fundamentals of gis lecture 4 written by austin troy, brian voigt and weiqi zhou, university of...

Fundamentals of GIS

Lecture 4

Written by Austin Troy, Brian Voigt and Weiqi Zhou, University of Vermont © 2011

Fundamentals of GIS

1. The Vector Data Model

2. Multi-layer Vector Queries

3. Spatial Join

4. Geoprocessing

Fundamentals of GIS

1. Vector Data Model

Fundamentals of GIS

The Vector Data Model• Three types of vector data

1. Points2. Lines / Arcs3. Polygons

• A given layer holds a single feature type (e.g. “roads” is a line layer, “counties” is a polygon or line layer, “weather stations” is a point layer)

• Shapefile vs Feature Class

Fundamentals of GIS

Point Features• A collection of records with (x,y) coordinates

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Fundamentals of GIS

Line Features• Each point has a unique location • 2 points define a line segment• One or several line segments define an arc• The endpoints of an arc are “nodes• The angle points are “vertices” (sing. Vertex)• The feature is the arc, not the line• Two arcs meet at the nodes

Fundamentals of GIS

Line Features• Points define lines (arcs)

Line segment

Vertex

Node

• Feature is the ARC, not the line segments

• Arcs meet at the nodes

Arc

Fundamentals of GIS

Polygon Features• Area of homogenous phenomena • In a polygon layer, lines (arcs) define areas• Closed region – first and last coordinate pairs are at the

same location• Line segments bound the polygon• Computer “knows” that interior belongs to shape

Lines (Arcs)

Points

Fundamentals of GIS

Rings• A series of line segments (a string) that close upon each other• NOT a polygon!!

Fundamentals of GIS

Vector TopologyDefinition1: Explicit encoding of spatial

relationships between objects: the spatial location of each point, line and polygon is defined in relation to each other

Definition2: Topology is a collection of rules and relationships that enables the geodatabase to more accurately model geometric relationships found in the world.

Two major purposes:1. Allows for powerful spatial analysis 2. Quality control

Fundamentals of GIS

Vector Topology: Types • Arc-node and node topology : the way that line

features connect to point features• Polygon topology: the way that neighboring

polygons connect and share borders• Route topology: the way that a line feature of one

type (e.g. commuter rail line) shares segments with line features of another type (e.g. Amtrack rail line)• Regions topology: the way that polygons overlap

(e.g. GIS layers with a time component) or when spatially separate polygons are part of the same feature

Fundamentals of GIS

Vector Topology: Quality Control

• Ensuring data quality and “logical consistency”• Defining complex and nuanced spatial rules. • Single layer quality control: • dangles • overshoots • polygons that don’t close • adjacent polygons that do not share a border

Fundamentals of GIS

Vector Topology: Quality Control

overshoots

slivers

dangles

Not sharing border

Fundamentals of GIS

Vector Topology: Quality Control• Mutli-Layer quality control: Defining spatial rules

between layers• Polygon rules: e.g. Must Not Overlap• Line rules: e.g. Must Not Intersect• Point rules: e.g. Must be Properly Inside Polygons

• Define and validate topology rules in ArcCatalog and ArcMap• (http://help.arcgis.com/en/arcgisdesktop/10.0/help/

index.html#/Geodatabase_topology_rules_and_topology_error_fixes/001t000000sp000000/)

Fundamentals of GIS

Topology Rules: Example• Say we have the following layers: property lots,

sidewalk, building footprints, zoning map• We can specify topological rules, like:• Lots must be enclosed polygons• Buildings must be entirely within a lot• Sidewalks must be outside a lot polygon• Lots must fall entirely within a single zone• Lots must either share a border with another lot or

with city land, including streets and sidewalks.• In a low-density zone, no more than 20 lots can be

touching• We can’t do this yet, but will be able to shortly

Fundamentals of GIS

Vector Topology Table1. Polygon topology table

• Lists arcs / links comprising polygon

2. Node topology table• Lists arcs / links that meet at each node

3. Arc, or “link” topology table• Lists the nodes on which each arc / link ends and

polygons to right and left of each arc / link, based on start and finish nodes

4. Table with real world coordinates for each point

Fundamentals of GIS

Vector Topology TableGraphical display of arcs, nodes, vertices and lines

Topology table for the ARCs making up the polygons

A table of the polygon topology

Fundamentals of GIS

Spaghetti Data Model• Non-topological data model that looks like a

vector data set• Collections of line segments and points with no

connectivity or topology• No relative relationships encoded in this model • Each line segment is “unaware” of the other line

