GEOGRAPHIC INFORMATION SYSTEMS Lecture 24: Spatial Analyst Continued

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1 GEOGRAPHIC INFORMATION SYSTEMS Lecture 24: Spatial Analyst Continued Spatial Analyst - Spatial Analyst is an ArcGIS extension designed to work with raster data - in lecture I went through a series of demonstrations illustrating the use of several important Spatial Analyst tools Density (revisited) used to create a continuous raster density surface from point or line features - a density surface can also be weighted by the value of an attribute - in the first example, the playa point values not weighted and show a simple density map - in the second example, the playa point values are weighted by the area of each playa (acres) playa density surface not weighted playa density surface weighted by area (acres per sq mile) darker areas represent areas with larger playas Interpolate to Raster (revisited) - used to interpolate a continuous raster surface from sample point values - interpolation methods include Spline, IDW, Kriging and Topo to Raster - These different interpolation methods produce similar results using different tools. - Spline - applies a smooth spline function to calculate the value of interpolated cells - IDW - applies an inverse distance weighting to calculate the value of interpolated cells - Kriging - applies a stochastic (statistical) function to calculate the values of interpolated cells Sample points Spline IDW Kriging 1

2 - Topo to Raster - interpolates topographic contours to create a raster surface Topographic contours Raster surface from contour lines Map Algebra - provides access to the Raster Calculator - the Raster Calculator allows you to apply arithmetic or algebraic functions to raster surfaces - can be used to convert the grid cell units of a raster layer (e.g. meters to feet) - can be used to subtract, add, multiply or divide two or more raster layers - note that the raster calculator is often used to create raster spatial models Map Algebra used to apply arithmetic or algebraic functions to raster surfaces - to create a new surface, ArcGIS applies the mathematical function to each cell (cell by cell analysis) - in this example, we subtracted the water table elevation in 2000 from the water table elevation in 2008 to calculate the change in the water table elevation Water Table 2008 Water Table 2000 Change in Water Table 2000 to = 2

3 - Viewshed (revisited) used to create a visibility surface relative to one or more observer points - user specifies the location and height of observation points - output raster shows cell that are not visible from the observation points (pink) and cells that are visible to one or more observation points (light green). - ArcGIS also calculates whether or not each cell is visible from each observation point - the cell values are number of observation points than can see that cell (in this case, 0, 1, 2 or 3) digital elevation model (DEM) viewshed derived from DEM Viewshed classified by cell value - Cut/Fill (used to calculate the change in volume between two elevation surfaces where surface material has been added or removed) 3

4 GIST 3300 / 5300 Spatial Analyst Continued Spatial Analyst Extension - distance - density - interpolation - map algebra - surface analysis Spatial Analyst Extension to ArcGIS - Density tools - Distance tools - Interpolation tools - Map Algebra - Surface Analysis 1

5 Spatial Analyst Interpolation tools - IDW - Kriging - Spline -Topo to Raster - used to interpolate a surface from sample points or contour lines Spatial Analyst Map Algebra tools - access to the Raster Calculator used to: - convert units in a raster (meters to feet) - add, subtract, multiply and divide rasters - build raster spatial models (site suitability) 2

6 Spatial Analyst Reclass tools - used to convert a continuous raster surface to a discrete raster e.g. convert a DEM to classified elevation ranges 3