raster analysis. learning objectives develop an understanding of the principles underlying lab 4...
TRANSCRIPT
Raster Analysis
Learning Objectives Develop an understanding of
the principles underlying lab 4 Introduce raster operations
and functions Show how raster analysis can
be applied to topographic surfaces
Raster Data
Derived Data By taking rasters and
operating on them we can create additional data
inRaster * 2 = outRaster Slope(inRaster) = outRaster
Creating New Data With Rasters
Digital Elevation Model Slope Model
Raster analysis
Typically 4 steps Base data Derived data Classified data Combined data
Reclassifying raster data One reason is to set specific values to NoData to
exclude them from analysis. Another reason is to assign values of preference,
priority, sensitivity, or similar criteria to a raster.
Operations
Boolean (AND, OR, NOT, XOR)
Arithmetic (+,-,*,/) Mathematical (trig, log, etc.) Logical (<,>,=,<>,etc.) Overlay Derivation Transformation
Boolean (AND, OR, NOT, XOR)
Boolean And
Boolean Or
Arithmetic (+,-,*,/)
Arithmetic (+,-,*,/)
Raster Calculator
Working with NoData Similar to logical values, NoData (Null) values also
influence the evaluation of expressions. This special value indicates that there is no
information associated with the cell.
In general, a Map Algebra expression will return NoData for a cell if any of the corresponding input cells have NoData.
Functions
Local Global Focal Zonal
Local
Very simple Operate on each
cell individually Arithmetic and
boolean are examples of Local functions
Global
Perform operations based on an entire input grid
E.g. Global statistics
Focal
Evaluates a new grid by summarizing statistics in the neighborhood around each cell.
Filters
Low-pass filter
Also referred to as a mean filter
Low-pass filter
DEM Low-pass filter
High-pass Filter
Also referred to as edge enhancement
High-pass filter
DEM High-pass filter
Additional Filters
Zonal
Operations based on zones of like values in a grid
Zonal geometry
Zonal stats
Zonal
Input
Output
Recap
Distance
Euclidean Rectilinear Weighted
Weighted Distance
Sometimes referred to as cost path or travel cost
Surface Analysis
DerivativesSlopeAspectHillshade
•Visibility- Viewshed- Line of Sight
•Feature Interpolation- Interpolate Shape- Interpolate Poly To Patch- Surface Length- Surface Spot- Contour
•Volume- Surface Volume- Cut Fill- Surface Difference- Polygon Volume- Extrude Between
Slope: steepness
Aspect: direction of steepest slope
Hillshade: steepness and directionrelative to light source
Slope Slope is calculated as the maximum rate of change
in values between each cell and its neighbors. Slope may be expressed as either degrees (e.g., 45
degrees) or percent (e.g., 50%).
Measures of slope in degrees can approach 90 degrees and measures of slope in percent can approach infinity.
Aspect The cell values in an aspect grid are compass
directions ranging from 0 to 360. North is 0 and in a clockwise direction, 90 is
east, 180 is south, and 270 is west.
Input grid cells that have 0 slope (flat areas) are assigned an aspect value of -1.
Hillshade Hillshading creates a
hypothetical illumination of a surface by setting a position for a light source and calculating an illumination value for each cell based on the cell's relative orientation to the light, or based on the slope and aspect of the cell.
Viewshed The viewshed identifies the cells in an
input raster that can be seen from one or more observation points or lines.
Each cell in the output raster receives a value that indicates how many observer points can see the location.
Line of Sight
Distance
Ele
vati
on
0 75 150 225 300 345
Observer
15
61
81
20
62
31
Target
Volume Below Plane
Summary Most of the power of the Spatial Analyst
is found within Map Algebra. The Raster Calculator is your friend. You can construct a extremely complex
Map Algebra expression using this interface