Waves within an Image
Raster images - What are we talking about?
Remote Sensing often makes use of sunlight that is reflected by the surface of the earth. Sensors that are attached to aircraft or satellites detect this light, transform it into image data and then store these data. As a next step, the data can be analysed by scientists.
A sensor collects light reflected by the earth and stores it as numbers in a grid. The numbers mirror the reflecting intensities of an area.
Thus, sensors produce image data. But image data are not just pictures; behind such a picture are thousands of numbers arranged in columns and rows. This arrangement is called a raster (and can also be referred to as a grid). The cells in such a raster are called pixels.
Intensity in a raster
Each number within a raster indicates the strength (= intensity) of the light reflected by a certain area on the surface of the earth and then collected by a sensor. Note that reflection and absorption differ according to the area:
If the scanned area is white (see image below left), a lot of light is being reflected and then detected by a sensor. Accordingly, a high number is assigned to this area.
In contrast, a black area absorbs a lot of light itself, so this light cannot be measured by a sensor. Accordingly, a small number is assigned to this area (see image below right). As you can see, the numbers in a raster image allow you to draw conclusions about the characteristics of an area scanned by a sensor.
If an area reflects a lot of sunlight, a high number is assigned to this area in the raster.
If an area reflects little sunlight, a small number is assigned to this area in the raster.
The image below shows the product of a satellite sensor or, to be more precise, of one of its bands: a so-called grey-scale image (in this case of Bonn). Such an image displays areas reflecting a lot of light which is then measured by a sensor in white. In contrast, areas reflecting no light at all are black. Shades of grey indicate areas ranging between little and a lot of reflected light.
Grey-scale image of Bonn produced by a red band of a sensor. (© USGS/NASA Landsat Program).
The grey-scale image above was produced by the so-called red band of a satellite sensor. This band is sensitive to the red range of visible light. The blue and the green ranges of visible light are processed by the other bands of the sensor.
Digital grey-scale image of New York: The arrangement of the single pixels in rows and columns is clearly visible.
The animation above demonstrates how a digital raster image is set up by taking the example of a grey-scale image of the Statue of Liberty in New York. In order to assign an exact position to every pixel, a digital image is divided into columns and rows like a chess board. An image column is as broad as one pixel and an image row is as high as one pixel. The origin of the image coordinates can be found in the top left corner.
If you would like to find out more about the different bands of a satellite, or if you are asking yourself why the satellite images that you know are colourful and not grey, click here!
Many sensors work with sunlight which has been reflected by the surface of the earth. They collect it according to wavelengths. Thus, there are specific bands for the blue, green, red and infrared light. The intensity of the reflection is being measured and then stored in a raster. According to the level of intensity of the reflection, high or small numbers are being assigned to the respective raster cells. The final product is a grey-scale image for each band.