Overview
Ground Sampling Distance (GSD) is a fundamental concept to any drone flight conducted for survey or inspection purposes. It underpins many key decisions for the flight planning:
1. What altitude should I fly at? (safety),
2. Am I going to get the required data resolution from this flight? (quality)
3. How does my camera affect safety and quality of the flight? (equipment)
But what really is GSD? How is it calculated? How do we ensure it's set the correct value during flight? We explore all of these questions below.
What is GSD?
Ground Sampling Distance, as the name implies, refers to the amount of ground / surface area covered by a single image in flight. If you're flying a mapping flight with the camera facing down (nadir position), then this distance is basically the amount of ground covered by your drone per image in flight. If you're flying vertically and mapping a tower or a facade, then GSD is basically the amount of facade surface area covered by a single image in flight.
You might see GSD frequently expressed as cm / pixel. As implied, this refers to the amount of ground or surface area covered by a drone image.
Lower the GSD, higher the resolution of the flight and the more details you can see in captured images.
GSD primarily depends on 2 main inputs:
Altitude / Distance to the structure of Interest - Intuitively, this is easy to understand. The larger your distance to the ground / structure, the higher is the amount of ground covered by a given camera's lens. So you'd expect the GSD to increase at higher altitudes or greater distance to the structures.
2. Camera's Parameters - This involves the sensor width, focal length & image resolution of the camera. Again, it's intuitive that a camera's field of view, focal length and image resolution would have an effect on the GSD. Higher the focal length of the drone, lower will be the GSD.
Image credit: vision-systems.com
How much is this effect and how calculate it? We'll explore this in the next section.
How to calculate GSD?
From the points above, we know that GSD has to be directly proportional to the working distance (distance from target) and indirectly proportional to the drone's focal length. The formula to calculate GSD is as follows:
GSD = Sensor Width * Working Distance / Focal Length * Image Width
where:
1. Sensor Width (mm) = width of the drone camera's sensor
2. Working Distance (m) = Distance to target structure i.e. Altitude in mapping missions and horizontal distance in vertical inspection missions.
3. True Focal Length (mm) = The true focal length of the drone's camera (in mm).
Note - this is the true focal length of the camera - NOT the 35mm equivalent! You can always calculate the true focal length of the camera using the sensor width and the horizontal FoV (field of view) below:
T. Focal Len = (0.5 * Sensor Width)/ TAN((0.5 * H.FoV)/ 57.296)
4. Image Width (pixels) = Number of pixels in the drone camera's image resolution. e.g. 5472
Let's take an example drone and work it out below. For a DJI Mavic 2 Pro flying at 50m altitude, the numbers come out as follows:
GSD = 12.8 * 50 / 10.26 * 5472 = 0.0114 m/pixel = 1.14 cm/pixel
GSD = (Sensor Width * Working Distance / Focal Len * Image Width)Want to calculate this for yourself? Get your free and comprehensive spreadsheet designed with Drone Mapping and Inspection formulas over here
Whilst it's good to be armed with the knowledge of how to calculate the Ground Sampling Distance in flights, the key question is: Should you be calculating GSD by hand for every flight?
In Hammer Missions, GSD calculations are automatically done for you every time you plan a flight. Hammer also takes into account whether the flight is a mapping mission or roof inspection and therefore uses the altitude as the working distance, or if the flight is a vertical mapping or tower inspection mission, it uses the horizontal distance to the structure as the working distance.
Get your free and comprehensive spreadsheet designed with Drone Mapping and Inspection formulas over here
Planning Missions with required GSD
To plan your missions with the required GSD, all you have to do is pick the camera type in Hammer and Hammer will show you the calculated GSD based on the set altitude or horizontal distance in flight.
Additionally, if you have other parameters of the flight affecting GSD - for instance the Ground Offset used frequently in roof inspection missions, Hammer will also take those parameters into account while calculating the GSD.
From the post above, you might notice that to achieve your required GSD, you basically have 2 main parameters to play with:
1. The Flight Altitude (or horizontal distance in vertical flights) AND
2. The Camera Type.
Hammer helps you play around with both seamlessly so that you can achieve the best tradeoff between safety, quality and costs.
Summary
We hope this post helps you calculate GSD in your upcoming flights or better yet - provide a tool to automatically calculate it for you! If you’d like to learn more about other missions supported in Hammer, please visit our mission tutorials.
To learn more about our enterprise solutions, including mission collaboration, flight analytics and more please contact us at team@hammermissions.com