|
|
Exercise 3 - Change detection analysis using subtract function  In this section you will use the Image Arithmetic function in LeoWorks in order to undertake a change detection of our study area. Open band 5 of the 1984 image, selecting the following crop parameters (Step 2, section 3.1 of the LEOWorks tutorial): Left=260, Right=660, Top=410, Bottom=920. This subset is centred on the Sacalin Island. Repeat the process for band 5 of the image from 2000 using the same crop limits.
Use the Arithmetic function of LEOWorks (section 3.6 of the LEOWorks 3 tutorial) to subtract band 5 of the 1984 image from band 5 of the 2000 image. The result should look like this:
The image created as a result of the subtract operation is a grayscale with values from 0 to 255. As it is difficult to make a visual difference analysis using a grayscale image, we will convert it to a colour image. We will do that using the LEOWorks LUT editor. Following the steps outlined in section 4.5 of the LEOWorks 3 tutorial, edit the LUT range using the following values: 0-15 (blue), 16-135 (green) and 136-255 (red).
In order to reduce the maximum number of different colours to 256, some image formats (e.g. 8-bit TIFF) use an 8-bit Look Up Table (LUT). Using this method, it is only necessary to store an 8-bit index into the colourmap for each pixel, rather than the full 24-bit colour value. Thus 8-bit colour image formats consist of two parts: a Look Up Table describing what colours are present in the image, and the table of index values for each pixel in the image.
Last update: 16 April 2013
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
