I work with ArcGIS 10.1 (advanced license), and I am finding difficult to grasp how the horizontal factor works in the Path Distance tool. I have read the help documentation several time. These are the sources: (1) http://desktop.arcgis.com/en/arcmap/10.3/tools/spatial-analyst-toolbox/understanding-path-distance-analysis.htm (2) http://desktop.arcgis.com/en/arcmap/10.3/tools/spatial-analyst-toolbox/how-the-horizonal-and-vertical-factors-affect-path-distance.htm (3) http://desktop.arcgis.com/de/arcmap/10.3/analyze/arcpy-spatial-analyst/an-overview-of-horizontal-factor-classes.htm
Now, sticking with the ArcGIS help documentation's example of a car moving from location A to location B, I do understand that the use of a horizontal factor in the context of path distance analysis is actually aimed at increasing a given movement's cost on the basis of an additional horizontal friction. In other words, to stick with the car example, if we only consider the slope as a cost and the car is moving uphill, there will be a given cost (say 10); if we add an additional friction caused by the wind blowing at 180 degree offset (i.e., in a direction opposite the car movement's direction), the total cost will be increased by a factor f (in total: fx10).
So far so good. YET, I am rather confused here: To determine the HF for moving from one cell to the next, the prevailing horizontal direction at the processing cell must be established from the horizontal direction raster. The horizontal relative moving angle (HRMA), the relationship between the moving direction with respect to the horizontal direction, is calculated and plotted on the horizontal factor graph to determine the horizontal factor. The resultant value establishes the horizontal factor from the center of the processing cell (the FROM cell) to the edge of the TO cell (source: http://desktop.arcgis.com/de/arcmap/10.3/analyze/arcpy-spatial-analyst/an-overview-of-horizontal-factor-classes.htm).
That paragraph is totally obscure to me. In particular, I do not understand what is the horizontal direction raster to which reference is made. Is that the cost raster which is one of the input required by the tool?
In summary: let's assume I have a raster representing the cost of the car movement in a given area, and I want to use the Path Distance tool to calculate the accumulated cost from a origin location also accounting for the additional horizontal friction caused by the wind.
(A) Am I correct in understanding that I need to only input the main cost raster AND the horizontal factor data?
(B) is the main cost raster equivalent to the aforementioned (rather obscure) horizontal direction raster?
(C) As for the horizontal factor data, assuming I want to use a customized table in which I assign a factor value to each wind direction, am I correct in understanding that I have to use 0 degrees to "represent" wind blowing from behind (i.e., blowing in the car movement's direction)? I ask this because this point is not clear to me from the help documentation:To define the HRMA, the angle between the horizontal direction defined on the horizontal direction raster and the moving direction to the cell to which the cost distance is being calculated is determined. Even though the values range from -180 to +180, the HRMAs are symmetrical with respect to the horizontal factor axis; therefore, they are represented from 0 to 180 on the x-axis of the horizontal factor graph. The HRMA is the number of degrees the travel direction is from the specified horizontal direction, regardless of positive or negative orientation. Therefore, the range of HRMAs is 0 to 180 degrees. (source: http://desktop.arcgis.com/de/arcmap/10.3/analyze/arcpy-spatial-analyst/an-overview-of-horizontal-factor-classes.htm).