copyright © 2006 by maribeth h. price 8-1 chapter 8 geoprocessing

Copyright © 2006 by Maribeth H. Price

8-1

Chapter 8

Geoprocessing


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Outline

• Spatial analysis and overlays

• Types of overlays

• How to perform overlays

• Other geoprocessing tools– Clip, dissolve, buffer, append

• Tips for overlay

• Geoprocessing methods


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Geoprocessing• Executing analysis functions and tools• Stringing together analysis functions to achieve

a result


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Geoprocessing Environment

• Familiar ways to execute functions– Menus– Tools in ArcToolbox

• Environment settings• New ways to execute

functions– Command line– Creating new tools with

ModelBuilder– Writing scripts


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Key analysis functions

• Overlay– Without attributes (clip, erase)– With attributes (intersect, union)

• Dissolve

• Buffer

• Append


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Without attributes (extraction)

Extracts portions of features based on an overlay layer.

Clip keeps the features inside the feature boundary.

Erase keeps the features outside the feature boundary.clip/erase layer

Ignores interior boundaries; brings attributes through unchanged.


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On-the-fly clipping

• Temporary clip applied to a map layout

• Does not create new layers

• Can be performed on many layers simultaneously

• Can be removed when no longer needed

• Set as a data frame property


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Overlay with attributes

• Similar to a spatial join

• Combines attributes based on common location (inside join)

• Enforces one-to-one cardinality between features


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The problem with spatial joins

Associate the land use type with the road

BUT

The road does not stop at the land use boundary.


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Map overlay

A map overlay forces features to split at polygon boundaries.


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Now a land use type can be associated with each road.

Sum to give total length of each land use.


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Steps to overlay

• Combine features spatially, producing all possible new features

• Combine attribute tables, bringing original values from each table and assigning to each new polygon

• New spatial data set is created with features and attribute table


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1. Combine spatial features by producing all possible polygons

Geology

Slope class


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Geology

Slope class

2. Combine tables, bringing original values from each table for each new polygon


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Intersect: keeps common areas

Union: keeps all areas

Overlay types


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Geology

Slope class

Union example

Find combinations of geology and slope that are high risk for landslides


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Intersect example

Find residential areas that are at risk of radon infiltration from the Opeche formation

Residential areas

Opeche formation


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Hazards mapping

Residential areas Opeche formation Areas of both


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Habitat analysis with Intersect

Use of multiple input layers to define regions with specific characteristics


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Other overlay examples

Point-in-polyWells in geology

Line-in-polyRoads in landuse

Poly-on-polyZoning on geology


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Output geometry choices

Intersecting polygons: choose polygons, lines, or points

Intersecting lines: choose lines or points

Output dimension must be less than or same as lowest input dimension.


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Output geometries

Intersecting polygons and lines


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Overlay operations summary

union

clip

erase

intersect

outputoverlayinput

Attributes not joined

Attributes joined


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Using a tool


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The Intersect tool


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Dissolving

Choose to summarize other

attributes

Only attribute preserved in output layer


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Dissolving

Dissolve polygons on habitat class

Dissolve lines on street name

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Main St Choose an attribute to dissolve on…


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Dissolving eliminates all of the attributes in the table except the dissolved one…

However, you can choose to summarize the other attributes…

Avg_ crown_cov_percentSum_acres


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Buffering

Constructs polygon areas within a specified distance of features.

Single buffers around linesMultiple buffers around points


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The Buffer tool


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Buffering with the NONE option

Buffering with the ALL option

The Dissolve option


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Append


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Appending attributes

• Combines feature classes without overlay

• To bring attributes along, tables of input features classes must match exactly.– same classes, same order, same definitions


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Efficient overlay

• Overlay is time intensive

• Minimize number of features– Intersect geology

and elevation first– Dissolve vegetation

before intersecting with other layers


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• Settings that influence function inputs and outputs


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Environment Settings

• Hierarchical—can be set for the document or models or individual tools

• Most specific setting takes precedence


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Using environment settings

• You are clipping a series of layers in order to place them in a new geodatabase with a different coordinate system.

