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Improve accuracy of surfaces' coordinates

Hi, I want to improve the accuracy of point’s coordinates (specially in the z axis).

Problem: When I compare the same location for two different flights, the difference in the z coordinate is ~ 0.4 - 1 m.

Context: I’m trying to calculate the cut/fill of earthworks.

My procedure: Using pix4d Capture, I do 2 missions at different times (details in the images).

Then, I open the project in Pix4d Mapper and for each mission, I produce a DTM (details at images):

Then, I export both .dxf files to Civil3D and generate surfaces with the contour levels. However, I’m getting differences of ~1m of elevation at the same point. I want the error to be ~10 cm.

How can I improve the accuraccy?

Thanks in advance. If you need more info, just tell me.



My advice is to use GCP’s or PPK/RTK capable drone. Thats the basically only two ways to get the desired accuracy. 

EDIT: or you can manually select GCP’s from your first project and use those in project 2. Choose objects/shapes that are easily identifiable in both projects