Module 4.3
NSF DUE-1205110;
0903270
GeoTEd Partners
Module name and number
NSF DUE-1205110;
0903270
Overview
 Buffers
 Overlays
 Clip
 Intersection
 Union
 Dissolve
 Merge
 Spatial Joins
Module 4.3, Overlays and Buffers
NSF DUE-1205110;
0903270
Buffers
 Can add a buffer to a point, line, or polygon
 Pay attention to map units when specifying buffer
distance
NSF DUE-1205110;
0903270
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 way
 Specific overlay techniques
 Create an output layer
Layer 1
Layer 2
New
Layer
NSF DUE-1205110;
0903270
Projections
 Before 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
NSF DUE-1205110;
0903270
Overlaying Vector Data
 Common vector overlay/geoprocessing operations
 Clip (cookie cutter)
 Intersect – keeps only common features between two
layers
 Union – combines all features in two layers
 Dissolve – combine (simplify) features in one layer
 Merge – combine two layers into one
 Always recalculate areas/perimeters of polygons after
one of these operations
NSF DUE-1205110;
0903270
Clip
 Use one layer as a boundary to clip (cut) out features
from another layer
NSF DUE-1205110;
0903270
More on clip
 Be 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 layer
 Can use selected features or all features as a clip layer
NSF DUE-1205110;
0903270
Intersect
 Combines two layers and outputs only the features
common (spatially) to both layers
NSF DUE-1205110;
0903270
More on Intersect
 Be sure you know which layer is the input layer and
which is the overlay layer
 The overlay layer must be polygons
 Attributes will come from both layers
 Will need to re-calculate polygon areas
NSF DUE-1205110;
0903270
Union
 Combines two layers and all features (whether or not
they overlap)
NSF DUE-1205110;
0903270
More on Union
 Be sure you know which is the input and which is the
overlay layer
 May use all or selected features as overlay
 Overlay must be polygon
 Attributes will come from both layers
NSF DUE-1205110;
0903270
Dissolve
 Combine features in a layer based on one attribute
 Example: aggregate counties in US by state name
NSF DUE-1205110;
0903270
More on Dissolve
 Have to specify common feature
 Points, lines, and polygons
 Need to specify how to deal with remaining attributes
 First or last
 Sum, min, max, mean
NSF DUE-1205110;
0903270
Merge
 Combine features from two or more layers into one
layer
NSF DUE-1205110;
0903270
More on Merge
 Can have multiple layers
 Must specify which layer will define attribute table
fields for the output
 Need to be aware of:
 Differing attribute tables
 Presence of gaps (spatially) in merged layers
NSF DUE-1205110;
0903270
Summary: vector overlay
 Powerful tools for creating new information
 Consider attributes
 Recalculate area and perimeter
 Files with different coordinate systems can cause
problems – convert coordinate systems first
NSF DUE-1205110;
0903270
Spatial Join
 Spatial joins match the join layer to a target layer –
creating a new output layer that contains attributes
from both layers
 Based 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
Questions?
Module name and number
NSF DUE-1205110;
0903270