UNIT 34: OVERLAY OPERATORS

Written by Teresa A. Pitts, Department of Entomology, Virginia Tech, Blacksburg, Virginia

Context

Overlay operations are powerful spatial analysis tools used to combine layers of data. They are generally used to determine which features in one coverage lay within polygon features of another coverage. Different operators allow the user to add (or subtract) all or part of a coverage's features to (or from) another coverage.

Example Application

The Slow The Spread Gypsy Moth Project generates grids of moth trap sites across eight states (Indiana, Illinois, Ohio, Michigan, North Carolina, Virginia, West Virginia, Wisconsin). The project is a cooperative effort between Federal (Forest Service) agencies and state agricultural agencies. Each agency is responsible for placing, checking, and removing traps located in its area of responsibility. The STS GIS manager must determine how many of which kind of trap fall within each agency's boundaries. Trap sites that fall into water bodies must be removed from the grid. Statistics related to acreage counts can be generated on a project, agency, or county level.

Example Operations:

Determine U.S. Forests that fall within the project area
Add county boundaries
Add trap sites (point data layer) to project area (polygon data layer) Eliminate traps that fall in water bodies

Learning Outcomes

Awareness:

To gain a general understanding of the implementation and potential applications of overlay operators and to gain a working knowledge of overlay operation terminology.

Competency:

To determine the appropriate application of different overlay operations and be able to manipulate the command parameters to control overlay operation results.

Mastery:

To integrate knowledge of overlay operations into the decision making/analysis functions of a GIS.

Preparatory Units

Recommended:

Unit 10 - Projecting Data

Unit 28 - Editing polygon data

Unit 33 - Using Buffers

Unit 39 - Performing Statistical Analyses

Unit 41 - Using Boolean Search Techniques

Awareness

Learning Objectives

Student can define a basic vocabulary of overlay operator terms.
Student can differentiate between overlay operators.
Student can describe an application of an overlay operator.

Vocabulary

Attribute
Layer
Overlay
Projection
Registration
Topology

Unit concepts

Adding/subtracting all or part of a coverage to another coverage

Restricting the addition/subtraction spatially
Limits of overlay operators with different data models
Knowledge of specific software commands related to overlay operations, their proper use and parameters
Pre-overlay processes
Projection (equal area)
Registration
Topology requirements

Competency

Learning Objectives:

Student can use overlay operations to perform:

Addition of features from one coverage to another
Determination of which features in one cover lay within the area of another cover
Subtraction of features within an area of one cover from another cover

Mastery

Student will be able to use post-overlay statistical analyses to determine:
Acreage of a land use type within a greater boundary
Number and attributes of features which are in specified territories

Comptency Tasks

4.2.1 Pre-Overlay Tasks

VA_CNTY = a coverage of ten Virginia counties in the example area

PROJ_QUADS = USGS 7 1/2 minute quads covering the example area

USFSLAND = a coverage of Federal Forest land in the example area

PROJ_SITES = a generated grid of trap site locations

Generic List of Pre-Overlay Tasks

Section 1. Check each layer to be used for topology errors, proper registration, and accuracy of contents. Graphically display the layers individually to check for topology errors; display them together to check registration.

Section 2. Verify that each data layer has the expected and correct attributes. Use the data base query commands of your software package to list the items (fields) associated with each layer.

Section 3. View the descriptions of each data layer to be used in the overlay: all layers to be combined must have the same projection and projection parameters.

TIP: If post overlay manipulations include area calculations/comparisons, the layers must be in an equal area projection (such as Albers Equal Area).

Section 4. Compare attribute names between the coverages. The names of the items which you wish to retain must be unique. When performing an overlay, items with the same name are dropped from one or the other coverage; this can result in data loss. Again use the software's data base query commands to check the items between coverages.

A Summary of Pre-Overlay Tasks:

Check data layers for topology errors, proper registration, and accuracy of contents

Be sure all layers are in the same projection; use an equal area projection if area data will be generated.

Compare attribute names between coverages. Change attribute names, if necessary, to maintain data items during overlay operations.

4.2.2 Overlay Tasks

EXAMPLE:

trap_sites98 - a point coverage of generated nodes (trap sites) across the example area

proj_quads - a polygon coverage of USGS 7 1/2 min. quads across the example area

va_cnty - a polygon coverage of ten Virginia counties: this defines the example area

sm_mtn_lake - a polygon coverage of an inland water body

usfs_lands - a polygon coverage of some USFS lands in Virginia & West Virginia

All of parts of the above data layers will be combined, using different overlay operators, to create a composite coverage containing multiple attributes. From that composite coverage information will be derived for pretend project funding requests, and a county administration data base.

Generic List of Overlay Tasks

Section 1. Add the total contents of one polygon layer to another polygon layer.

EXAMPLE: Using your software's commands, combine the data layers "va_cnty" and "proj_quads" so that all polygons and the non-duplicated attributes are retained.

