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MABLE/Geocorr (2k version) PowerPoint PPT Presentation


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MABLE/Geocorr (2k version). John Blodgett OSEDA Missouri Census Data Center Rev. 12/06. What Is MABLE/Geocorr?. Geocorr is a web application that accesses the MABLE geographic database. A geographic utility that reveals relationships between geographic layers.

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Mable geocorr 2k version l.jpg

MABLE/Geocorr(2k version)

John Blodgett

OSEDA

Missouri Census Data Center

Rev. 12/06


What is mable geocorr l.jpg

What Is MABLE/Geocorr?

  • Geocorr is a web application that accesses the MABLE geographic database.

  • A geographic utility that reveals relationships between geographic layers.

  • Works for the entire U.S. with emphasis on Census geography.


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How Can I Access the Application?

  • http://mcdc2.missouri.edu/websas/geocorr2k.html

  • Linked to within the Quick Links navigation box on most Missouri Census Data Center(MCDC) web site pages.

  • This is the current version with 2000 census geography. An earlier version with 1990 geography is linked to (as “Previous Version”).


Quick example 1 l.jpg

Quick Example 1

  • We are interested in the relationship of the 2 geographic layers: county and Urban-Rural portion.

  • For each county in a state we want to know how much of the county is urban vs. rural.

  • We select our state and our two geographic layers--3 clicks. Then submit.


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Quick Example 1: Output


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Reading the Output

  • County is the chosen Source geocode; ur is the chosen Target geocode.

  • The source area (Fairfield co.) intersects with 2 target areas – hence 2 lines in the report.

  • 36,195 people in the county also lived in a block designated “rural” in 2k census. 4.1%.

  • The other 846,372 residents lived in urban areas. 95.9%


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Quick Example 2

  • A bit more complicated, but still fairly simple.

  • Choose 2000 census tract as Source geo and 2000 PUMA (for 5 Pct Samples) as target geo.

  • Use Geographic Filtering Options section to limit processing to 1 county.

  • Output to csv file (comma delimited) which IE automatically imports into Excel.


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Quick Example 2: Excel Output


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Quick Example 2: Notes

  • We chose Missouri for state. Then in the Geographic Filtering Options section we entered a single county code value in the text box provided for doing such filters.

  • There is a link to County codes on the form that we could use if we did not know the code. We typed in 29183 for St Charles (MO).

  • We hid the columns County & Cntyname in the spreadsheet because they were constants.


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Uses of “Correlation Lists”

  • Geocorr outputs are called “correlation lists”, “equivalency files”, “crosswalk files”, etc.

  • They have many uses. Such as:

    • Create new map coverages using a GIS with “geographic dissolve” function. E. g. tag tracts with PUMA codes and create a PUMA layer from a tract layer.

    • Select all ZIP codes (or counties) that are all or mostly rural.

    • See which counties comprise a metro area.


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Geocorr2k Input form

  • Has never won any awards for style.

  • Appears more daunting than it really is.

  • Has 5 major sections, not counting the very top (“top matter”) and bottom.

  • Of the 5 sections, the last 2 are rarely used and the 3rd only occasionally.

  • Most options have default values that rarely need to be modified.


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Geocorr2k Input Form: ”Top Matter”

  • Identifies the application, incl version and revision date.

  • Links to related applications:

    • Previous Version link takes you to version with 1990’s-era geography.

    • CCC link lets you look up geocodes for use in Geographic Filter section.

    • Direct access mable2k link is for power users who are willing/able to access the data without the geocorr interface.


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Help and Examples

  • These 2 links near the top of the page take you to web pages that were created in the late 90’s.

  • A few details have changed since these were written, but they are still very useful documents.

  • Where things have changed it’s pretty obvious.

  • We strongly suggest you read these two pages.


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Five Main Sections…

  • “Only the first 2 are required” it tells us at the top of the page.

  • The row of links across the top take you directly to each section.

  • The “Bounding Box” section, in particular, is rarely used.


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Input Options Section

  • By far the most important section of the form.

  • Choose the state(s) of interest. You can choose all 51 but you run the risk of timing out.

  • Note the MAGGOT link just above the geocode select lists.


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Input Options – 2

  • Select “1 or more” geocodes from the 2 select lists (“Source” and “Target”).

  • Selecting more than 1 geocode from a list creates a composite layer.

  • Parent geocodes are selected automatically for you. E.g. selecting tract automatically selects county; selecting place automatically selects state.


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Input Options - 3

  • Entire Universe [no code] selection a special case.

  • Use it, for example, to just get a list of the codes and names of all the [counties or places or CBSAs, etc] for the selected geographic universe.

  • --Missouri-only geocodes-- :the codes in this section are only available for Mo.


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Input Options - 4

  • The weighting variable is what geocorr uses to measure the size of the intersection between the base/target layers. By default we use 2k census pop.

  • The option to “ignore Census Blocks with a value of 0 for the weighting variable” saves some processing time. Avoids “trivial sliver” intersections. E.g. where 2 areas share some common space, but nobody lives in that space.


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Output Options Section

  • The “weighted centroids” option says to take a weighted average (using the chosen weight variable) of the lat-long coordinates of each census block contained in an output geographic area. Not widely used.

  • Generate 2nd allocation factor option is handy. For example, if you did a ZIP to Place (city) list with pop2k as weight you could tell what portion of the ZIP was in the city and what portion of the city was in the ZIP.


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Output Options - 2

  • Sort by target geocodes lets you override the default sort order of outputs.

  • You can accomplish this by just reversing the choices of source vs. target geocodes – but then the allocation factors would be different.

