Analysis in 9.2:
Overlay
Analysis
• Usually starts with selection…
• Why?
• Because
– Many steps in analysis are simply selection by
• Attribute OR
• Location
– Most of the OVERLAY functions’ operations
work with selected features
Here are some actions…
• Check that
– Layer(s) are the ones you want
– The selection options are what you want
• Check the bottom of the dialog and make
sure it is doing what you want it to
• That sounds easy but quite often NOT
easy
• Here are some of the options
Options: Intersect
Options:Touch Bndy of…
Options: Have their Center in…
Options: Are completely w/in…
Options: w/in a distance of…
Options: are contained by…
Selection Summary
• Allow the selection of
features based on
criteria
• Is a pretty rich set of
operations
• BUT
– Some share name with
the REAL overlay
operations but don’t
deliver the same result
•
•
•
•
•
•
•
•
•
•
•
Intersect
Within distance of
Completely contain
Completely within
Have center in
Share segment w/
Touch boundary of
Are identical to
Are crossed by outline
Contain
Are contained by
Overlay Operations
• Overlay operations are like the
“select by location” functions but not
exactly the same.
• Both use two layers
– Select by location just selects features
in one of the layers
– Overlay operations create new feature
classes
Toolbox
Overlay: Erase
• From Help: Erase feature removes a chunk of the
input feature
• It is important to keep Input and Erase feature
correct or GIGO
Overlay: Union
• This computes a geometric
intersection of the input and output
feature classes . ALL features will be
written to the output feature class
with the attributes from both.
• Or: This tool builds a new feature
class by combining the features and
attributes of each feature class.
Overlay: Union
UNION: another view
Overlay: Identity
• Computes a geometric intersection (logic)
of the input and identity features
• Input features (or portions thereof) that
overlap the identity features will get their
attribute features from the identity
features
• To put it another way: combines the
portions of features that overlap the
identity features to create a new feature
class.
Overlay: Identity
• Computes a geometric intersection (logic)
of the input and identity features
Clips by the Input
• Input features (or portions
thereof)
that
feature
and then
overlap the identity features
will get
UNIONS
what is
attribute features from the identity
left.
features
• To put it another way: combines the
portions of features that overlap the
identity features to create a new feature
class.
Overlay: Intersect
• Computes the geometric intersection
of the input features or portions of
features COMMON to all layers.
• This tool builds a new feature class
from the intersecting features
common in both feature classes.
Available with any ArcGIS license.
Overlay: Intersect
• Computes the geometric intersection
of the input features or portions of
features COMMON to all layers.
Intersect is NOT
• This tool buildsthe
a new
class
same feature
as
from the intersecting
Identityfeatures
common in both feature classes.
Available with any ArcGIS license.
Overlay: Symmetric Diff
• Computes a geometric intersection of the
input and update features. Any features
COMMON to ONLY one of the inputs will
be written to the output
• Or to put it another way: This tool creates
a feature class from those features or
portions of features that are not common
to any of the other inputs.
Overlay: Symmetric Diff
This is the olive drab
• Computes a geometric
intersection
of the
feature
showing through
input and update features.
a hole inAny
thefeatures
yellow
COMMON to ONLY one offeature
the inputs
will
!
be written to the output
• Or to put it another way: This tool creates
a feature class from those features or
portions of features that are not common
to any of the other inputs.
Overlay: Update
• Computes the geometric intersection of
the input and update features. The
attributes and geometry of the input
features are replaced by the update
features.
• This tool updates the attributes and
geometry of an input feature class or layer
by the Update feature class or layer that
they overlap
Overlay: Update
• Computes the geometric intersection of
the input and update features. The
attributes and geometry of the input
features are replaced by the update
features.
• This tool updates the attributes and
geometry of an input feature class or layer
by the Update feature class or layer that
they overlap
Overlay: Summary
IDENTITY
INTERSECT
SYM-DIF
UNION
UPDATE
Buffer
• Buffering is an additional tool used in
Not
Analysis.
Dissolved
Dissolved =
All
Summary
• The analysis tool in ArcGIS are very
powerful
• And fairly easy to understand … one
at a time!
• It is use by combining the overlay and
selection tools to solve a problem
that is the problem!
A Problem
• Find location for a laboratory
–
–
–
–
–
Within 300 meters of a sewer line
NOT within 200 meters of a stream
Parcel in a specific landuse (900 is forest)
Not on steep slopes (<=20%)
>=100 acres but less than 200 acres
• THIS IS A CLASSIC LOCATION
PROBLEM
Sewers
w/in 300m
Buffer
Sewbuf
300
Sewer
OK
ERASE
Streams
not w/in
20m
Buffer
Landuse:
Lucode =?
Query
Slope<=20%
Strmbuf
20
IDENTITY
IDENTITY
Query
Of selected
polys
Land OK
OK sites
Booby Trap
• Select by location using Intersect
does not give the same results as
Overlay Intersect
Sel by Location: Intersection
• Intersected
Parcels w/
Cover with Ag
Selected
• The red
outlines are
the selected
parcels
Lets Zoom In ----
Select by location
• You can see that
• Entire Parcels were
selected
• Including…
• Parcels with only a
little bit of Ag
landuse were
selected
Overlay Intersection
• BUT THE RESULT IS VERY DIFFERENT!
Red outlines
are the parcels
selected by
Select by
theme
Green are the
result of the
Overlay
Comparison
So you have to be careful!
• Exercises 7a and 7 b have you
experiment with some simple layers
• And I will be throwing some relatively
simple problems your way to provide
more practice
• And I will be changing the schedule at
bit too (what else is new?).