Bad Hslm (mean sea level) dsm model

PIX4Dmapper PIX4Dmapper Questions/Troubleshooting
1

hi…

i have created a lot of dsm but all of them has the same problem…

The hslm is wrong… the dsm has only negative hslm value…

 

May you help me ?

2

Hello Marco, 

We are not sure we completely understood the problem? Do you have image geolocation? Is your project georeferenced? 

Are you referring to the Mean Sea Level that has negative values?

Best Regards, 

 

3

hi…

i use a phantom 3 adv and all image are georeferenced.

i’m referring to the mean sea level that has negative values…

4

Hey Marco,

Since you are using Phantom 3 Advanced, the images that are saved on the drone’s SD card are geotagged by DJI. Regarding the Lat./Log. coordinates, the drone’s GPS saves reliable information in the image EXIF. There is no problem from that side. However regarding the altitude there might be some inaccuracies depending on the location when you are mapping.

DJI now measures the absolute elevation above sea level using EGM 96 as the reference. It used to record the elevation above ground level but this is no longer the case since the firmware released in March 2016. However the vertical coordinate is still not fully reliable. Indeed, we made testing in our office here and we found that the vertical coordinate is off by several meters that can reach an error of 100 meters. Some users noticed the same. Note that this is just an offset meaning that the within the model, the accuracy is not affected, only the absolute location. DJI released more recent drone firmware, but as far as we know, the issue remains. Therefore our developers suspect something wrong in the DJI EXIF regarding the vertical coordinates. As a consequence, we always recommend to process with ground control points (GCPs) in order to fix these uncertainties.

About using GCPs: https://support.pix4d.com/hc/en-us/articles/202558699 If you do have GCPs, you could get for instance 5 points from a Web Map Service server over your area (e.g. Daft Logic): https://support.pix4d.com/hc/en-us/articles/202560149

  1. Make sure to define the appropriate GCP coordinate system. The Daft Logic website takes the coordinates from Google Maps which refers to the mean sea level (MSL) egm96 to estimate the vertical coordinate.

  2. On the menu bar, click Project > GCP/MTP Manager… > GCP Coordinate System > Edit… > choose WGS 84.

  3. Then click Advanced Coordinate System > select MSL egm96.

  4. Then insert/import the points according: https://support.pix4d.com/hc/en-us/articles/202560349

  5. In the field Type make sure to set Ground Control Point, and manually enter the coordinates you took from the website.

  6. Then on the menu bar, click Process > Reoptimize.

Best Regards, 

5

to do this have i to use the pro version ?

6

Hi Marco,

This is fully included in our Discovery edition. The only limitation is the processing possibilities and resulting outputs. Basically, it is only possible to obtain a quality report and produce 3D video animations from the rayCloud scene.

For more information, please see our comparison table: https://support.pix4d.com/hc/en-us/articles/204162839

Regards,

7

Hi, I am using ebee X and processed images using Pix4Dmapper Pro and my recently generated DSM came out with all negative values, and didnt use  any GCP. But with Postflight Terra 3D I got DSM with farly accurate values (at least all positive) in earlier missions. So what is happening here. Thanks

8

Hi Chandima,

How good are your images coordinates? Please send us a screenshot of your images coordinates:

Do you fly nearby the sea level?

 

 

9

Hi Christina,

Sorry for the late response. Flew over inland area and according to SRTM DEM, elevation should vary from 10m-20m approx… Note that I haven’t input any GCPs during the processing.

 

During the last mission drone failed to maintain its altitude and subsequently  lost the radio link and fall down and broke its antenna. Would like to know why this happened and get an quotation to repair.

Whom should I contact in this regard?

 

Thanks

Chandima

10

Hello Chandima,

By looking at your altitudes, it is very likely that you get negative values in the DSM. 

As you say “you flew over an inland area and according to SRTM DEM, elevation should vary from 10m-20m approx”.

That means the altitude over the terrain is around 10-20 m. Is that so? what is the altitude over the terrain you flew?

As for the repair, please contact SenseFly directly.

Thank you very much.

 

11

Hi Daniel,

Flying altitude was 106m approx. 

Thanks

12

Thank you for your answer.

Your flying altitude over the terrain was 106 m approximately but as you see in your image properties editor screenshot, the altitude that your drone registered was around 25 m which means 75 m below.

I suppose you are getting negative altitudes of around 50-60 m approximately which is 75 m below your terrain. Is that so?

You should correct the altitudes before computing Step1 and check why your drone captured wrong altitudes.

Thank you very much.

 

13

I have solved the issue, emotion new version generated geoinfo.txt contains 2 altitude columns whereas in previous case it was only altitude_wgs84.

So I have to open this file when importing images to the project and that will get the correct drone altitude also gives positive DSM.

Thanks for your help. And one more thing my sensefly account (https://my.sensefly.com/login.html) is not working with the given credential and it redirect to https://my.parrot-business.com/.   I want to send my drone logs to see what happen during the flight because it fall down due to altitude control malfunction and broke antenna.

 

14

Hello Chandima,

Thank you for the feedback.

As for your SenseFly account, please contact SenseFly directly.

Regards.