segments

Fundamentals of GIS

2. Multi-layer vector queries in ArcGIS

Fundamentals of GIS

Selecting By Location• Let’s say we want to gather information about houses in

four sample neighborhoods to identify which houses are within fire hazard zones

Fundamentals of GIS

Select By Location

Selection Method

Selection layer

Selection overlay layer

Selection rule

Fundamentals of GIS

Select By Location• Now with “sample houses” active, we click

select by theme and tell it to choose features that intersect the features of fire hazard zone

Fundamentals of GIS

Select By LocationThose that overlay a hazard zone are selected

Selected

Not selected

Fundamentals of GIS

Select By Location…Zooming in to one of those neighborhoods

Fundamentals of GIS

Select By LocationNow we run statistics on the Price attribute of the

selected records. This tells us that 1955 houses overlay fire zones it also tells us that the mean price for these properties is $467,551!

Fundamentals of GIS

Select By Location• If we invert the selection with the Switch Selection

button, we see that on average houses outside the fire zones are worth less! Only $246,752

Fundamentals of GIS

Select By Location: Distance• Now, say we want to select features from layer A that are

within a distance of features in layer B. In this case we’ll select houses in our sample neighborhoods that are within 1 mile of a Starbucks

Fundamentals of GIS

Select By Location : Distance• This time we use a

different selection method

• Note how we can specify the distance for selection

Fundamentals of GIS

Select By Location: DistanceResults in the following selection

Fundamentals of GIS

Select By Location :DistanceZooming into a neighborhood…

Fundamentals of GIS

Select By Location :DistanceNow if we run statistics on price again…

Those within a mile of a Starbucks have a mean value of $504,972

Those not within a mile of a Starbucks have a mean value of $273,866!

By the way, these are real data, I’m not making this up!!

Fundamentals of GIS

Select By Location :DistanceFor that same selection we could get statistics on a

different variable—here we’ll look at lot sizeThose within a mile of a Starbucks have a mean size of 8776 square feet

Those not within a mile of a Starbucks have a mean lot size of 10,024 sq feet. Why might that be?

Fundamentals of GIS

Select By Location :DistanceYou can also select features in a point layer that are

within a specified distance to a linear feature in another layer. Here we’ll find houses in a neighborhood within a mile of a highway

Note that these smaller roads are in a different layer

Fundamentals of GIS

Select By Location :Distance• Scenario: There’s going to be a parade on Valley

Boulevard and the city needs to inform all those homeowners on or near the route.

• We want to select points located within a specified distance of a single feature within a layer?

• Here we’ll find all homes within 500 meters of Valley Blvd.

• First a simple query to identify Valley Boulevard

Fundamentals of GIS

Select By Location :DistanceOnce that feature is selected we can use “Select by

location” to identify the affected properties

Notice that this time we check “Use selected features”

Fundamentals of GIS

Select By Location :DistanceWe end up selecting only the houses within 500 m of

Valley Blvd

Fundamentals of GIS

Select By Location: Polygons• Generally polygons in one layer do not perfectly coincide

with those in another.

• Using the default selection method (Intersect) the entire polygon is selected even if only a small part is coincident.

• However, there are many other methods we can choose from that will change the number of polygons selected.

