Why Ground Sample Distance is Important for Photogrammetry and How to Calculate Your GSD
Ground sample distance (GSD) is a metric that is useful for photogrammetry and measurements in drone mapping and surveying projects. GSD is defined as the distance between the centers of two adjacent pixels measured on the ground. This metric is related to the camera focal length, the resolution of the camera sensor, and the camera’s distance from the subject. It is usually described in centimeters per pixel (cm/px).
Why is GSD Important to Understand?
In photogrammetry and aerial surveying, ground sample distance can affect the accuracy of your survey. The data collected during the survey will be transformed into real-world measurements and distances. Your GSD will help translate the distances in your images to distances on the ground.
A GSD of 5 cm/px means that one pixel in the image represents a 5 x 5 centimeter square (25 cm2) on the ground. A smaller square translates to more detail within the image. GSD will scale linearly with the drone’s altitude if it is carrying a camera with a fixed focal length lens. The GSD will scale linearly with focal length if the camera has a variable lens. If a drone flies closer to the ground, the GSD will improve and smaller details will be clearer in the image.
How Do You Calculate GSD?
Ground Sample Distance Formula
The formula to calculate GSD is:
(flight altitude x sensor height) /
(focal length x image height and/or width)
If you want to experiment with a calculator outside of flight planning, Pix4D has one available online. All you have to do to determine ground sample distance is plug in your flight altitude, camera sensor width, camera focal length, image width, and image height.
However, Pix4D’s calculator is not connected with real-world flight planning. Important considerations when planning a mission include obstacles you may need to avoid, flight times, or the actual surface area of your survey.
Fortunately, most drone mapping and photogrammetry software like QGroundControl calculates your GCS for you during mission planning.
Download the desktop version of QGroundControl for free here.
What is Considered a “Good” Ground Sample Distance?
A lower value for GSD means a more accurate survey. Your survey cannot be more accurate than your GSD. The range for UAV photogrammetry typically falls between 1.5 to 2.5 cm/px (.6 to 1 inch). Some recommend a ground sample distance of 1 cm/px for professional surveys, which is very low.
In order to get this level of GSD, the pilot would need to fly at a very low altitude, which would decrease the acreage per flight and increase the possibility of a collision with an object on the ground.
To get a higher level of GSD you can do a few things:
- Reduce altitude
- Increase camera sensor resolution
- Increase focal length
To achieve a higher level of survey accuracy, it is recommended to use a high-resolution image sensor, fly closer to the ground, and use it in conjunction with PPK data. Sub-centimeter data set accuracies with a 2.5 cm (1 inch) GSD can be achieved at an altitude of 122 meters (400 feet).
A Comparison Between Low and High Ground Sample Distance
| Low GSD | High GSD |
|---|---|
| Higher accuracy | Lower accuracy |
| More image detail | Less image detail |
| Lower flight height (altitude) | Higher flight height (altitude) |
| More data | Less data |
Conclusion
Ground Sample Distance is a crucial measurement to understand, as it directly affects the accuracy of photogrammetry mission outputs. In simple terms, it is the measure of a pixel in an image and how a pixel relates to a physical distance on the ground. The lower the Ground Sample Distance, the higher accuracy and more detailed your survey will be.
To learn how to measure Ground Sample Distance on a mission plan, check out the article: How to Plan a Survey Flight and Determine Your Ground Sample Distance in QGroundControl
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