How to use ground control points for accurate drone mapping

Drones seem to have the potential to revolutionize numerous areas of our lives, but today their biggest disruption is in the field of mapping. These small flying machines have made mapping any area or an object a matter of few minutes or hours, instead of the typical days or weeks it used to take before. In addition, this mapping equipment is accessible to even ‘non-experts’ because using a simple hobby drone like the DJI Phantom or DJI Mavic Pro will let you achieve survey-grade accuracy in a photogrammetric map or 3D model.

But what exactly is this ‘accuracy’? How do you define it, measure it, and verify it? And finally, what is the accuracy you actually need and how do you achieve it using UAVs? All these questions are really important if an aerial mapping business is your aim. In this industry, accuracy is the name of the game. And if you cannot define it and deliver it, you would end up offering your clients nothing more than a pretty picture from above.

For those new to UAV mapping operations or traditional airborne data collection, the idea of establishing Ground Control Points (GCPs) in a study area may be a new concept.  Let’s start with a definition: Ground Control Points are identifiable points on the surface of the earth with known locations, which are used to georeference the data being captured.  Clear as mud?  Okay, let’s break this down a bit further.  I’ve organized this post into a number of different questions that came to mind when I first started to work with GCPs on project sites.

Accuracy VS. Pixel size ?

One of the most important parameters of your survey is the spatial resolution, which in photogrammetry terms is described as GSD (Ground Sampling Distance). In literature, it is defined as the distance between two consecutive pixel centers measured on the ground. In practice, it is simply the size of the pixel in the field.

The GSD depends on the parameters of the camera (mainly camera resolution and focal length) and the flight altitude. For example, to achieve 1 cm (3 inches) pixel size using a hobby drone like the DJI Mavic Pro, you will need to fly at an altitude of 25 m. Using a professional device like PrecisionHawk’s Lancaster 5 will let you achieve 1 cm GSD at 60m.

However, if you manage to complete your flight with a pixel size of 1 cm2 (0.16 in2), it doesn’t mean that the accuracy of your overall survey (or to be more exact, the output of your survey: Orthophotomap, Digital Surface Model, or 3D point cloud) is also 1 cm.

In photogrammetry, accuracy is always relative to positional accuracy, which is defined as the degree to which the information on the map created from the data captured matches the actual real world.

What influences accuracy ?

The degree to which the absolute accuracy approaches the relative accuracy is determined by the overall quality of the photogrammetric process and the accuracy of the Ground Control Points.

The absolute accuracy of your survey can not be higher than the GCPs’ accuracy. Therefore, it is important to make sure the points are measured with an accuracy higher than the pixel size (e.g. if your pixel size is 1 cm, the GCPs should be optimally measured with sub-centimeter accuracy).

 

The absolute accuracy will also significantly depend on the relative accuracy of your model. When you stitch together hundreds or thousands of images taken with a small (and most often non-metric) drone camera, it is almost impossible to have each pixel on the map located exactly where it should be.

Additionally, there are a lot of factors that will influence the overall quality and accuracy of your survey. We will soon publish a dedicated post about these factors, but they include parameters like the terrain profile, drone hardware choice, image overlap, weather conditions, stability and the speed of the flight, and GPS conditions, among other things

What does a Ground Control Point (GCP) look like on site?

A GCP may take the form of a man-made target, or it may just be recorded on a pre-existing object in your study area. Whatever it is, it is important that it will be visible in your photos after you fly the drone mission. Figure 1 gives some different examples of how GCPs may look like in the field.  Details on how to set up targets are provided later in this post.  You can order air photo targets online, or make them yourself.  If you are going to make them yourself I would recommend using a material that can be folded and easily transported, such as pool liner, which can also be painted with your preferred markings.  Make sure grommets are installed in the corners to have a method for spiking them into the ground. 

 

 

Why should we establish ground control?

The main reason we want to set up ground control is to be able to accurately georectify the UAV imagery that we are stitching together. In other words, this process allows us to stretch and warp the imagery to an absolute real-world position, while removing distortions.  This improves the horizontal (X,Y) and vertical (Z) dimensions of the final dataset.

The process with dronedeploy

Placing the GCP Markers

Placement of GCP points should be scattered throughout the area of interest being mapped. If the area being mapped has noticeable elevation changes, make sure your GCP placement (hills, mines, valleys etc.) accounts for this by placing them in the different elevations on a relatively flat surface. Ensure that the region of interest (where you want the map accuracy to be highest) has GCP points surrounding it, as well as scattered throughout the region. When placing GCPs around the perimeter DroneDeploy recommends a 50 ft buffer zone between the edge of the map and the GCPs location. This will ensure that there is enough image coverage to carry out the processing.

Conducting the Flight

GCP flights are flown just like any other flight. The only difference is that you are now capturing GCP targets in your imagery that will later be used to increase the accuracy of your map. Make sure that your camera is in focus and setup correctly. If the GCP targets are not in focus, the process will not result in the same levels of accuracy. It is always a good idea to make sure the area you will be flying that day has good weather. High-winds, low-clouds or precipitation can make flying and mapping difficult. When the flight is done, upload your images to dronedeploy.

Processing GCPs

  • Once you have selected your images (but before you start processing your map) select the Ground Control Points drop-down.
  • Download the GCP file template that better suits your needs and add your coordinates to the template. The template will contain example coordinates for you as a guide. You can also upload your GCP data in a .csv file directly but it must follow our required formatting.
  • Once you’ve added your GCP information to the template, save it as .CSV and then upload it into DroneDeploy.
  • Next, add your EPSG code. A full list of codes can be found at Spatial Reference. Again, most GCP maps are in WGS84 or one of the NAD83 (typically not HARN, CSRS, or NAD27) State Plane Coordinate Systems.
  • Once you have finished the above steps, click Add GCPs. You are now ready to start processing your images by clicking the “Upload Images” in bottom left corner of your screen. DroneDeploy will begin pre-processing your GCP map. Once the pre-processing is complete you will receive an email that will take you to our in-browser GCP tagging tool.
  • If your GCP .CSV was successfully read by our auto-tagging tool then this step will be skipped and you will proceed directly to step 4.

    If we encountered a problem with your .CSV or EPSG code, you will need to fix this data using our GCP .CSV File Formatting. Please see our GCP .csv File Formatting documentation for a full walkthrough of this process.

  • Once you have located and tagged the correct GCP target. You will be prompted to carry out a secondary refinement to make sure the exact pixel is selected. This is important to ensure the highest accuracy is achieved within the map. There will be up to 8 images validate in the secondary refinement stage. It is important to complete all of them to achieve the highest accuracy within the map.If for some reason one of the markers is undetectable during the refinement you can exclude the marker by unchecking the blue square in the top right corner of the image.
  • Once you have tied in all of the markers on the map, you can click Preview in the top right corner. This will preview how many tie points each marker selected has within the map.
  • Once you have validated the tie points, press submit. That’s it, you’re done! You will receive an email once your map has completed the final steps to increase the accuracy of the map on our servers.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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