Fundamentals of GIS

Selecting By Location: PolygonsExample: let’s select any census tract that intersects a

fire zone; here’s the pre-selection map

Fundamentals of GIS

Select By Location: PolygonsUsing the “intersect” selection method we get this

Fundamentals of GIS

Select By Location: PolygonsUsing “that are completely within” method, we return no

selected features. With “have their center in” we get

Fundamentals of GIS

Select By Location: Selected Records

Intersect

Completely within

Fundamentals of GIS

What can be done with multi-layer selections?Once a selection has been done using “select by

location” you can do all the same things you would do with a single-layer selection:• Make a new layer from the selection• Do statistics on it• Make a new field in that layer • Calculate or recalculate a field for a selection

Fundamentals of GIS

3. Spatial Join —assigning attributes by location

Fundamentals of GIS

Spatial Join• Assigns attribute data from features in one

layer to spatially coincident features in another• polygon attributes to point features• point attributes to point features • point to line distances between two layers

• Simply adds attributes to the attribute table

Fundamentals of GIS

Spatial Join• Right clicking on the “to” layer and

selecting Joins and Relates >>> Join• Specify what you want to join by location

and choose which layer we are joining from

Fundamentals of GIS

Spatial Join• In this case we are

going to join tracts to the houses from our sample neighborhoods. Each house inherits all the attributes of the tract in which it falls. This is a great way to assign data from layer to another

Note that this creates a new layer

Fundamentals of GIS

Spatial Join• Now we can plot houses by any of the attributes

that were in the tracts database. Here’s a plot of houses graduated by percent unemployment of the tract to which they belong

Fundamentals of GIS

Spatial Join: Distance• We can also do spatial joins based on distance.

Whenever we join a point or line layer to another point or line layer, for each feature in the TO layer it gives us the attributes of the nearest feature in the FROM layer PLUS the distance between those features in whatever map units we specify

Fundamentals of GIS

Spatial Join:Distance• Suppose we want to assign the name of the nearest

major road to our housing layer.

Fundamentals of GIS

Spatial Join:Distance• Two options

1. numerical summary of the lines that intersect each point

2. assign all attributes from the nearest line.

• We choose the latter

“from” layer

Fundamentals of GIS

Spatial Join: Distance

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• Name of nearest highway is an attribute for each housing point; here I’m plotting out categorically (by road name) after completing the join

Fundamentals of GIS

Spatial Join: Distance

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• The Euclidean distance from each point to the nearest road feature is recorded in the attribute “Distance.” Here I’m plotting out distance to nearest major road.

Fundamentals of GIS

Spatial Join: Distance

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• We can also do a join to get the distance from a series of points in one layer to a series of points in another: here is distance of houses to nearest Starbucks

Fundamentals of GIS

Spatial Join: Polygons• Intuitive when it comes to assigning attributes to

points and lines• What about polygons?

Layer A

Layer B

Fundamentals of GIS

Spatial Join: Polygons• Problem: A polygon in layer A may overlay several

polygons in layer B. Whose attributes do you give it? • Spatial join and summarize (e.g. by average) the

values of all the polygons of layer B that overlap the polygons of layer A.

• Example: Say we have a census tract layer with all sorts of demographic info (e.g., population, race) and we have a zip code layer with no demographic info attached to it. Our client is doing a marketing study and needs to have a map showing median age and percent Hispanic by zip code. We have both these attributes in our tract data layer and need to “transfer” them to the zip code layer

Fundamentals of GIS

Spatial Join: Polygons• Unfortunately, the tract boundaries and zip code

boundaries do not match up. Note that tracts are not nested within zip codes—they cut across

Fundamentals of GIS

Spatial Join:Polygons

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• To deal with this we join two polygon layers based on their spatial location and choose the “summarize” option (the first radio button). This allows us to choose a statistic by which to summarize the value of all the constituent tract polygons for each zip code polygon. In this case we’ll choose “average” as our statistics

Fundamentals of GIS

Spatial Join:Polygons• This results in a new output zip code layer with the

average of every census tract variable; here median age is plotted; ArcGIS calls it Avg_MEDAGE so that we know what statistic this is based on

Fundamentals of GIS

4. Vector Geoprocessing

Fundamentals of GIS

Purpose of Geoprocessing• Tools for breaking down the size of map features: • Union, Intersect, Clip

• Tools for increasing the size of map features:• dissolve and merge (indirectly)

• ArcInfo and ArcToolbox include various other geoprocessing overlay operations, such as Update and Dissolve Regions

Fundamentals of GIS

Union• Combines features of two themes• Each theme is treated the same• Goes to extent of largest theme• Keeps all line work, creates new polygons• Breaks down features into smaller minimum mapping units• Can use selected features option too• Keeps all attributes

