©2005 Austin Troy

Lecture 9:

Introduction to GIS

1. Vector Geoprocessing

Lecture by Austin Troy, University of Vermont

©2005 Austin Troy

Geoprocessing in Context• Last time we learned how to select features based on

locations and to join attributes based on location• This time we will learn about how to actually change the

geometry of vector features to more efficiently answer our questions

• We have three general classes of tools at our disposal:– Breaking features into smaller features (usually polygons)– Aggregating features into larger features (ditto)– Creating new polygon features through buffering

• First I’ll introduce the tools, then I’ll show how they can be integrated to dramatically increase your GIS toolbox.

Introduction to GIS

©2005 Austin Troy

Tools: Geoprocessing• In ArcGIS, the GP wizard allows you to

dissolve, clip, merge, intersect, union

Introduction to GIS

©2005 Austin Troy

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)

• Arc/Info and Arc Toolbox include various other geoprocessing overlay operations, such as Update and Dissolve Regions

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©2005 Austin Troy

Union vs. Intersection• Remember Set Theory from Geometry?• Union is the union of two overlapping set of features

and intersection is the intersection• What SQL query operators do these correspond with?

Introduction to GIS

Layer 1 + Layer 2

Intersect:

Layer 1 + Layer 2

Union:

“1 AND 2”

“1 OR 2”

©2005 Austin Troy

Union• Combines features of two theme• 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

Introduction to GIS

©2005 Austin Troy

Intersect• Yields polygons representing areas that are common

to both layers• Preserves line work within common extent • Usually creates many new, smaller polygons• Preserves all attributes from both

Introduction to GIS

©2005 Austin Troy

Union vs. Intersection: Example• Here’s an example. Say we have deer wintering areas

in one layer and conserved lands in another.

Introduction to GIS

©2005 Austin Troy

Union vs. Intersection: Example• Union gives us land that is EITHER conserved OR

that is a deer wintering areas

Introduction to GIS

©2005 Austin Troy

Union vs. Intersection: Example• Intersect gives us land that is BOTH, and preserves

all polygon boundaries within that common extent

Introduction to GIS

©2005 Austin Troy

Clip• This uses one theme to “clip,” or serve as the outer

boundary of another theme• EG, clip the CA highways theme to include only

highways for Merced County• Can clip by a single

selected feature in a theme

Introduction to GIS

•This is another way to break down features into smaller units

©2005 Austin Troy

Clipping highways for Merced

Introduction to GIS

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

©2005 Austin Troy

Dissolve• This is a single layer operation• Tool for aggregating polygons—making them

bigger. Allows you to join together into one polygon multiple adjacent polygons with the same value for a specified attribute

Introduction to GIS

©2005 Austin Troy

Dissolve• Example: Zip

codes are nested within counties. The zip code layer has county as an attribute. Therefore I can dissolve zip codes (small) into counties (large)

Introduction to GIS

©2005 Austin Troy

Dissolve• I choose to

dissolve based on the County field

Introduction to GIS

• Then I must choose how to summarize the resulting field values, since, for a given field, many values are being merged into one field. For instance, I could sum population for each county

©2005 Austin Troy

Dissolve• Now we have

created a county map, and for each county we have as an attribute the sum of population of the constituent zip codes

Introduction to GIS

©2005 Austin Troy

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

adjacent themes into the same layer

• Like “tiling”

• Best when attributes match

Introduction to GIS

©2005 Austin Troy

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 brings the tiles back together and dissolving joins together polygons that were split up in the process

Introduction to GIS

©2005 Austin Troy

Tools: BufferingBuffering is when you draw a polygon around a feature

(point, line or polygon); Here we’re buffering a stream

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Tools: Buffering • ArcGIS has a “buffer wizard”• Allows for variable width based on attribute• Allow multiple buffers

Introduction to GIS

©2005 Austin Troy

Tools:Variable Width BufferingWhere width of

buffer varies with an attribute

Often we must recalculate that attribute to make the buffer width meaningful

Example from lab: a buffer based on traffic volume

Introduction to GIS

©2005 Austin Troy

Combining Buffering and Geoprocessing

• Last time we learned how to select a feature in layer A if it was within a distance X of a feature in layer B

• Buffer offers a different approach for doing the same. • By building a buffer of a given distance around a

feature, we can then do overlay analysis and select features from another layer that are inside or that intersect the buffer

• The difference is that with selecting by location, we can only do 2 layers at a time. When we combine buffering with geoprocessing, we can ask questions across unlimited numbers of layers.

Introduction to GIS

©2005 Austin Troy

Combining Buffering and Geoprocessing: Example

• From Lab 6: say we made fixed buffers around deer wintering areas and water bodies, and a variable buffer

around roads, based on traffic:

Introduction to GIS

(note the lab is a bit

different):

©2005 Austin Troy

Combining Buffering and Geoprocessing: 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 those.

• Then we would union the resulting with the traffic buffer and run an attribute query to determine which areas meet the criteria

Introduction to GIS

©2005 Austin Troy

Combining Buffering and Geoprocessing: Example

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

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©2005 Austin Troy

Combining Buffering and Geoprocessing: Example

• The union of that intersection with the traffic buffer:

Introduction to GIS

©2005 Austin Troy

Combining Buffering and Geoprocessing: Example

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

Introduction to GIS

©2005 Austin Troy

Combining Buffering and Geoprocessing: 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.

Introduction to GIS

©2005 Austin Troy

Buffering and selectingOf course, we can also use buffers to assist with “select by

location” but we could do that using the select by location distance function more easily

Introduction to GIS

©2005 Austin Troy

Combining Union and Dissolve• In many cases you will want to join two layers with

different polygons so that you can attach an attribute from one coverage to the second coverage

• Use Union, to “squash” them together. Then dissolve the theme back to the unit of geography you care about, based on that unit’s unique ID.

• When you dissolve, you can summarize the new attribute by mean, max, min, stdv, etc.

• This is how we take attribute info in polygon layer A and assign it to differently shaped polygons in layer B

Introduction to GIS

©2005 Austin Troy

Combining Union and Dissolve• Confused? Here’s an example:• Say you have a hill slope polygon theme with small polygons

and a vegetation theme with larger polygons and you are using the vegetation stands as your management units; you want to get an average slope value for every vegetation polygon without having thousands of slope polygons.

• Do a union or intersect of the two • Now dissolve polygons based on vegetation polygon ID; all

those new polygon fragments that formerly belonged to the same vegetation fragment will be reunited.

• Choose “average of slope” as the option to summarize by when you merge; it averages all the slope values for each vegetation polygon, based on area.

Introduction to GIS