5 • Geographic Analysis

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Modeling flow over a surfaceThe Hydrology toolset in the Spatial Analyst toolbox contains tools for modeling the flow of water over an elevation surface. You can, for example, define hydrologic basins, identify stream channels, or calculate the distance along a flow path. Another set of tools models the flow of water or other material (such as a contaminant) through the subsurface. These tools are included in the Groundwater toolset in the Spatial Analyst extension.

The Basin tool (left) calculates hydrologic basins, or watershed areas. The Flow Accumulation tool (right) can be used to identify and create stream networks. The results of the these tools are rasters—you can convert the basins to polygons and the streams to linear features.

Basin, Flow Accumulation, and other tools in the Hydrology toolset take a flow direction surface (left) as input. This surface is in turn derived from an elevation surface (bottom), using the Flow Direction tool.

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Using ArcGIS Desktop

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Raster surfaces represent phenomena that have values at every point across their extent. They are created from values sampled at a limited set of locations, such as surveyed height values (for an elevation surface), or temperatures collected at weather stations (for a temperature surface). ArcGIS Desktop includes tools for interpolating values between the sampled locations to create a continuous surface.Another type of surface created from sample points shows concentration per unit area (density), such as crimes per square mile. Unlike an interpolated surface, a density surface doesn’t predict a value at each location—there may, in fact, not have been any crimes within a particular square mile area during the time period being analyzed. Rather, the density surface provides an indication of the distribution of features or values.

Creating an interpolated surfaceInterpolation tools create a continuous surface from samples with measured values, such as elevation or chemical concentration. There are several interpolation tools, and each has a variety of parameters that influence the resulting surface. The tools are included in both the Spatial Analyst toolbox (Interpolation toolset) and the 3D Analyst toolbox (Raster Interpolation toolset).

The Topo to Raster tool is specifically designed to create elevation surfaces. It allows you to input elevation contours, spot heights, and streams to create an accurate digital elevation model (DEM).

Creating raster surfaces

The same set of surface interpolation tools (including Natural Neighbor) is available in both the Spatial Analyst and 3D Analyst extension products.

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