A remote sensing sensor detects the reflected radiation of the Earth's surface and stores it as numbers in a raster. In accordance, each area that has been detected constitutes a cell in a raster. These raster cells are called pixels. The size of an area represented in a pixel depends on the capability of the sensor to detect details.
Raster with low and high spatial resolution.
Low and high spatial reolution
The ability of a remote sensing sensor to detect details is referred to as spatial resolution. The spatial resolution is stated in metres. The more pixels are included in a remote sensing image of a certain area, the higher the spatial resolution meaning the more details can be observed.
The swipe below shops two satellite images of Bonn. You can clearly distinguish between a higher spatial resolution of 30 metres and a lower spatial resolution of 300 metres. In the image with the lower resolution, much more different objects must be included in one pixel.
Satellite images of Bonn with a spatial resolution of 30 metres and 300 metres respectively (images by courtesy of USGS/ NASA Landsat Program).
In almost every satellite image, objects that are close together must be included in one pixel. Such pixels are called mixed pixels.
The image below shows a house and a garden included in the same pixel. Due to the low spatial resolution, the colour components of both objects (brown and green) result in a brown-green mixed pixel, which is very hard to analyse.
The lower the spatial resolution, the more mixed pixels and the harder it is to tell areas apart.
Formation of a mixed pixel caused by different objects in the same raster cell.
Each remote sensing sensor produces raster image data. In turn, each raster consists of raster cells, which are also referred to as pixels. The bigger a pixel, the more objects on the surface of the earth are captured and the lower the spatial resolution of a raster image. The higher the spatial resolution, the smaller the amount of hard-to-analyse mixed pixels included in a raster image.