walter

PS: If there is a solution for the case, please post it here in the community ok?

Hi Walter,

Did the DSM and DTM generated from Pix4Dmapper have correct elevation values? To be honest, this is the first time I hear about calculating the plant height using elevation or surface models generated from Pix4Dmapper. How did you measure them in Global Mapper?

Cheers,
Teodora

Hi Teodora,

GM has a distinct way of calculating the height of objects… It uses only the DSM and takes as reference the nearest soil surrounding the plants. It is quite accurate when compared to field measurements.

Cheers,

Walter

Hi,

The DSM and DTM generated from Pix4Dmapper should be correct, but measuring plant height is tricky because of the interpolation algorithms used by Pix4Dmapper. Also, don’t forget that there is a certain incertainty in the x, y, z directions. From a research perspective, I would rethink the strategy. However, I have no better suggestion at the moment. Hopefully, you’ll get feedback from some users who have similar research questions.
Cheers,
Teodora