This tool alters a digital elevation model (DEM) by streamlining it in a specified direction. That is, the tool will modify the elevations of grid cells in the DEM that are in a leeward orientation and have a horizon angle (or maximum upwind slope) that is greater than the specified maximum value. When an obstacle is in the path of the wind, there will be an area of turbulence and reduced wind speed behind the obstacle. Similarly, there will be an area of high pressure in front of the obstacle, such that the wind will start to ascend some distance before it actually reaches the obstacle. Streamline Topography can be used to mimic this pattern. The following images provide an example of a DEM that has been streamlined from approximately a north-eastern direction (Azimuth of 36-degrees) using a maximum leeward slope of 2.8-degrees
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The user must specify the name of an input DEM raster, a horizon angle raster image, the hypothetical wind azimuth, and a maximum leeward slope (in degrees). Notice that the specified azimuth must be the same as the azimuth used to calculate the horizon angle raster. As guidance, the distance that an obstacle to the wind effects wind patterns in the leeward direction ranges from about 15 to 20 times the obstacle height. This translates into a maximum leeward slope of between 2.86-degrees and 3.81-degrees, although in reality this range is highly variable and likely dependent on the wind speed and characteristics of the obstacle. It is possible to further streamline the DEM to include the wedge of high pressure in the windward direction of topographic obstacles by calculating the horizon angle a second time, using the leeward streamlined DEM as the input and specifying an azimuth that is 180 degrees from the original. Then run Streamline Terrain a second time using the new horizon angle image and the streamlined DEM as inputs (accounting for the 180 degrees altered azimuth). The maximum leeward slope should be much higher in the windward direction. The high-pressure wedge in front of an obstacle will typically extend at least twice the height of the obstacle (again this varies significantly), translating into an angle of 26.6 degrees.
The algorithm works by tracing a ray from grid cells that have a horizon angle greater than the maximum leeward slope in the specified direction. When a grid cell is encountered along the ray that has a horizon angle less than the specified maximum slope, the algorithm will use the elevation of this cell and the desired maximum leeward slope to estimate a new elevation for the origin grid cell. 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. In order to provide a satisfactorily smooth result, the process is repeated using rays that are +/-3 degrees and the estimated elevation is determined by an inverse-distance-to-a-power interpolation scheme.
An example of the application of Streamline Terrain for modelling exposure/sheltering to wind is described in: Lindsay, JB and Rothwell, JJ (2008) Modelling channelling and deflection of wind by topography, in Advances in Digital Terrain Analysis, edited by Qiming Zhou, Brian Lees, Guo-an Tang, pp. 383-405.
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. Maximum upwind slope is best displayed using a Grey scale palette that is inverted.