DTM altitude values are less by 17.5 meters from a known survey points

PIX4Dmapper PIX4Dmapper Questions/Troubleshooting
1

Hello!

After checking generated DTM points and contours over my known survey points, I found a difference by 17.5 meters in elevation for ALL checked points which are well spread out within the study area. The entire DTM point fabric over 2 km2 has the same difference in elevation (less by 17.5m than reality).

DSM points and contours generated from DSM look good (reflect the reality of elevation) for same open areas without trees or man-made objects.

I don’t have a Pix4D processing window screenshots, but I used generic options such as DSM and DTM both x5 GSD, Contours 0.5m with resolution 1m and min line 10 vertices.

There is a screenshot from ArcMap window showing DTM contours and known survey points (red dots):

I would like to hear if anybody has experienced the same problem or can tell me what could cause that error which looks like an algorithm error by the same value.

2

Hi Denis,

Thank you for sharing a screenshot of your results.

My understanding is that you found that:

  1. There is a consistent 18.5-meter error in your DTM’s Z-axis across your entire project area, but that you do not see this same error in your DSM.
  2. There is a consistent 18.5-meter error in the Z-axis of the contour lines that were derived from your DTM across your entire project area, but that you do not see this error in the contour lines that were derived from your DSM.

Please let me know if I misunderstood something.

The most common reason that there is a constant shift in Z-axis across the entire project area is that a geoid model is applied to the reconstruction when it shouldn’t be or third-party software fail to account for a geoid model. With that in mind, please ensure that the correct Vertical Coordinate System is applied to your project’s Ground Control Point Coordinate System and Output Coordinate System.

You can learn more about managing your project’s Ground Control Point Coordinate System and Output Coordinate System at How to select /change the Image / GCP / Output Coordinate System.

If you make any changes to your project’s coordinate systems, please ensure that you Reoptimize prior to regenerating your results. You can learn more about Reoptimize at Menu Process > Reoptimize.

3

Hi Andrew,

thank you for getting back to me!

I actually wrote a request (#57074) to your support team with more details about my challenge and got some feedback from Sylvain.

Please see the link to our correspondence here: https://support.pix4d.com/hc/en-us/articles/hc/en-us/requests/57074?flash_digest=1a5254d1a55ffb394c73ce8bb28e7a0280c05f6a&flash_digest=eb46f7377282c92779af4c4bf3f1e65204794945 

I would appreciate if you also take a look at the problem from your perspective, as I still don’t understand what should be the process in my case.

The video tutorial that Sylvain recommended did not explain how do I find the correction number to input for Geoid Height Above the ellipsoid section? In tutorial, the user said that he knows that MSL egm96 should be checked to correct the difference between datums (ellipsoids). I am wondering if there is any information available about those correction numbers or is it always depends on the particular spatial location of the study area?   

I am attaching the screenshot of GCP coordinate system window showing the number I have figured out using my known survey points but what if I don’t have any known reference points? 

Thank you in advance!

 

 

4

Unfortunately, I still have a question on how to figure out or measure the Geoid Height Above the ellipsoid if using different coordinate systems for imagery and GCP?

5

Hi Denis,

We worked this case together through our ticketing system, I comment also here to give updates for our Community users:-)

The DSM had the same error as the DTM.

The problem gets solved when applying the correction number for Geoid Height Above the Ellipsoid. Knowing this correction number (offset) can be challenging:  it requires either having GCPs (survey points) or some surveying knowledge. 

6

In the US,  I use this website to determine my Geoid Height Above Ellipsoid value.  I can also get this value from my GPS data collector but the process is less straight forward than using the website. You will have to determine if it will serve your needs in Canada or find a suitable alternative source for the geoid value.

https://www.ngs.noaa.gov/cgi-bin/GEOID_STUFF/geoid12B_prompt1.prl