  • Think of a ZIP to CD list that you want organized (sorted) by CD but you want to know what portion of each ZIP is in the CD.


Output options 3 l.jpg

Output Options - 3

  • You can (de)select the option to output a delimited file. You can also select to have tabs instead of commas as the delimiter between fields. (Not recommended if you to import into Excel.)

  • You can (de)select to have a listing file output. You have a small pull-down to choose listing format: plain txt, html, pdf.

  • For both output types (csv/listing) you can opt to not keep codes or names associated with codes.


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Output Options - 4

  • The final option in this section allows specifying a title for your output report. (Avoid using special characters such as & and % in this title)

  • Finally, we see our first pair of Reset Defaults/Run Request buttons. It is quite common for your request to be completed at this point with nothing specified in the last 3 sections.

  • Nothing really happens until you click a Run Request button.


Exercise l.jpg

Exercise

  • Generate a report showing what cities (places) correspond to what ZIP codes (ZCTA’s) in your state.

  • The report should be sorted by place and should indicate both what portion of the place population is in the ZIP and what portion of the ZIP population is in the place.


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Typical Output, this Exercise


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Exercise Variations

  • Modify your report as follows

    • Select County as a Source geocode (in addition to place)

    • Drop the 2nd allocation factor option.

    • Request your report in HTML format.

    • Use Geographic Filtering Options section of the form to filter based on place code: enter a value of “-9999” to indicate you want to ignore “unincorporated remainder” areas.


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Exercise-With-Variations: Output


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The MABLE Database

  • The underlying database for the application is the Master Area Block Level Equivalency file.

  • It’s a collection of state-based tables. Each row of these tables describes a small unit of geography – a 2000 census block.

  • The columns of these tables are geographic codes or attributes.


The geocorr2k geographic universe l.jpg

The geocorr2k Geographic Universe

  • Starts with a choice of state(s), 1st thing.

  • Each of last 3 sections have to do with specifying within-state limits to the “GU”.

  • You can limit the GU to specified (1 or more) counties, places, ma’s, UA’s, or places (“geographic filters”).

  • Last section allows filtering using a “bounding box”.

  • Each block in MABLE has a lat-long internal point. Blocks are spatially filtered using that point.


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Point and Distance Options

  • Allows limiting the GU to a circular area.

  • Allows defining multiple “ring” areas in addition to limiting GU (to largest ring).

  • User needs to specify latitude-longitude coordinates. (links to web pages given).

  • The GU is limited to blocks with int pts within Radius miles of (lat,long), where …


Radius and lat long l.jpg

Radius and (lat,long)

  • Radius is the value entered in the 1st text box (“Value for radius of Circle or radius of largest Ring:”).

  • Lat, long are the values entered in the 2 boxes on the “Coordinates of Point:” line.

  • Enter lat/long values in decimal degrees.

  • Use the Melissa link to enter a street address and get lat/long coordinates. (right click).


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Sample Melissa Geocoding Results

  • Latitude & longitude coordinates in decimal degrees are on the next to last line shown here.

  • Use copy / paste to insert these values into the geocorr2k form. (assumes 2 browser windows).


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Specifying a 5-Mile Radius of Address

  • Partial view of the section with Radius value specified as 5, and with the coordinate values as obtained via the Melissa geocoding link.

  • The Label of Point value is strictly optional.

  • West longitude assumed. Entering a leading “-” would make no difference – it is assumed.


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Complete Specs to Go With These Point and Distance Options

  • Choose Missouri as state (default).

  • Choose County as Source geocode.

  • Choose Entire Universe as Target geo.

  • Choose HTML format for output report.

  • Specify a custom title.

  • Point and distance options per previous slide with no further entries in that section.


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Which Generates This

  • It generates a 1-line report because the entire 5-mile circular area is entirely within Boone county.

  • The sum of the weights gives you the total pop (2000) within the 5-mile circle.

  • The Distance and weighted centroid coords are automatically added when a pt is specified.

  • The label of the Distance variable uses the “Label of point” value we entered.


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Same Specs Except for…

  • Use your browser Back button to return to the specs just entered. Make these changes:

    • Change the value for Radius from 5 to 50.

    • Further down in the Point-and-Distance section enter a value of 5 as the “# of equi-distant rings”.

    • Change Target geo selection to Concentric Ring Pseudo-Geocode

    • Edit the title.


Ring pseudo geocodes rpg s l.jpg

Ring Pseudo-Geocodes (“RPG”s)

  • These are dynamically-generated geographic areas. The first one generated is a circular area centered at the specified point with a radius of the first specified ring value.

  • Any subsequent RPG is a ring (i.e. donut) with an outer radius of ring N and inner radius of ring (N-1).

  • Ring values are entered manually (in pt-and-distance section) or using Radius and # of equi-distant rings (as in our example).

  • A maximum of 10 RPGs may be generated.


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Output From Revised Run with RPGs

  • The RPGs are labeled with their outer radius values.

  • 16,431 people found in Audrain co between 20 & 30 mi.

  • 14,970 people found in Boone co in the 10-to-20 mile “donut”.

  • Probably better to switch Source & Target geocodes here.


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Exercises for the Student

  • (Use geocorr to) determine what portion of the land area of North Carolina is rural.

  • Which unified school districts in New Jersey cross county boundaries?

  • 63101 is the ZIP code for downtown St. Louis, MO. What counties fall (all or partly) within a 50 mile radius of downtown St. Louis?

  • Generate a pdf report showing the counties that comprise the Regional Planning Commissions for the state of Missouri.


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Thank You!

Address questions and comments to the author at: [email protected]


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