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