2. Add the partial contents of one layer to another

EXAMPLE: For the Gypsy Moth Slow-The-Spread project, knowledge of which agency (state or Federal) is in charge of an area is of utmost importance, as the responsible agency must oversee trap placement and trap checking.

Some of the land within the ten county example area is owned by the U.S. Forest Service. This example uses an overlay operator which adds only those features within an area of interest: here we wish to retain only that Forest land which is inside the project area. Using your software's commands, combine the data layers so that only those polygons of the "usfsland" data layer which fall inside of the "proj_coqds" are retained in the output data layer.

Section 3. Subtract features falling within an area of one layer from another layer.

EXAMPLE: Trap Sites for the Gypsy Moth Slow-The-Spread Project are generated across the entire STS area (which covers parts of eight states). For this section, we need a coverage containing only those traps which are in the example area (the ten counties in Virginia). Using your software's commands, and using the example area as a "cookie cutter", extract those needed sites from the "sites98" data layer.

Section 4: Subtract the area of one layer from another layer.

EXAMPLE: A recreational lake, Smith Mountain Lake, is within the example area. Any trap sites that fall within that water boundary should be eliminated from our sites coverage. Use your software's commands to remove features in one coverage, which are encompassed by polygons of another coverage.

Summary of Overlay Tasks

Add the total contents of one polygon layer to another polygon layer

Add the partial contents of one layer to a polygon layer

Subtract features falling within an area of one polygon layer from another layer

Subtract the area of one polygon layer from another layer

Mastery

Learning Objectives:

Student will be able to use overlay operators to perform the following tasks:

Combine data layers to achieve predicted results
Given verbal instructions, determine the methods and commands to use to derive needed data from various layers
Create data base ready files from attribute tables
Generate descriptive statistics from combined data layer
Perform spatial analysis on combined data layers
Prepare combined data layers for further manipulations

EXAMPLE: Create database ready files from attribute tables

The composite polygon data layer "proj_coqdsfed" contains county, USGS quad, and USFS land information. The data points layer "sites_final" contains UTM coordinates and site numbers for the trap sites within the example area.

PROBLEM: A Craig County, Virginia administrator has contacted the GIS technician. She wants a list of traps sites within her county, so that any citizen inquiries can be knowledgeably answered. She has asked that the list include the coordinates, the name of the responsible agency, and the USGS quad name for each trap.

SOLUTION: Using your software's commands, generate a data base ready file of trap sites within one county. Suggested steps:

Step A. Combine the trap site layer with the composite layer

Step B. Choose only those trap sites within the given county

Step C. Output, from the layer's attribute table, a file of trap site UTM coordinates, site number, agency, and grid

EXAMPLE: Budget estimates are due, and for the sake of this example we'll say that participating agencies are funded in proportion to the number of acres for which they are responsible. The agencies contact the GIS technician, each wanting to know how many acres in the example area are within their agency boundaries. Using your software's commands, calculate the number of acres in each agency's area. Suggested steps:

Step A. Be sure the polygon data layer is in an equal area projection

Step B. Output area values from the data layer's attribute table

Step C. Convert the area values from map units to acres

Using your computer's file editor and a hand calculator, the area in square meters can easily be converted into square acres by multiplying the square meter values by 0.0002441054:

AGENCY AREA SQUARE ACRES SQUARE METERS

George Washington Nat'l Forest 100,544.73 406,890,068.387467

Jefferson Nat'l Forest:Blacksburg 115,312.39 466,652,662.325159

Jefferson Nat'l Forest:JNF_GL 74,363.79 300,939,578.135164

Jefferson Nat'l Forest:New Castle 139,976.45 566,464,549.283207

Jefferson Nat'l Forest:Wytheville 2,493.66 10,091,498.234800

Virginia Dept. of Ag. & Cons. Serv. 172,282.82 16,884,628,275.990430

Alternatively, the values can be exported to a spreadsheet software package and converted there. From the above values, VDACS should receive the majority of the available funding.

EXAMPLE: The EPA in the Hardy quadrangle is conducting some biological monitoring. Franklin County is working closely with the EPA on this monitoring project and is concerned that the weekly trap inspections will disrupt the delicate monitoring equipment. Franklin County's EPA project liaison person has contacted the GIS technician to request a list of sites and the coordinates that are in both Franklin County and the Hardy quad. Use your software's commands to extract the sites that fall in the area overlapped by both Franklin County and the Hardy quadrangle. Suggested steps are:

Step A. Extract the Hardy quad polygon from the quads data layer

Step B. Using the appropriate overlay operator, create a coverage of sites common to both areas

Step C. Output a file to send to the County EPA liaison

For a look at the entire Gypsy Moth Slow-The-Spread Project, including current trap placements and moth catches plus historical data, click here:

Slow The Spread Project