Slow way:

Clip each file

Project each clipped file and put into final geodatabase

Delete intermediate files

Clever way:

Set the current workspace and output geodatabase in Environments

Clip each file. Output automatically projected into the final geodatabase.


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Coordinate systems

• Output coordinate system rules– If output is placed in a feature dataset, the output CS

matches the dataset CS– If the environment setting is set to a coordinate

system, that CS is used.– If the environment is not set, the CS of the first input

data set to the tool will be used.

• Using projected coordinate systems is best– More convenient when areas/lengths are needed– Fewer distortions to worry about


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Managing slivers with settings

• Tiny areas of overlap created during geoprocessing

• Result of slight differences in boundaries

• Can build up as a result of multiple operations

• Nuisance for some applications

You can reduce sliver problems by specifying an XY tolerance in the Environment Settings


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Tolerances• During geometry operations (like buffering, intersect,

editing, etc) tolerances are used to determine whether two features should be the same. The tolerance is the maximum distance a feature vertex can be moved to coincide with another vertex.

Tolerance

ac

b

ac

b

gap

overlap

Coincident lines?

Clustering


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The XY tolerance• XY Tolerance is a property of a feature class

– Set at time feature class is created– Used during geoprocessing operations such as clip,

buffer, intersect, etc.– Avoids tiny slivers and dangles– Can override it using geoprocessing environment

settings if needed (General Settings)

• Default setting– 0.001 meter– 0.003281 feet (0.03937 inches)– 0.0000000556 degrees

Defaults are calibrated to preserve coordinate precision, not resolve differences between data sets during geoprocessing.


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Setting the tolerance

If slivers are a problem for your analysis, you may wish to increase the XY Tolerance in the Environments settings to several meters or more.

The XY Tolerance box is way down the list of settings under the General Settings heading.

Measure to see typical size of slivers. If 5 meters, then an XY tolerance of 8-10 meters or so might work well.


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Other ways to execute functions

Menus

Tools

Command line

ModelBuilder

Scripts


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Command line geoprocessing• Type command instead of using tool or menu• Can be faster than using the mouse• Smart interface helps you complete commands

correctly with right syntax

Type commands here

Messages/progress here


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ModelBuilder• Create models built from sequences of tools• Store processing steps for later reference• Execute models repeatedly with different inputs• Share models with others


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Scripts

• Programs to execute geoprocessing steps

• Include looping, if-then control etc.

• Choose from several languages

• Python used by ESRI for examples

• Can convert models to scripts and then modify them


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Areas and lengths

• Often analysis involves areas or lengths– Total length of

streams in each watershed?

– Total snail habitat area?


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Geodatabase geometry measures

• Automatically created and maintained– Usually appear at end of table

• Shape_Area field• Shape_Length field

– Units will match units of the coordinate system


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Coverage geometry measures

– Automatically constructed– Updated whenever the BUILD or CLEAN

command is used to update topology• LENGTH field in arc tables• AREA field in polygon tables• PERIMETER field in polygon tables


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Shapefile geometry measures

• Shapefiles DO NOT create or maintain these fields automatically!– Must be created and updated manually


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Be careful!

Before After

AREA/LENGTH/PERIMETER fields in shapefiles are NOT automatically updated if features change.

Don’t use one of these fields unless you are CERTAIN that they are correct.

These area/perimeter fields are likely from a coverage


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User geometry calculationsCreate field to hold values

Choose type, coordinate system, and units

Not automatically updated if features change!!!


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Test yourself: True or False?

• A field named AREA in a shapefile will always have the correct area?– False. Shapefile area fields are not automatically

maintained.

• A field named AREA in a geodatabase will always have the correct area?– False. Automatically updated fields in a geodatabase

are called Shape_Area.

• A field named AREA in a coverage will always have the correct area?– True. Automatically updated area fields in a

coverage ARE called AREA.

copyright © 2006 by maribeth h. price 8-1 chapter 8 geoprocessing

Documents

spatial features

overlaycombine features

land use boundary

geoprocessing copyright

land use type

pricehabitat analysis

erasewith attributes

priceoutlinespatial