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Roof Highs are wrong

Hello Support-Team,

i have a Problem with Roof hights. Some I have controlled with the Tachymeter.
I have differeneces on
Garages: Tachymeter: 168,31m, Pix4d 168,14m, Ground is about 165.85
Roof: Tachymeter 174,79, Pix4d 174,26, Ground is about 165,78
I have controlled 5 Roofes, all have the same Problems.
GCp are messured with Tachymeter and Nivellement.

On a other Project (Smaller, computed as 3d Modell, not as 3dMap) has the Problem not, there the high is exact to 3 cm

What is the Problem? if i compute the Tachymeterpoints on the Roof as GCP there is a error RMS Error 0.28m.

The Points on Ground like sewer covers are correct (+/-1-2cm)

What could be wrong?

Hello Benjamin,

This issue has already been solved in a personal support request, but we will summarize the solution here to allow other users benefit:

First of all, the project that had inaccurate results was processed with the option Keypoint Image Scale set to 1/4.Therefore, the number of keypoints extracted is much smaller than in the second run that was processed with original image scale. This leads to much fewer observations for the calibration and therefore poorer results. Of course, processing at 1/4 image scale is faster and, depending on the purpose of the project, this might be sufficient.

Also, the distribution of the GCPs both horizontally and vertically is very important to achieve accurate results. It is recommended to have some GCPs on different altitudes of the model to increase the accuracy. 

We would recommend you to add some more GCPs to the project considering their distribution and process again with original image scale for the keypoint extraction.

Best regards,