Copyright © 2006 by Maribeth H. Price 15-1 Chapter 15 Raster Analysis

Copyright © 2006 by Maribeth H. Price

15-1

Chapter 15

Raster Analysis


15-2

Outline

• About rasters

• The grid raster format

• Raster analysis basics– Map algebra– Boolean overlay– Other raster functions

• Using Spatial Analyst


15-3

Outline

• Differences between the raster and vector data models and the benefits and drawbacks of each

• Basic facts about the storage and use of the grid data model

• Getting familiar with basic raster analysis functions

• Using Boolean map algebra to perform overlay analysis with rasters

• Controlling the analysis environment when using Spatial Analyst

• Using Spatial Analyst for raster analysis


15-4

The raster data model

Rows

ColumnsX, Y location

Raster data fileN rows by M columns

X, Y location

Georeferenced to earth’s surface


15-5

Types of raster data

Discrete raster: land use

Continuous raster: DEM

Continuous raster: image

Discrete raster: roads


15-6

Raster Properties

Scroll down for more info


15-7

Raster vs Vector models

• Raster strengths– Simple, robust format– Implicit georeferencing– Storage of continuous

data– Surface analysis– Faster analysis

• Raster drawbacks– Storage space– Lower precision

• Vector strengths– Coordinate precision– Attribute management– Storage space– Flexible cartography

• Vector weaknesses– Surface analysis– Continuous data

storage

“Raster is faster, but vector is correcter.”


15-8

Raster resolution

• Measured by cell size

• Storage space increases dramatically with precision

Vector format 200 m raster 50 m raster


15-9

Raster formats

• Grids– Special format for Spatial Analyst– Required to perform analysis– Integer or floating-point values

• Other formats– JPEG, TIFFs, MrSID, etc.– For display only


15-10

Naming grids

• Names must be 13 characters or less

• Do not use spaces or non-alphanumeric characters to name grids

• No spaces in folders or grids– (underscore is OK)

Remember. No spaces.

http://en.wikipedia.org/wiki/Image:Vader.jpg


15-11

Managing grids

• Working directories tend to get clogged with grids after a while and will need to be cleaned out

• Grids have complex data format

• Copy, move, and delete grids ONLY with ArcCatalog!!!!


15-12

Grid attribute tables

• Created for discrete integer grids only

Geology


15-13

Converting features to raster

Choose one attribute

Grid valueText attribute carried over


15-14

Converting raster to features

Convert only discrete data without too many polygons

To complex to convert

OK to convert


15-15

NoData

• Specific grid value to indicate lack of data

• Often replaced by zero in other formats

• Used to create masks

NoData

NoData


15-16

Georeferencing rasters

Rows

Columns

x,y location

Raster data fileN rows by M columns

x,y location

Georeferenced to earth’s surface


15-17

Georeferencing of images

• Some raster data comes already georeferenced, with an assigned coordinate system.– Landsat image– Digital elevation models– Digital map products from the

USGS

• Other rasters have no georeferencing and must be prepared before use– Scanned maps or photos,

pictures grabbed from Web


15-18

The Georeferencing toolbar is used to link image locations to corresponding locations in an already georeferenced data set. After the links are established a final georeferenced map can be produced.


15-19

Raster analysis

• Basic concepts

• Map Algebra

• Boolean overlay


15-20

Map Algebra

• Rasters are essentially arrays of numbers

• Can be added, subtracted, etc

• Line up matching cells vertically

5 7

2 4

3 2

1 6

8 9

3 10

Ingrid1

+

Ingrid2

=

Outgrid

Fig. 15.4. Map algebra


15-21

Map Algebra expressions

• Convert precipitation in cm to inches– [Precip] / 2.54

• Compute earth volume to be moved– [InitialSurface] – [Finalsurface]

• Enter models based on multiple inputs

[Precip] * 2 + [Slope] * 4 / ( [Erode] – [Vegcover]

• Logical expressions– [Elevation > 1400]


15-22

Conversions

[Precip_cm] / 2.54

Precip in cm Precip in inches


15-23

Cut and fill on a site

[Initial surface] – [final surface]

Cut

Fill


15-24

Model expressions

Complex expressions with multiple inputs to calcuate risk or hazard index.

Runoff in cm based on four input grids: precip, slope, soil infiltration, and vegetation cover.

[Precip] * 2 + [Slope] * 4 / ( [Erode] – [Vegcover]


15-25

Logical expressions

• Produce a Boolean grid of 1’s and 0’s– 1 = True– 0 = False

[EarthMove] > 0

1

0

Elevation > 14001

0


15-26

Boolean expressions

A AND B

A XOR B A NOT B

A OR B

A B

0

1

Blue circles represent areas where condition holds

(True) = 1


15-27

Boolean Map Algebra

True (1)False (0) A AND B A OR B A NOT B A XOR B

InputsA

B

Output

Compare values of corresponding cells and determine output value


15-28

Boolean overlay using map algebra

0

11

0

11

0

11

AND =

Find intersecting areas where all conditions are true to determine lodgepole habitat. Equivalent of vector Intersect


15-29

Another way to look at Boolean intersection

0

11

0

11

0

11

x =

Boolean AND is equivalent to multiplication. So multiplying layers works just as well.

