This tool calculates the relief for each grid cell in a digital elevation model (DEM) in a specified direction. Directional relief is an index of the degree to which a DEM grid cell is higher or lower than its surroundings. It is calculated by subtracting the elevation of a DEM grid cell from the average elevation of those cells which lie between it and the edge of the DEM in a specified compass direction. Thus, positive values indicate that a grid cell is lower than the average elevation of the grid cells in a specific direction (i.e. relatively sheltered), whereas a negative directional relief indicates that the grid cell is higher (i.e. relatively exposed). The algorithm is based on a modification of the procedure described by Lapen and Martz (1993). The modifications include: (1) the ability to specify any direction between 0-degrees and 360-degrees, and (2) the ability to use a distance-limited search, such that the ray-tracing procedure terminates before the DEM edge is reached for longer search paths. The algorithm works by tracing a ray from each grid cell in the direction of interest and evaluating the average elevation along the ray. Linear interpolation is used to estimate the elevation of the surface where a ray does not intersect the DEM grid precisely at one of its nodes. The user must specify the name of an input DEM raster file, the output raster name, and a hypothetical wind direction. Furthermore, the user is able to constrain the maximum search distance for the ray tracing. If no maximum search distance is specified, each ray will be traced to the edge of the DEM. The units of the output image are the same as the input DEM.
Ray-tracing is a highly computationally intensive task and therefore this tool may take considerable time to operate for larger sized DEMs. NoData valued grid cells in the input image will be assigned NoData values in the output image. The output raster is of the float data type and continuous data scale. Directional relief is best displayed using the blue-white-red bipolar palette to distinguish between the positive and negative values that are present in the output.
The following is an example of a Python script that uses this tool:
wd = pluginHost.getWorkingDirectory()
demFile = wd + "DEM.dep"
outputFile = wd + "output.dep"
azimuth = "215.0"
maxDist = "5000.0"
args = [inputFile, outputFile, azimuth, maxDist]
pluginHost.runPlugin("DirectionalRelief", args, False)
This is a Groovy script also using this tool:
def wd = pluginHost.getWorkingDirectory()
def demFile = wd + "input.dep"
def outputFile = wd + "output.dep"
def azimuth = "215.0"
def maxDist = "not specified"
String[] args = [demFile, outputFile, azimuth, maxDist]
pluginHost.runPlugin("DirectionalRelief", args, false)