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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
Distance - used to create a continuous raster distance surface from point, line or polygon features
- you can also create a direction surface and allocation surface (Thiessen polygons)
- to create a distance surface, ArcGIS calculates the distance to the nearest point, line or polygon
- in this example, the grid cells values are the distance to the nearest point (selected west Texas cities)
- to create a direction surface, ArcGIS calculates the direction to the nearest point, line or polygon
- in this example, the grid cell values are the direction to the nearest point (selected west Texas cities)
- to create an allocation surface, ArcGIS allocates cells to the nearest point (creates Thiessen polygons)
- in this example, the grid cells values are feature class codes corresponding to the nearest point
Copyright© 2015, Kevin Mulligan, Texas Tech University
Density - used to create a continuous raster density surface from point or line features
- for a density surface, ArcGIS calculates the density of points or lines per unit area
- in this example, the grid cell values represent the density of playa wetlands per square kilometer
- to calculate the density of playa polygons, we must first calculate the playa polygon centroids (points)
- then we calculate the playa density surface from the playa centroids
>
playa wetland polygons
>
playa polygon centroids (points)
playa density surface
(number of playas per sq kilometer)
- a density surface can also be weighted by the value of an attribute
- in this example, the playa point values are weighted by the area of each playa (hectares)
playa density surface weighted by area
(hectares per sq kilometer)
Copyright© 2015, Kevin Mulligan, Texas Tech University
Interpolate to Raster - used to interpolate a continuous raster surface from sample point values
- interpolation methods include Spline, IDW, Kriging and Topo to Raster
- to interpolate a surface, ArcGIS estimates values for all of the grid cells between points
- in this example, we used well measurements to estimate the water table surface in 2008
measurement wells in Castro County
interpolated water table elevation surface
- we then classified the water table elevation surface and created contours to depict the surface
original surface
classified surface
classified surface with contours
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 elevation 2008
=
water table elevation 2000
change from 2000 to 2008
Copyright© 2015, Kevin Mulligan, Texas Tech University
Surface Analysis - particularly useful set of tools used to analyze digital elevation models
- a few of the most useful tools in the Surface Analysis toolbox include the following:
Hillshade - used to create a shaded relief layer from a digital elevation model
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digital elevation model (DEM)
shaded relief (hillshade)
classified DEM overlaid on a hillshade
Contour - used to create vector contour lines from a DEM
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digital elevation model (DEM)
contours derived from DEM
Copyright© 2015, Kevin Mulligan, Texas Tech University
Slope - used to create a slope map from a DEM
- the cell values in the slope raster can be calculated in either degrees or percent slope
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digital elevation model (DEM)
slope raster derived from DEM
Aspect - used to create an aspect map from a DEM)
- the cell values in an aspect raster give the compass direction the slope is facing
>
digital elevation model (DEM)
aspect raster derived from DEM
Viewshed - used to create a visibility surface relative to one or more observer points
- user specifies the location and height of observation points
- ArcGIS then 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
Copyright© 2015, Kevin Mulligan, Texas Tech University
GIST 3300 / 5300
Geographic Information Systems
Spatial Analyst Continued
Spatial Analyst Extension
- distance
- density
- interpolation
- map algebra
- surface analysis
Geographic Information Systems
Spatial Analyst Extension to ArcGIS
- Density tools
- Distance tools
- Interpolation tools
- Map Algebra
- Surface Analysis
Geographic Information Systems