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Surveying very large area and processing images


My name is Antriksh.

I’m running a small drone survey business and getting in to the survey industry. 

We are working for water management department on canal project. They are building a Dam in the area which will distribute water for irrigation purpose in surrounding villages through canals. The area of interest is 3400Ha or 34 Km square. For covering the area we used Phantom 4 drones and did ~102 flights all over the area. This area has elevation difference arond 50 meter. So i did flights from different areas of different elevation. 

My client need 0.5 meter contour map of the area.

I divided the compete area in to sub areas of 100Ha each. So total 34 sub parts. Each flight has following details-

75m height AGL

70% frontlap and 60% sidelap 

4000X3000 pixel images

I got around 1400 (7GB) images in a patch of 100Ha covered with single drone using ~3 batteries.

Now when i process these images i need to do it one by one sub part (100Ha area)

Here is the tricky part-

If keep contour base 0 in each map than because i flew from varying elevation the contour line will not match while merging all contour maps.

My client want the Dam base as 0 and need 0.5 meter elevation contour line respective to that. How it can be done?

I observed DTM doesn’t remove human build structure like houses, which create noise in the contour map. How can i get rid of that? Mostly of the survey area is crop field having small villages.


HI Antriksh,

Your project sounds very interesting.  Due to the large size of your project (so many flights and sub projects), I believe you’ll need to establish Ground Control Points (GCPs) across your area of interest.  There are a number of support articles on the Pix4D site that can help, a great start is Using GCPs


Hi Antriksh, did you solve your problem? I would like to know how you managed all of your data! Its a great project!


About your DTM for contourn lines, you should edit your points cloud and classiffy those points you dont want in your DTM under DISABLED. The run again process 3. That way you will have a better DTM and contourn lines just from you ground and road surfaces.

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Hi Victor,

I am mapping 3000 Ha at the moment and broke the whole area into sub-area with 45Ha each.

With so many sub-projects, i can say that almost all of them them consist of un-calibrated images. 

Is there a best practice when it comes to mapping large area (on the ground & on PixDesktop?) 

Hi Chin,

Could you upload the Quality Report of your project to our OneDrive so we can investigate the problem?


HI Antriksh,

   Did you all ever find out the best workflow for your project? I too have mapped a very large area and had to do sub-projects. I also had differences in starting altitudes so would like to know the best method of establishing the contour baseline. Regarding GCPs. I captured points using a Trimble Geo XH unit and used WGS84 (Lat/Lon) for my horizontal and HAE (Height Above Ellipsoid WGS84) for vertical. I did not see the Geoid options offered in Pix4d in the Trimble unit so thought of capturing HAE and then converting using Pix4d. I usually set images coordinates to defaults. I set the GCP coordinates to wgs84 and  Height Above Ellipsoid at 0 meters. Finally,the output is selected at wgs84 (UTM zone 10n),Geoid egm96. Does this set-up look okay to make sure datums for both images, gcps, and outputs are aligned? Thank you for any help.




I have not done a project with that many flights - however I have done projects with our eBee that have 4 or 5 flights that I need to merge.  What i have done in the past is complete the initial processing for each sub project - then I add the GCPs available for each sub project and then reoptimize them.  Your flying altitude should not matter if you are using GCPs or MTPs to tell the software matching points.

You should then be able to then merge each of the sub projects together - you may need to define some MTPs to help the software match along the perimeter of each sub flight. 

Now I would think if you wanted all the elevations related to 0 - the top of the dam - I would determine the true elevation of the top of the dam - then adjust each GCPs elevation according to the difference and set the vertical coordinate system to arbitrary.  IE if 375=0, then all GCP elevations are reduced by 375

“I observed DTM doesn’t remove human build structure like houses, which create noise in the contour map. How can i get rid of that? Mostly of the survey area is crop field having small villages.” - use the Pix4d clipping tool to remove your noise from the point cloud- then create a polyface surface over that area and enable it to be used in the DTM in the properties menu