There are many applications of directional analysis relating to surfaces. These include: analyzing and mapping surface aspect (e.g. the d.rast.arrow function in GRASS, the aspect generation tools of Landserf, SAGA, TAS (now superseded by Whitebox, ArcGIS and TNTMips), which is defined as the direction of the surface gradient (as opposed to the magnitude of the gradient — see Figure 4‑80); analyzing and modeling flow directions and friction as part of hydrological and similar studies (e.g. local drainage directions such as the lddcreate function in PCRaster, the Flow function in Idrisi, and the FlowDirection function in the ArcGIS Spatial Analyst hydrology toolset); and analyses relating to the lighting of surfaces, as part of solar energy analysis, lighting for visualization, and for visibility analysis (e.g. in the ArcGIS 3D Analyst and Spatial Analyst toolsets). Each of these topics is discussed in some detail in Chapter 6, Surface and Field Analysis.
In general the representation of direction in such instances is strongly affected by the underlying model or representation of the surface — for example, with grid (raster) surfaces there is often directional bias as a result of the limited range of directions available in immediate neighborhoods (e.g. only 8 directions in a 3x3 neighborhood).
For vector-like grid data (single or dual-grid source data) a vector plot may be informative. An example of such a plot is shown in Figure 4‑80 where gradient vectors have been plotted using Surfer for a sample grid file of the Mt St Helens volcano. In this case color has been used to indicate vectors of greater magnitude (from white through blue to red). Similar mapping of simulated wind directions is performed by software tools such as WindWizard and WindNinja designed for use in conjunction with wildfire modeling and management (Figure 4‑81). In this example wind flow has been modeled for the OS NT04 DEM (Pentland Hills area) with 100 meter output grid resolution and input direction of 230 degrees and speed 50 mph. Output here is displayed using Google Earth (.KML file).
Figure 4‑80 Slope and aspect plot, Mt St Helens data, USA
Figure 4‑81 Wind flow grid simulation using WindNinja