GSD (Ground Sample Distance, or spatial resolution of an image) is a value that defines the size of the area of terrain corresponding to one pixel of the image obtained from the camera. In other words, it can be expressed as the linear distance between the two centers of two adjacent pixels, as shown in Figure 1. It is measured in meters or centimeters per pixel. For example, if GSD is 1 cm/pixel, each pixel in the image covers a 1×1 cm land area.
Figure 1. GSD.
GSD is one of the key parameters in photogrammetry, as it directly affects the accuracy and detail of the created terrain models, maps, or measurements.
- The smaller the GSD (e.g., 1 cm/pixel), the more accurately the dimensions of objects in the image can be measured. High resolution is necessary for tasks that require detailed analysis, such as surveying construction sites or monitoring infrastructure.
- In photogrammetry, images are used to create 3D terrain models. The higher the resolution (lower the GSD), the more detail the model will contain, which is essential for design, landscape analysis, and architecture.
- With a low GSD (e.g., 50 cm/pixel), small objects such as road signs, cracks in the asphalt, or wires may remain invisible. High resolution allows you to detect such details.
GSD depends on several factors:
– Focal length of the camera. The longer the focal length, the higher the resolution.
– Flight altitude or distance to the object. The lower the camera is relative to the ground, the lower the GSD.
– Pixel size of the camera sensor. Smaller pixel sizes allow for higher resolution.
The principle of calculating GSD is shown in Figure 2.
Figure 2. The principle of calculating GSD.
h – Height [meters]
IMGw – Image width [pixels]
IMGh – Image height [pixels]
F – Focal length [mm]
Sw – Sensor width [mm]
The calculation is carried out according to the formula (1):
(1)
Knowing the GSD value makes calculating one image’s coverage area easy. To do this, we will use the formulas (2):
(2)
where Dw and Dh are single image footprints on the ground. S is an area of the image footprints on the ground.
Calculation example:
Calculating Ground Sampling Distance (GSD) is essential for understanding the spatial resolution of aerial or satellite imagery. You can determine how much ground area each pixel represents using the correct formula—GSD = (Sensor Height × Pixel Size) / Focal Length. This knowledge is crucial for planning missions, selecting the right equipment, and ensuring that the imagery meets the resolution requirements of your project. A clear understanding of GSD empowers surveying, mapping, agriculture, and remote sensing professionals to make informed decisions and achieve reliable results.