Ground Sampling Distance (GSD) in Photogrammetry
Essentially, the Ground Sample Distance (GSD) determines the spatial resolution of a map or 3D model.
GSD is the distance between the center of two consecutive pixels on the ground when looking at a photo. A higher GSD value means lower spatial resolution and less detail in the image.
Therefore a 1cm GSD will be a higher resolution map or 3D model than a 2cm GSD
To calculate GSD, consider:
- Flight Height (H): The distance from the camera to the ground or object.
- Camera Specifications:
- Image Width (ImW): The pixel dimensions of the captured image. This determines the number of megapixels
- Sensor Width (SW): The width of the camera sensor.
- Focal Length (F): The focal length of the camera lens.
Determining the GSD before beginning image capture of a drone mapping project is essential for adjusting flight height and camera settings to meet project requirements. For detailed area reconstruction, a lower GSD with a closer flight height is preferable. For larger areas with less detail, a higher GSD and greater flight altitude can reduce acquisition and processing time, as well as time on site. Although, a camera with a larger sensor and megapixel count can provide high detailed maps and models, even from higher/further distances.
Examples:
- A GSD of 1cm means each pixel in the image covers 1cm on the ground (1×1 = 1 square centimetre).
- A GSD of 3cm means each pixel represents 3cm on the ground (3×3 = 9 square centimetres).
You can see in the image below the ground control point chequered marker which is 80m above ground level. If we zoom right in on the image, you can view each individual pixel that make up the image. In this example, one pixel in the image represents a 2.36cm square in the real world. This number is the GSD
