4. Calculating ground sampling distance (GSD)

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After you've decided on an image acquisition plan, all you need to do is determine your ideal flight height for the maximum detail visibility in the acquired images.

Image accuracy can be quantified using a value called ground sampling distance (GSD) - the distance between the centre of two adjacent pixels measured  in centimetres on the ground. To give you a better idea of what GSD stands for, let’s consider the following example. Say, you know that your GSD equals 10 cm. That means that the distance between the two consecutive pixels in your drone captured photo is 10 cm if we were to measure it on the ground. So, if you zoom into a rock that spans across 20 pixels, in real life that rock would be 200 cm (or 2 m) long. 

To calculate GSD, you need to know the flight height (m), the drone camera focal length (mm), the camera sensor width (mm), and the image width and height (pixels). 

The higher you fly your drone, the bigger GSD value you will get, which means that your final images will be less precise, with fewer details clearly visible. On the other hand, flying at higher altitudes means that you’ll be able to take fewer photos to cover the same area, which makes imagery stitching more straightforward. 

The best practice recommendation for most professional drone surveyors is to aim for GSD of 1 cm or less, but that of course depends on the project aim and the area that needs to be covered. So before you fly, make sure you know what the survey will be used to measure and what the accuracy requirements of these measurements are. This way you can strike the balance between:

  • Flying sufficiently low to keep the GSD as small as possible for your required accuracy, and
  • Flying sufficiently high to keep your final photo count manageable

The good news is that you don’t need to calculate GSD by hand or to memorise a formula, as there are many online tools available to automate the process. For instance, you can use this online GSD calculator. Let’s try it out by completing the check-point tasks below.

GSD check-point

Well done - you are almost there! Our final task for this week is to go through essential safety regulations for operating a drone in Australia.