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ammenbergWatching Lakes From Space

Petra Ammenberg
Centre for Image Analysis
Uppsala University
June 2002

I am using digital images collected from satellites and airplanes to try to determine the water quality in lakes and coastal zones in Sweden. This is done by analysing the colour of the water, since different substances give the water different colour. Different species of algae will, for example, make the water look green, yellow or even red. Other substances, including non-living substances, will also affect the colour of the water. If one knows the components and the amount of each component in the water, one can draw conclusions about the water's quality.

The goal of this project is to be able to apply a recently developed water quality model to digital satellite or airplane images and derive concentration maps of different substances in the water. The substances of interest can, for example, be a concentration of chlorophyll, which is a pigment in algae, or a concentration of organic (vegetation) and inorganic particles (clay, minerals, etc.). At this point the model has been adapted to Lake Malaren in Sweden, since a number of images from the eastern part of the lake were available for testing the model.

To obtain the correct result from the model, it is necessary to pre-process the images to remove noise/errors and, especially, any signal recorded in the image that is not a result of the substances in the water. The main problem at this stage is to correct for the signal contribution from the atmosphere between the water surface and the satellite or airplane. To make the correction an atmospheric model is used. The model calculates the disturbance from the atmosphere based on a number of input values, i.e. the amount of water vapour, dust and soot in the atmosphere. The result after the atmospheric correction can be verified by comparing it to measurements made at the surface. These measurements correspond to removing the sensor out of the satellite or airplane and placing it just above the surface of the lake, hence removing the effect of the atmosphere.

The water quality model can then be applied to the images after pre-processing. The resulting concentration maps can be evaluated using water samples taken in the lake at the same time as the image collection. These water samples are analysed in a laboratory to derive the actual concentrations of chlorophyll, etc. The timing between image collection and water sampling is very important since the water is a moving target and the situation can be quite different a few hours later. The evaluation of the results after applying the model to the images from Lake Malaren indicated good correspondence between modelled concentrations and concentrations from the laboratory analysis. The next step will be to investigate whether the same model can be used on other images and other lakes with the same promising results.

In Sweden today, there are several large water control programs managed by the county government boards and local water quality associations. The assignment for these programs is to collect and analyse water samples and report the results. For some lakes and some parts of the coastal zone, sampling is done more than 10 times per year at each sampling station and approximately 30 variables are measured, indicating the large amount of work. Some of these variables could be estimated using satellite or airborne images instead, hence minimising the number of water samples necessary. However, the most important contribution is that this technique can generate an image describing the concentration levels in the whole lake or coastal area, not just in a number of sampling points.