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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    • Thank you Jeff,

      is your utility exe  Ground Control Station / MS Windows run off-line / on-line , since it supports MAVLink, during the flight ?

      Does it access done camera's sd card during the flight ?

      Sorry for basic question but I would like to set more exif tags for the image, more tags from sensor data.

      I am just testing exif read library (js)

      Looking for exif write library (js)

      to work in off-line mode (batch processing) controlled via html page.

      I need to save GCS, RTK data as exif tags.

    • The utility is an exe which takes as inputs the .tlog from a pixhawk powered vehicle and exports a csv with image names corresponding to GPS tags (lat, lon, alt). It's a PC application which would be run after the flight only.

  • I am a survey pilot based in Sweden.
    I do very high resolution surveys for engineering applications and high grade survey applications, as well as ultra high resolution, georeferenced ortho photos.
    Without good GCP (Ground Controll Points) you will never get good results.
    The GPS on a mobile phone is useless.

    Using Google Earth / Google Maps to "generate" GCP's is useless, as the map data can vary a lot depending on location, and you will never knowwhat to trust and what not. I have seen differences of a few hundred meters in Greenland. (Google Earth X,Y,Z versus surveyed data)
    I have gotten results of 0.8mm internal accuracy, but in my standard mapping missions i get between 5mm and 15mm internal accuracy. By internal accuracy, I mean the accuracy between points in the model, relative to each other, and not Total accuracy measured against a total station, but my total accuracy is generally within 2.5cm if I only use my RTK GPS. If I use a total station to measure in my GCP's I will get a total accuracy of 7 to 17mm.

    Attached is a screen dump from a "standard" survey, flown at 40 meters height.

    GCP errors.jpg

    • Hi Cedric,

      The area we surveyed was very remote in Africa. Google earth image was very poor. 

      Impressive accuracies you got there. Have you done a comparison between survey by total station and by drone for engineering design.

      Can you say that in most cases you can replace total station and other ground instruments with drone survey? The issue for us is that right now ground surveys take too long due to various challenges of operating in Africa. If we can use drones for engineering survey then that will cut our time in the field significantly. 

      I am on quan.le@gmx.com for further discussion.

      Best. Quan

    • Hi there Quan, yes I did compare my drone data with a total station. As you can imagine, I did get a lot of raised eybrows when I claimed that my drone data was as accurate as it is. I contacted Leica Geosystems, and we did a comparison test with their MS50 total station, and laser scanner combination. Attached is the paper I wrote on the matter.

      To answer you question. Yes you can replace total station survey with drone survey in a lot of cases, but not all. Bare in mind that you are taking photos from the top, and that you can only measure hard surfaces with photogrammetry. If you cannot see it, you cannot measure it. If you need the ground surface in tall grass, a total station is your best way. 

      Another point to consider, is that your survey will only be as good as your ground controll points. If you want total station accuracy, you need a total station to measure in your GCP's...

      Today is my last day at Geosigma, and will be starting at Clinton on monday. You can get hold of me at cedric.pieterse@clinton.se for further questions.

      Aibotix accuracy test 150318 version 3.pdf

    • Many thanks Cedric just a final point before I move to the direct email, if the ground is a mixture of bare ground, glass of different heights, crops and some trees (typical of Africa arable land) the would the accuracy be OK?

      We will go through the paper but it looks good. 




    • Hi Quan,

      On the bare ground, you will get good data, but on the trees and grass, you will only get the surface..

      Water is obviously a no-no, and anything that moves or reflects.

    • Hi Cedric

      But any wild agriculture land will certainly include all type of vegetation. I guess through post processing/photogrammetry you can correct the distortion created by tall grass and trees?

    • Nope.

      You can only rely on the hard surface. To get reliable ground surface points in the vegetation areas, you are going to need a VERY good LIDAR or the tried and trusted method of physycally measuring with RTK/Total station.

      Drones are not the answer to everything, they can do a lot, but not all of it...

    • @Quan,

      you can get 1cm GPS accuracy and 1cm resolution with RTK GPS diff and corrections, modern GPS chips can be clocked 1Hz up to 50Hz.

      To establish an array of Ground Control Points for a large area ortho surveyed you don't need to spend $Ts.

      Mobile Control Point with your drone mount RTK GPS can offer you 1cm GPS accuracy and 1cm GPS resolution.

      But going back to your problem.

      Your imaginery can still be repaired since image overlap can be used to correct err in alt, step by step (basics of trigonometry) image by image.

      Since you are not interested to have your imaginery repaired, I really don't know how could I be of any help.

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