Fundamentals of GIS

Tools: Union

only accepts polygon features

Fundamentals of GIS

Finding Geoprocessing Tools

Fundamentals of GIS

Intersect• Yields polygons

representing areas that are common to both layers• Preserves line work

within common extent • Creates new, smaller

polygons• Preserves all attributes

from both

Fundamentals of GIS

Union vs. Intersection

• Union is the entirety of two overlapping sets of features• Intersection identifies the common area• Continuous and exhaustive vs. “island” polygons

have different ramifications for these tools

Layer 1 + Layer 2

Intersect:

Layer 1 + Layer 2

Union:

“1 AND 2”

“1 OR 2”

Fundamentals of GIS

Union vs. Intersection: Example

• Suppose we have deer wintering areas in one layer and conserved lands in another.

Fundamentals of GIS

Union vs. Intersection: Example

• Union gives us land that is EITHER conserved OR that is a deer wintering areas

Fundamentals of GIS

Union vs. Intersection: Example

• Intersect gives us land that is BOTH, and preserves all polygon boundaries within that common extent

Fundamentals of GIS

Tools: ClipInput Features = Point, Line or Polygon

Clip Features = Polygon

Fundamentals of GIS

Clipping highways for Merced

Note that the “Use selected features only” option was used

Fundamentals of GIS

Tools: Dissolve

Fundamentals of GIS

Dissolve: Example• Dissolve zip codes (small) into counties (large)

Fundamentals of GIS

Dissolve: Example• Choose the dissolve field: e.g. Dissolve based on

the County field

Fundamentals of GIS

Dissolve : Example• Summarize the resulting field values. For instance, I could

calculate the average area of the zip codes that intersect each county.

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Fundamentals of GIS

Dissolve : Example• Now we have created

a county map, and for each county we have an attribute that equals the sum of the populations of the constituent zip codes

Fundamentals of GIS

Merge• Allows you to “join” two adjacent or non-adjacent layers into a

single layer• Like “tiling”• Best when attributes match

Fundamentals of GIS

Merge

• Often when you merge you will want to follow up by dissolving.• This is because artificial polygon boundaries

were created at the borders by the act of splitting the data up into tiles• Merging joins multiple tiles into a single layer

and dissolving reunites polygons that were split up in the tiling process

Fundamentals of GIS

Tools: Buffering

Buffering is when you draw a polygon around a feature (point, line or polygon); Here we’re buffering a stream

Fundamentals of GIS

Tools: Buffering Based on distance

Based on attribute

Fundamentals of GIS

Tools: Buffering• Width of buffer can vary with an attribute value• We can recalculate that attribute to make a

meaningful buffer width• Example from lab: a buffer based on traffic volume

Fundamentals of GIS

Combining Multiple Geoprocessing Tools: Example• Suppose we create fixed buffers around deer

wintering areas and water bodies, and a variable buffer around roads, based on traffic:

Fundamentals of GIS

Combining Multiple Geoprocessing Tools: Example

• Then we could, for instance, find areas that are near deer wintering areas and water bodies but far from traffic: • First we would intersect the deer wintering and water

body buffers, yielding areas that are near both.• Next we union the result with the traffic buffer.• Finally, query the attributes to determine which areas

meet your criteria.

Fundamentals of GIS

Combining Multiple Geoprocessing Tools: Example

• The intersection of deer wintering buffers and water buffers is the area in the red

Fundamentals of GIS

Combining Multiple Geoprocessing Tools: Example

• The union of that intersection with the traffic buffer:

Fundamentals of GIS

Combining Multiple Geoprocessing Tools: Example

• Now we can query for polygons that were created from the intersection (met the two good criteria) and for areas that are not within a traffic buffer

Fundamentals of GIS

Combining Multiple Geoprocessing Tools: Example

• We can then create a layer from that—Note that we have created entirely new polygon boundaries and geometry by cutting and splicing these buffers together.

Fundamentals of GIS

Combining Geoprocessing Tools • Involve multiple tasks performed in sequence,

such as those that clip, buffering, intersect, union, then select datasets.

• Build and run a “model” in Model Builder–Step by step instructions

–Specify input, output, other parameters, order of operation

–Create and run a script

Fundamentals of GIS

Model Builder