Geocoding 101

1. What is a Geocode and Geocoding?

Geocoding is the process of assigning a precise latitude and longitude to a location on the face of the earth. The resulting point of intersection is the location's "Geocode".

Geocodes are the basic building blocks of all maps. Every point, line, object and shape must be defined on the map by its location. A single point is placed by its geocode. A line, can be made by connecting the geocode of 2 points. This is how street segments are built. By connecting 3 or more points on the map, one can define a shape on a map. Political jurisdictions, such as city boundaries, school districts and legislative districts are drawn on a map by locating the geocodes of their borders and connecting them. The result will be a polygon that defines the shape of a district. The district borders can be shown as a line, "outlining" the borders of the district. The resulting polygon showing the districts can then be "filled in" with a hue or shade to show it up on a map.

2. How are Geocodes used?

Geocodes are used to describe a point's location on a map. They are also used to determine distance between points and their relationships to other features on the map.

3. What does a Geocode have to do with a political jurisdiction?

Political jurisdictions, such as House of Representatives and Senate voting districts, can be represented as shapes on a map. Each Geocode can be within many jurisdictions. For example, every California address is within a voting district for the US House, state senate, state assembly, and County Board of Supervisors. It may also be within a city that elects municipal officials as well as in various districts like a school district that elects board members

In computerized mapping, the computer sees each jurisdictional shape as a layer on the map. Overlaying each type of jurisdiction forms a virtual stack of layers on the map. Metaphorically speaking, the computer looks up from the Geocode and sees which shapes are on top of it and reports a list of those shapes to the user using the jurisdictional name of each shape.

4. Can Geocoders be used to determine if a location is effected by something other than a political jurisdiction?

Yes. They can map and report whether a particular location is within any area that can be defined on a map. For instance, let's say you want to develop a certain parcel of land and need to know whether it is within an area in which development is prohibited to protect the endangered red-toed frog. You could gather reports of red-toed frog sightings and use the Geocoder to depict their locations as a shape layer on a map. The Geocoder then determines whether the shape overlays the parcel in question.

5. Are jurisdictional names the only political information obtainable from a Geocode?

No. Any statement of fact potentially associated with a jurisdiction can be reported by the Geocoder. The program correlates the statement of fact with all jurisdictions for which the statement is true and produces them as a list. For example, the Geocoder can correlate a list of U.S. Congress members with a list of Congressional districts and return a table showing not only the district in which a particular Geocode is located, but also the name of the member who represents the district

6. Can a Geocoder be used to measure distances?

Yes. Any information that can be displayed on a map can be returned and organized through a Geocoder. It can determine the distance between any two points and can use the information to, for example, generate driving directions between the points

7. Can a Geocoder be used with any kind of map?

Yes. A Geocoder can be used with any type of map to display anything that can be located on the map. The Geocoder can find two or more locations and determine whether any specified relationship exists between them.

8. What are "layers" and how are they used on a map?

Geocodes can be classified by a map maker into any grouping. For instance, all fire stations serving a map area can be grouped in a table, called "fire stations". This table can be loaded to the map as a "layer" which can be turned on (displayed) or turned off (not displayed).

Lines can also be grouped on a map. For instance, all streets can be described as linked points, called "segments". These linked points together describe the curves and straight lines of a street. When all the streets for a map are thus described, they can be loaded into a table, usually called a "shapefile" and loaded up to the map as a layer. By turning on and off the layer, the roads can be displayed or not.

Labels for features can also be linked to the features. for instance, the label for a street can be linked to the segments describing that street. All these labels can be compiled to a table with their links to the segments they describe. When a feature is to be displayed, the user can choose to see only the featuers, or the features with their labels. This linking is done in standard database relational formats. Thus, as the streeet is updated with a better description of its twists and turns, (I.e. segments are updated with more accurate geocodes.) the labels linked to those segments will automatically display with the updated map.

9. Can datasets be transferred from one program to another?

Yes, just as tables of data in a standard relational database can be copied to other databases, the files describing features for a map can be copied to other mapping databases. These tables are aggregations of geocodes. These types of files are called shapefiles. Specialized databases are used to store and read shapefiles so maps can be displayed.