Raster Analysis

• Raster math

• Topography: Slope, aspect, contours

• Reclassify

• Raster / Vector Conversions

• Statistics: min, max, mean, std. dev.– Local, Neighborhood, Zonal

• Histograms

• No Data / Null Value Issues

• Future:– Distance (cost)

Jim’s Secrets for Rasters

• Keep the overall size of the rasters below 200 Meg

• Use “Resample” to convert to relatively small “IMG” files (or GRIDS)– Data Management Tools -> Raster ->

Raster Processing -> Resample

• Watch “Results” for errors– Geoprocessing -> “Results”

• Use “Task Manager” when Arc crashes

Task Manager

• Right Click on Menu Bar, “Start Task Manager”

• “End Process” on all “Arc” processes

Raster Statistics

• Local– Operate on one pixel in each raster

• Neighborhood (or Focal)– Operate on a few pixels around each pixel

• Zonal– Collection of regions– Region is a contiguous area of the same

pixel values

• Global– Operate on the “whole” raster

Local Stats

• “Cell Statistics” computes stats on all the values for a pixel in a multi-band raster.

• Example for “Sum”:

Esri ArcGIS 10 Help

Neighborhood (or Focal)

• Result=Operation on pixels nearby

12 20 23 34 40

15 23 30 31 39

15 22 29 30 40

14 20 28 29 38

13 19 25 32 37

Columns

Rows

Focal Statistics

• Computes stats on pixels around an existing pixel

• Example for Sum:

Esri ArcGIS 10 Help

Zonal Statistics

• Computes statistics for defined regions (features)

• Tools:– Zonal Statistics:

• Outputs a raster (not sure of the value)

– Zonal Statistics as Table:• Outputs a table• Used to compute all kinds of valuable things:

– Percent land cover– Percent impervious cover– Percent water below a certain depth– Etc.

Zonal Statistics

Esri ArcGIS 10 Help

Zone Layer

Value layer

Output layer (or table)

• Con(<condition>,<true>,<false>)

• Given a raster “condition”:– Puts the true value where true and false

value where false

Conditional Operator

Con!

Elevations over 3000 meters

• Con("W100N40.DEM“>3000,0,1)

Elevations over 3000 meters

Issues with No Data

Issues with Signed Data

Convert Values to “No Data”SetNull("W100N40.DEM"==55537,"W100N40.DEM")

Rasters Draw as “Solid” Color

• Click on “Type:” and select “Minimum-Maximum”

• ArcGIS will compute statistics

Histograms

Exporting Histograms

Titles and Axis Labels!