What is GSD? Ground Sampling Distance
The spatial resolution of a sensor is defined by a measure called GSD or pixel size.
The Ground Sampling Distance is the distance, mostly in centimeter units, between the centres of two neighbouring pixel in the image on the ground
As bigger the number of the image GSD, the lower the spatial resolution of the image and the less visible details.
The GSD is related to the altitude of flight: the higher the flight altitude, the bigger the GSD number.
- GSD of 10 cm means that one pixel in the image represents 10 cm on the ground (10 x 10 = 100 square cm).
- GSD of 40cm means that one pixel in the image represents 40 cm on the ground (40*40 = 1.600 square cm).
Even when flying at a constant height, the images of a project may not have similar GSD due to terrain/ground elevation differences.
Typical GSD selected for mapping projects:
- for 1 : 1000 GSD 5 cm – 10 cm
- for 1 : 10 000 GSD 50 cm – 1 m
The real resolution of an image is related to its image quality, additional factors such as image noise, sensor quality and weather conditions will appear.
When scanning the analogue aerial film, typical with 12,5 or 21 micron scan resolution, a GSD is introduced for each pixel as well:
GSD = photo scale x scanning resolution
For digital aerial cameras the pixel size of the CCD varying from 3 to 12 micron, depending on the CCD manufacturer.
The Operation Profile of your digital aerial sensor is determined by the combination of calibrated focal lenghts and it’s CCD pixel size.
The Mission Parameters for photo flights with digital cameras are determined by the required Ground Sampling Distance for the application, mostly defined by the client specs.
Selection of GSD
The selection of GSD has a major impact on flying cost and data processing cost.
If the GSD is specified too high (high GSD means small pixel size on the ground, e.g. 5 cm) more flight lines will be required and the amount of data per area will increase.
It is recommended to specify a target GSD and a minimum GSD to allow some flexibility because of terrain elevation variation.
The overlap (forward and side overlap ) has to be specified as well. Since film and it’s processing costs are eliminated in a modern digital workflow, it looks alluring to specify higher overlaps ( e.g. 80% or even 90% forward overlap). The downside is that the user has to deal with a huge amount of data, longer processing time, higher required hardware resources.
The benefit for modern large format digital aerial cameras is the superior image quality, especially the advantage of the 12 bit pixel depth minimum. Especially for automated feature collection, a higher bit depth per pixel will give better results. (12 bit original sensor, 14 bit A/D converters & electronics, 16 bit file format)
Before, when aerial film cameras has been used widely (Zeiss RMK Top, Wild RC30, LMK) the image was defined by Photo Scale, the ratio of the distance between two points on a photo to the actual distance between the same two points on the ground(ie. 1 unit on the photo equals “x” units on the ground). If a camera’s focal length is 153mm, and the airplane’s altitude Above Ground Level (AGL) is 5240 feet, the photoscale would be 1:10.500.
- Large Scale – Larger – scale photos (e.g. 1:5.000) cover small areas in greater detail.
A large scale photo means that ground features are at a larger, more detailed size.
The area of ground coverage that is seen on the photo is less than at smaller scales.
- Small Scale – Smaller – scale photos (e.g. 1:20.000) cover large areas in less detail.
A small scale photo means that ground features are at a smaller, less detailed size.
The area of ground coverage that is seen on the photo is greater than at larger scales.
During digital conversion of an aerial film, various scanner pixel may be selected to achieve specified GSD.
For example, 20 cm GSD equivalent can be achieved by:
- scanning @ 1:20,000 scale imagery at 10u pixel size, or
- scanning @ 1:10,000 scale imagery at 20u pixel size.