Cloud Detection

To improve the definition of useful areas under clouds, it is necessary to detect cloud and shadow in images.

This tool is intended to speed up the cloud and shadow detection process by means of image processing specific techniques applied together and following pre-defined pattern recognition resulting in a vectorial data representing analyzed objects.

Different from other algorithms for cloud/shadow detection, this tool is able to detect these objects in an image of any satellite, not being specific to some sensor type. It is also possible to use some image extension types, such as TIFF, JPEG, etc.

To detect such objects it is necessary to define which image bands will be analyzed, i.e., the analysis is done in distinct bands of the image because each image band has a better response to certain targets. For instance, LANDSAT satellite images have a better definition of high radiance objects such as clouds in the blue band while in near infrared band it has a better definition for low radiance objects such as shadows. 

It is acessible throught:

Processing-> Raster Processing-> Cloud Detection...

• Cloud Layer: Click on the  button to select the layer to be used for cloud detection and corresponding band.

• Shadow Layer:  Click on the  button to select the layer to be used for shadow detection and corresponding band.

Observe the Histogram for cloud and shadow by clicking on an  the Minimum/Maximum values on the tables below.

• Cloud: Define an adequate interval (Minimum/Maximum) for cloud detection. For instance, Minimum: 60 / Maximum: 255.
• Shadow: Define an adequate interval (Minimum/Maximum) for shadow detection. For instance, Minimum: 8 / Maximum: 10.
• Hint: Use button on Menu Tools to help you define the best parameters to detect clouds and shadows.

•   check this box and then go to the Map Display and draw an area containing some clouds and shadows. Check the polygons of clouds and shadows over the map display.  Change  the previously defined intervals  (min/max) if necessary, according to the sensor response for clouds and shadows. and apply again.  Use  to change the default color of polygons (red) if necessary.

Morphologic Operator

• Closing: this filter usually chains Dilation and Erosion operators to obtain closing effects. It is executed according to the number of Iterations (default is 2).

• Opening: this filter usually chains Erosion and Dilation operators to obtain opening effects. It is executed according to the number of Iterations (default is 2).

• Filter order: to change the filters order, select one of them and click up  or down    arrows.

Output

• Repository - click on

• to select the output directory and also inform the new layer name to store the result, or
•  to select the Data Source and
• Inform the new Layer Name to store the Cloud Detection result.

• Click OK to perform the operation or Cancel to close the interface.