We (www.gmxconsulting.co.uk) are developing a large-scale agriculture project in Nigeria (5,000ha) and started to use drones to survey and map topography.

The intention was then to use the Digital Elevation Model for irrigation planning and design. However, when we post processed the images we got very large x y z errors - please see the photos below.

1. Could anyone tell us where we have done wrong? Can we correct NOW (after the aerial survey)?

2. Also we look to outsource the drone surveying and post processing for our projects in Africa - Anyone interested please let us know.

Here are some more details:

Platform: E384

AGL: 120 - 180m 

Overlap: Front 60% Side 60%

GCPs: No

Flight time: <60 mins

Cameras: Canon S110 RGB and NGB

Number of photos: 300-400

Photo taken out before processing: None

Software: Photoscan

Many thanks in advance!




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    • Hi Ian,

      Many thanks for your comments. Thought I covered some of the points you raised in my previous response. I am on quan.le@gmx.com and we are looking for partners to outsource the flying and processing part if possible. Best. Quan

  • Hi Quan Le

    Like many people have mentioned, work in Africa can be exciting.

    Also, like many people have mentioned Ground Control Points are necessary for accurate results.  A recent project we complete was to generate a flood line assessment for a mine in South Africa.  Through GCP's we were able to improve our results down to 1.5cm accuracy (x,y & z).

    Saying this, the density of the vegetation on the ground will also influence the accuracy of your results, as you mentioned you need to plan for irrigation.  At the end of the day the combination between using a UAS/RPAS and an accurate survey grade GPS will give you the results you want.

    Give me a shout if you have anymore questions.



    • Hi Gareth

      Looks like GCPs are the way to go. As mentioned in one of my responses, we couldn't exactly established 'where we are in the world'. We were working in a remote location so we had to make do with some estimated datum.

      My question for you is that, given the accuracy that you get can you actually replace traditional survey (e.g. with total stations) with drone topo surveys? In that case could you produce 1:500 map accurately?  

      Right now it takes 1 surveyor in our team 1 month to cover about 1000ha to produce a lower resolution map. So if drones can replace this cost-effectively the then our irrigation planning and design work in Africa will be transformed.

      In that case we of course still need someone on the ground 'to have a feel for the place' and do some geological survey. But our field work would be cut short and time instead be spent in the office to do the design.

      Please do send some further info to quan.le@gmx.com. Thanks

  • Hi Quan Le,

    We are a Survey Company located at Paarl in South Africa and have been using UAV's for the last 3 years to do Aerial Topographical Surveys throughout Southern Africa. It has been an expensive "learning curve" through which we had to go to iron out mistakes and get a working "Work Flow" in place.

    At this stage GCP are necessary to have an accurate survey of the area that has been flown, but we are looking at possible solutions where GCP are not necessary. The method entails having a Base Station on the ground and then a Rover on your Plane.

    You could correct the Survey by visiting the area again and fixing (X, Y & Z) features over the area that are visible on your Photos.

    We would be very interested in doing the Surveys for you and we can be contacted at info@aerialpix.co.za.



    co.za Domain Homepage
    The co.za Domain Homepage - contains pertinent and detailed information specific to South African domain registrations
    • Dear Michael

      That is exactly what we are planning to do. We still have a team on the ground. Instead of re-flying we will take the xyz at visible landmarks to see if the accuracy improves. We still have a lot of area to cover so in future surveys we will use GCPs.

      We are agriculture developer and manager, not an UAV survey company. Our interests in UAVs are in the end-user applications - produce topographic map for irrigation planning; taking stock of the farm/plantations; count trees and animals; monitor crop growth (e.g nutrient or water deficiencies) and the specific management actions to be taken from these.

      We will retain the interpretation of data, growing prescriptions in house and look to outsource the data acquisition (flying) and post- processing when we can. 

      Right now UAV uses will need to make our work cheaper and faster - that is the key consideration.

      For UAV surveys the key outputs we need for irrigation planning and design are:

      - contour map with x y z at scale of 1:500

      - orthophoto

      As we are designing, developing and managing agriculture projects in Africa we would like to partner with UAV surveyor, companies or individuals. Our projects are generally >1000 ha. 

      Please could you send a proposal and some example of your work to quan.le@gmx.com ?

      Thank you.

  • Thank you Quan,

    pls email me directly.

  • Thank you Quan,

    pls email me directly.

  • Hi Quan Le,

    First of all, it looks like worse precision areas are located in the border of your mission region, probably where the plane turns and where you are taking (and afterwards processing) no nadir images (ie, not zenital, with large camera location error). In those cases you can exclude these images from your analysis in Photoscan. That should improve overall precision of your models.

    If that region is important for you, you must extend your missión farther next time to keep that area in level flight (ie, make the turns outside there). You can also increase image overlapping to 70 or 80% in at least one side. If your camera is fast enought that should not increase your flight time very much (you can also flight slightly slowly, if possible).

    Another way to correct that is using some GCPs at easily recognized items of your images. Even with a conventional GPS you will improve your models (at least, its geographical position).

    Hope that helps you.


    • Thank you Eladio for your advice. We thought lacking GCPs could be the issue. As this was a remote area we didn't have a recognised datum so could never quite confirm the exact geographical position reference point. We will next time use DGPS first to work out "where we are in the world" then put down the GCPs as recommended. We will also try to delete the photos around the edges. All the images looked quite sharp though.

      We were reluctant to increase the overlapping because of flight time constraints. We did see that people use 80% for both. That would increase the processing time significantly though.

      When you process the image with Photoscan, does quality option 'medium' or 'high' affect precision? Again we were limited by computing power - just a Quadcore i7 16GB Ram laptop -  so couldn't go for the 'high' option. Any suggestion on building a cost-effective high-power processing hardware would be appreciated. Or can we lease computing power?

    • Hi Quan,

      I really love business opportunities and 

      like your

      Drone topographical survey for agriculture in Africa


      since as Climate Change Consultant with UNFCCC I join projects by UN agencies,

      World Bank, UN Foundation and other public agencies.

      I need to study a number of your images, geolocated as Google Earth/ Maps overlay first.

      I need geolocations for your regions to study with available Google Map, Earth, OpenMap mapping systems and more.

      Could you provide me with technical specification of your drones to let me check if they are fit to accomplish your task.

      Let me know your deadlines now and size of your imaginery database in MB, GB

      and if you can upload images over the Internet if necessary.

      I am sure, we can get a lot of invaluable materials from IRENA in support of your irrigation planning and design. 

      pls email me




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