Chapter 10 - John Donoghue 4 Map Overlay: Symmetric Difference •Combines and keeps parts not in common –Works on Points, lines, or polygons Map Overlay: Clip •Used to cut out

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Chapter 10

Geoprocessing

(Continued)

Copyright © McGraw-Hill Education. Permission required for reproduction or display.

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More map overlay geoprocessing functions

Copyright © 2015 by Marbeth H. Price

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Map overlay

• Joins features based on common location

• Forces features to split when they overlap each other, creating new features

• Enforces one-to-one cardinality between features in order to join attributes

Copyright © 2015 by Maribeth H. Price

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• Pick this back up with

– Symmetric difference

– Clip

– Erase

– Dissolve

– Merge

– Buffer


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Map Overlay

• Intersect: combines and keeps common features in both layers

– Points, lines, or polygons

• Union: combines and keeps all features from both layers

– Polygon-on-polygon overlay only

Copyright (c) 2008 by John Wiley and Sons

Map Overlay: Symmetric Difference

• Combines and keeps parts not in common

– Works on Points, lines, or polygons

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Map Overlay: Symmetric Difference

• Combines and keeps parts not in common

– Works on Points, lines, or polygons

Map Overlay: Clip

• Used to cut out a piece of one feature class using one or more of the features in another feature class. Works like a cookie cutter

• Does not combine attributes from both layers

– Only the attributes from the input layer retained in the output layer

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Map Overlay: Clip

• Useful for creating a subset of a larger feature class for a study area or area of interest

• Clip Features can be points, lines, and polygons, depending on the Input Features type.

– When the Input Features are polygons, the Clip Features must also be polygons

– When the Input Features are lines, the Clip Features can be lines or polygons

– When the Input Features are points, the Clip Features can be points, lines, or polygons

Map Overlay: Clip

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Map Overlay: Erase

• Creates a feature class by overlaying the Input Features with the polygons of the Erase Features.

• Only the parts of the input features falling outside the erase features outside boundaries are copied to the output layer

• Does not combine attributes from both layers– Only the attributes from the input layer retained in the output layer

Map Overlay: Erase

• Works on Points lines or polygons

– A polygon erase feature can be used to erase polygons, lines, or points from the input features

– A line erase feature can be used to erase lines or points from the input features

– A point erase feature can be used to erase points from the input features.

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Map Overlay: Split

• Splits features in an Input layer to create a subset of multiple output feature classes

– Creates individual output feature classes for each split feature

– The Split feature Field's unique values form the names of the output feature classes


– Only the attributes from the input layer retained in the output layer


Map Overlay: Split


– Only the attributes from the input layer retained in each of the output feature classes (or layers)


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Length/area changes


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Clip/erase can change the lengths or areas of output features

Geodatabase feature classes have Shape_Length and Shape_Area fields that are automatically updated

All other length or area-based fields are NOT automatically updated and will need to be recalculated

A user-defined MILES field would contain incorrect lengths after the clip.

Other map analysis functions

Dissolve

Buffer

Append and Merge


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Dissolve

• Creates a new feature class by merging adjacent features (polygons, lines or points) that have the same value for a specified attribute field


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Dissolve

• Used to create a simplified feature class from one that is more complex


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Dissolving


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Dissolve polygons on habitat class field

Dissolve lines on street name field

Main St Choose an attribute to dissolve on…

Dissolve

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

• However, you can choose to summarize the other attributes– Eg. Sum_acres


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Buffering

• Creates polygon areas around a specified distance of input features


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Single buffers around linesMultiple buffers around points

Append


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Places features from one feature class into an existing feature class of the same type.

Unlike union, it does not attempt to split or change features.

The features are simply brought in. If they overlap, you’ll have double features.

To bring the attributes with the features, the attribute tables of the two layers to be merged (input layers) have to match exactly

This function has many caveats and issues. Read the Help before attempting for the first time.

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Merge


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Combines two feature classes of the same type into a new combined feature class

Does not attempt to split or change the features. Overlapping features are permitted.

You can use a Field Map option to control which input fields are included in the output feature class.

The attribute tables of the input features can be different.

This function has many caveats and issues. Read the Help before attempting for the first time.

Performing efficient map overlay

• Map overlay is time and memory-intensive

• Minimize number of features to be processed

– Use clip to work with a subset of features in a small smaller area of interest

– Use definition queries to exclude features you don’t need

– Dissolve layers if you are only interested in one attribute

– Dissolve complex layers before other map overlay functions

– Use the appropriate overlay technique• Don’t use UNION when INTERSECT will do


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Map Overlay problems: Slivers

• Tiny polygons created during geoprocessing

• Result of slight differences in boundaries

• Can build up as a result of multiple operations

• Nuisance for some applications of GIS

– Maps from overlays


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Slivers

• Tolerances can be used to reduce problems with slivers.

– Tricky to set.

– Not good if tolerance is set too small

– Will degrade accuracy if tolerance is set too large

• Affects output layer only, not the input layer(s)


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Multiple geoprocessing steps can progressively degrade the accuracy even with an appropriate tolerance—so be careful.

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

END LECTURE


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Documents

Chapter 10 - John Donoghue 4 Map Overlay: Symmetric Difference •Combines and keeps parts not in common –Works on Points, lines, or polygons Map Overlay: Clip •Used to cut out