NSF DUE-1205110; 0903270

Buffers, Overlays, and Spatial Joins

Module 4.3

NSF DUE-1205110; 0903270

GeoTEd Partners

Module name and number

NSF DUE-1205110; 0903270

OverviewBuffersOverlays

ClipIntersectionUnionDissolveMerge

Spatial Joins

Module 4.3, Overlays and Buffers

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BuffersCan add a buffer to a point, line, or polygonPay attention to map units when specifying

buffer distance

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What is an overlay?Take two or more layers

Typically with the same extent (they are one on top of another)

Combine the layers in some waySpecific overlay techniques

Create an output layer

Layer 1Layer 2

New Layer

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ProjectionsBefore beginning any overlay type operation,

be sure you have the same coordinate systems throughout the map document

This will avoid crashing the program or inaccurate results

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Overlaying Vector DataCommon vector overlay/geoprocessing

operationsClip (cookie cutter)Intersect – keeps only common features

between two layersUnion – combines all features in two layersDissolve – combine (simplify) features in one

layerMerge – combine two layers into one

Always recalculate areas/perimeters of polygons after one of these operations

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ClipUse one layer as a boundary to clip (cut) out

features from another layer

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More on clipBe sure you have correct layers as the input

layer (the one to be cut) and the clip layer (the one to use as a cutting guide)

Must use polygons as the clip layerCan use selected features or all features as a

clip layer

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IntersectCombines two layers and outputs only the

features common (spatially) to both layers

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More on IntersectBe sure you know which layer is the input

layer and which is the overlay layerThe overlay layer must be polygonsAttributes will come from both layersWill need to re-calculate polygon areas

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UnionCombines two layers and all features

(whether or not they overlap)

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More on UnionBe sure you know which is the input and

which is the overlay layerMay use all or selected features as overlayOverlay must be polygonAttributes will come from both layers

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DissolveCombine features in a layer based on one

attributeExample: aggregate counties in US by state

name

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More on DissolveHave to specify common featurePoints, lines, and polygonsNeed to specify how to deal with remaining

attributesFirst or lastSum, min, max, mean

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MergeCombine features from two or more layers

into one layer

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More on MergeCan have multiple layersMust specify which layer will define attribute

table fields for the outputNeed to be aware of:

Differing attribute tablesPresence of gaps (spatially) in merged layers

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Summary: vector overlayPowerful tools for creating new informationConsider attributesRecalculate area and perimeterFiles with different coordinate systems can

cause problems – convert coordinate systems first

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Spatial JoinSpatial joins match the join layer to a target

layer – creating a new output layer that contains attributes from both layersBased on their proximity Based on inputs from you

To learn more about Spatial Joins, open ArcMap help

Search for “Spatial Joins by Feature type”

NSF DUE-1205110; 0903270

Module name and number

Questions?