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Mapping at the N&O


Introduction Mapping by Street Address 'Thematic' Mapping Where to Find It


Quick Intro

Mapping is a great way to put information in context.
The right map, with the right data to back it up, can show you:
-- where a particular type of crime has occurred most often
-- which neighborhood sends its kids farthest to school
-- the spread of a disease
-- a city's growing boundaries over time
-- the area affected by a spill of toxic waste
-- the distribution of people by race, income, education, political party or any other demographic fact at your disposal.

To make a good map, you need a location -- an address -- tied to your data, and a map that can match your address. The mapping process, and the resulting image, differ depending on whether your locations are keyed to a street address (123 Main Street) or a geographic region such as voting precinct, police beat, census block group, city or county boundaries.
Mapping by street address or X,Y coordinates will give you a picture of points scattered across the landscape.
Mapping by region creates 'thematic' maps with varying shades or colors to indicate different values in each region.

Mapping by street address

This process matches addresses in your database to street segments on a map.
On a street map, each stretch of road between two intersections is identified by the name of the street and the address range.
Here's what a typical data table for a MapInfo street map looks like:

StreetFromLeftToLeftFromRightToRight
Brooks Av100198101199
Brooks Av200298201299
W Peace St3103315731003158
The 'Street' column contains the complete street name.
The columns 'FromLeft' and 'ToLeft' contain the address range for even numbered addresses on Brooks and odd numbered addresses on Peace.
The columns 'FromRight' and 'ToRight' contain the address range for odd numbered addresses on Brooks and even numbered addresses on Peace.

To find the address 120 Brooks Avenue on this map, MapInfo would first search for Brooks Av, then find the segment with the even numbered range between 100 and 198, and then place a symbol on the map screen about 1/5 of the way between . Next, it calculates that 120 is about 1/5 of the way between 100 and 198, and it places a symbol on the map at about that point.
This process, called 'geocoding', relies on having two clean data sets: the data you bring in, and the data in the map itself.

Here's an example of a table of crimes in Raleigh that can be matched to a street map of the city. Note that all the elements of the address are in one column (1234 W Peace St).

OffenseDateAddressBeatTime
Auto Theft09/10/96215 S McDowell StRPD*12714:23
Rape09/10/96123 Gardner StRPD*13520:37
Larceny09/10/96320 Dixie TrailRPD*15715:15
Auto Theft09/16/96400 Glenwood AvRPD*15702:10
Rape09/17/96550 E Bragg StRPD*14211:57
Drugs - Possession 09/19/96320 Dixie TrailRPD*15715:03

From a table like this, you could map crime by the type of crime, the time or date when it occurred, the police beat, or even selected neighborhoods that you identify by street address.

Here's an example of a crime map:


This map shows three types of crime -- drugs, sex offenses and aggravated assault -- committed in Raleigh during a given time period.
The crimes were geocoded separately, using different symbols to indicate each crime.
Then the three maps were layered together, along with a map of major highways, to get this image.


Thematic Mapping

This process matches data to regions. It's a good method for looking at aggregate numbers -- percents, sums, changes over time -- rather than single events at unique points.

Here's an example of a thematic map:


This map shows the racial makeup of elementary students in Wake County.

The colored areas -- blue, lavender and white -- represent hundreds of tiny 'nodes' or neighborhoods, defined by the school system for assigning students to schools.
We used a table that contained the percent of minority students in each neighborhood, matched it to the map of 'nodes', and colored each node according to the racial proportions.
Then we overlaid the locations of Wake's 67 elementary schools (seen here as black dots).

The map was created to give an indication of the schools' racial balance if the system reverted to neighborhood schools.


Where to Find It

Most of our maps are located on Carnet, at f:\maps\
But you'll probably want help.

If you have other questions about mapping, please ask Donna Seese or past CAR Fellows (particularly Craig Jarvis) for help.


This page was created by Donna Seese
Last updated May 7, 1997