The area must be 50 hectares or larger.
The area must be outside the 50-meter protective buffer zone around water bodies and streams.
The area must not already be developed. Acceptable current uses for the area are crop land, forest land, pasture land, or open undeveloped.
The land must not contain slopes in excess of 15%.
We are provided with the following data layers to work with:
A raster land use map
A raster digital elevation model (DEM)
A vector streams layer
A vector roads layer
Let's start by taking a look at our land use map:
We start by reclassifying our land use categories to create a boolean image, pixel values of 1 for all suitable land uses (crop land, forest land, pasture land, open undeveloped land), and pixel values of 0 for all other classes. This is the first of our boolean images representing one of the criteria.
Our land use map contains water bodies such as ponds and lakes, but does not contain streams and rivers as a separate land use. Here is the land use map again. This time, the vector streams layer is superimposed for display:
We need to create a suitability layer based on buffers around both lakes and streams. The first step in creating such a layer is to reclassify our land use map (using the RECLASS module in Idrisi) to create a data layer where pixels representing water bodies have a value of 1 and all other pixels have a value of 0:
Now we will rasterize our vector streams into this layer (using the LINERAS module in Idrisi) to create a raster layer with streams and other water bodies all represented as non-zero pixels:
We now create buffers around all of the water features. The suitable areas we are looking for are those areas OUTSIDE the 50-meter buffers around water features. We can use the BUFFER module in Idrisi and specify that the areas within the buffers will have pixel values of 0 and areas outside the buffers will have values of 1. This image is the second of our boolean criterion images:
The third criterion we wish to work with is the restriction that slopes in excess of 15% are not permissible within our target areas. We start with a DEM of the study area:
From the DEM we can determine the slopes throughout the study area. In Idrisi this is done using the SURFACE module. This is the result of running SURFACE to create a slope image. The value of each pixel is the slope of that pixel in percent, which is calculated from the DEM based on the elevation value of that pixel relative to its nearest neighbors. The continuous slope image has been reclassified into slope categories for display:
We reclassify the slope image to create a boolean image to represent our slope criterion; areas with slope less than or equal to 15 percent get a pixel value of 1, areas where slopes exceed 15 percent get a value of 0:
We have created boolean images to represent three of our criteria: (1) areas outside 50-meter buffers around water bodies and streams, (2) areas with current land use of crop land, forest land, pasture land, or open undeveloped, and (3) areas with slopes less than or equal to 15%. At this point we perform overlay-multiply steps between the three images. This can be done by performing overlay-multiply between two of the images and then overlay-multiplying the result of that operation by the third image. In Idrisi, Image Calculator permits the three images to be overlay-multiplied in one step. Regardless of the sequence of operations, the following boolean raster image is produced:
Now that we know what parts of our study area satisfy the three criteria, we need to evaluate which of the satisfactory areas are greater than 50 hectares. In Idrisi we would use the GROUP and AREA modules. The GROUP module looks for contiguous blocks of pixels with the same value. For each block that it finds, each pixel within that block is given a unique ID value. GROUP produces this image:
The AREA module is now applied to the raster image created by GROUP. AREA creates an image in which each pixel value is equal to the area of the group to which that pixel belongs. One thing to note is that GROUP also considered the background areas, areas which did not satisfy our criteria, as possible groups. Now that we have run AREA, the AREA module calculated areas for non-target portions of our study area. To keep these areas, which we rejected from consideration based on our boolean analysis, from confusing our analysis, we overlay-multiply the result from AREA by the previous boolean multiplication product. This essentially "zeroes-out" the areas we are not interested in. After performing this "zeroing-out" step and reclassifying the image into categories for display the result of our area analysis looks like this:
We can reclassify this image to exclude all areas which do not exceed 50 hectares. When we do so the following areas remain for consideration:
If we were to continue with our attempt to evaluate a potential site for our proposed activity, we would probably be prudent to consider other contextual variables of the candidate locations. If we display water bodies, streams, roads, and our candidate properties together, it may provide some perspective on the areas we are considering.