1 AND 1 = 1

1 AND 0 = 0

0 AND 1 = 0

0 AND 0 = 0

1 × 1 = 1

1 × 0 = 0

0 × 1 = 0

0 × 0 = 0


15-30

Additive Boolean overlay

0

1

2

0

11

0

11

+ =

Instead of multiplying, add the layers together to create a ranked probability result.


15-31

Other raster analysis techniques

• Reclassification

• Surface functions

• Distance functions

• Density functions

• Interpolation

• Neighborhood functions

• Zonal functions


15-32

Reclassify

Convert one set of grid values to another

Manual or classify

Slope High slope/low slope


15-33

Surface analysis

DEMSlopeAspectHillshadeContouring


15-34

Viewshed analysis

• Areas visible from a set of observation points


15-35

Distance functions

Straight line distance

Cost path distance

Buffers


15-36

Lowest cost path1. Create start/stop

shapefiles

2. Create cost grid

3. Calculate cost distance grid and cost direction grid

4. Find lowest cost path

Elevation Slope

Cost distance Cost direction


15-37

Density

• Calculate from point distributions


15-38

Interpolation

• Estimate gridded values between known points

• Three options– Inverse distance

weighted– Kriging– SpliningRainfall grid from

weather station data


15-39

Neighborhood functions

Output gridInput grid

3 1 4

6 2 1

5 4 3

3 1 4

4

1

3

4

3 1 4 4

1

2

4

1

1

2.0 2.5 3.4 2.8 3.6

3.8 3.2 2.9 3.0 2.3

3.1 3.3 2.5

WindowTarget cell

Averaging function


15-40

Neighborhood majorityHigh slope/low slope areas

Before 5x5 majority filter After two passes of 5x5 majority filter

Useful for simplifying rasters prior to conversion to polygons


15-41

Cell statistics

Calculate statistics for a stack of grids


15-42

Zonal statistics

• Zones defined by a layer

• Value grid

• Calculates value statistics for each zone

Watersheds

Average slope


15-43

Using Spatial Analyst

• Executing functions

• Temporary vs permanent output grids

• Resampling and coordinate systems

• Analysis environment options– Workspace– Extent– Cell size– Masks


15-44

Menu tools

• Most commonly used functions are in the Spatial Analyst toolbar

• More functions can be found in ArcToolbox

• Some tools are slightly different than the menu tool—may have more or different options


15-45

ArcToolbox

• Hundreds of additional functions found in ArcToolbox

• Use them in scripts and models if desired

• Spatial Analyst is an incredibly powerful raster processing package


15-46

Raster Calculator

Use for executing map algebra expressions

Use to access Toolbox functions by directly typing tool command and arguments into the calculator.


15-47

ModelBuilderUse Spatial Analyst tools in ModelBuilder just as you can use any other tools.

This model generates watersheds and streams from a digital elevation model (DEM) using the Spatial Analyst hydrologic functions.


15-48

Analysis output types

• Permanent grids– Given name and location at time of creation– Can be used by other map documents

• Temporary grids– Created in working directory*– Rename in map document once created– May be converted to permanent grids later– Deleted from disk when removed from map!

*Set location where grids are created by default


15-49

Output coordinate systems

• Grids best stored in projected coordinate system for analysis

• Coordinate systems will be resampled or reprojected on the fly during analysis if needed– Convenient, but time-consuming

• Coordinate system handling– Output matches first raster input– Vectors will be reprojected to match rasters– User can override defaults by specifying that all raster

output will be in data frame coordinate system


15-50

The analysis environment

• General options– Working directory– Mask grid– Coordinate system options

• Analysis extent

• Analysis cell size


15-51

General options

• Working directory

• Analysis mask

• Analysis coordinate system


15-52

Analysis mask

Mask grid Elevation after masking

NoData

Data

Applies to ALL output grids

Masks may be grids, polygon shapefiles or feature classes. Coverages may not work properly.


15-53

Clipping a grid with masks

[Elevation]

mask

“Clip” grid by using Raster Calculator with a mask. Specify the output grid is the same as the input grid.


15-54

Clipping a grid with the toolbox


15-55

Analysis extent


15-56

Cell size and extent

Analysis extent Analysis cell size


15-57

Options/Environments

• The Spatial Analyst options set in the menu take precedence over the Environment Settings for functions executed from the menus.

• Tools, however, will honor the environment settings instead of the options settings


15-58

Did I mention not to use spaces in folder names?

http://en.wikipedia.org/wiki/Image:Vader.jpg

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Copyright © 2006 by Maribeth H. Price 15-1 Chapter 15 Raster Analysis