**5.1 Lakes**

190 Environmental Monitoring

Fig. 4. Available acquisitions of Landsat images for the year 2011: L5:1/January B. L5 17/January C. L7 10/February D. 30/March E. L5 7/April F. 23/ April G.2/June

database fields so as to join the satellite data to the observation database.

verification purposes.

Data fusion techniques have been used in creating enhanced images at ~ 15 meters resolution for the Landsat ETM images for the watershed areas of the lakes. Digital image processing techniques are applied, plus some necessary image enhancement. The next step in extracting image data for lakes used an unsupervised classification method based on a clustering K Means algorithm with 10 classes and 20 maximum iterations that were then aggregated to land and water classes. Raster to vector conversion techniques were used to outline the polygons of the water surfaces. Due to intense topographic relief of the area shadows were also classified as water surfaces especially in winter scenes. These were eliminated as they have small area extent using vector editing techniques. Auxiliary information was also used to guide AOI selection and correction and this included the use of vector (GIS) layers of map coastlines, bathymetric maps and sampling point locations, Figure 5. Each Lake\_AOI polygon was assigned a unique identification number and

The Lake\_AOI polygons were used to create the water-only images of the lakes. Multitemporal water-only images of Macro Prespa, Vegoritis and Ohrid lakes have been created and these have been stored as a raster database. Metadata information describing the image acquisition information was also included. Lake surfaces have then been further analyzed using unsupervised classification techniques. Self-organizing Map Classifier – unsupervised classification using neural network techniques proved quite effective in analyzing the lake water surfaces. Available SDT and Cl data are not readily available for the lakes of our region and so some ground measurements are used just for general

 Fig. 5. Auxiliary information: Lake bathymetry of Macro Prespa lake (left), Vegoritis lake (right)
