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Point Files and Geocoding This document summarizes concepts involved with "geocoding" address oriented data and developing geocodes for inclusion on address-oriented data records. For assistance in geocoding or to acquire geocoding resources, contact Proximity. Geocoding is the process of determining and assigning latitude-longitude coordinates and associated geocodes for address-oriented data. The geocoding process reviewed here applies to the U.S. Geocodes are numeric codes that are shorthand references for geographic areas. For example, a census tract geocode is a 6 character numeric code. When an address is geocoded, a unique census tract can be assigned to that address/address record. Taking this a step further, a census tract is unique only to a specific county. So, the complete geocode for a census tract coding requires the census tract code, county code, and state code. There are at least three types of address-oriented data that are important candidates for geocoding: student addresses, schools and school district buildings in the school district, and all housing units within a school district. How Geocoding Works. For an address record to be geocoded, two data resources are required: the address from the address record and a street/road geographic base file to use in determining the geocodes from the address. One widely used street/road geographic base file is the Census Bureau TIGER/Line. TIGER/Line files contain data records that describe attributes of line (street and other) segments. There are more than 33 million street segments included in the national scope TIGER/Line files. Consider the geocoding application where it is desired to geocode the address 105 Oak Avenue in some city/school district. The map presented below can be used to describe the type of data contained in TIGER/Line files (TLF). In this map, the segment of Oak Avenue is shown as existing with the 100-block address range extending from a start node point to an ending node point. The TLF contains a record for this specific segment of Oak Avenue (actually many related records). In that record, the name of the segment is given as Oak, it is coded as a street (e.g., as differentiated from a stream), it is given street type of Ave, and right- and left-side address ranges are given. The start node and end node are (x,y) coordinates, longitude and latitude, in decimal degrees. In addition, the record contains all of the relevant geographic codes (geocodes) for each side of the line segment. 
The TLF data record contains these data values in a manner as characterized by the following record sample.  The data field names are real field names for fields located in TIGER/Line record type 1. The field FENAME contains the Feature Name, and so on. In a geocoding operation, software determines that the candidate address, 105 Oak Avenue, is on the left side of the TLF line segment record shown above. This enable the software to extract the geocodes from the left side TLF record. It enables the software to use the latitude-longitude of the "from node" and the "to node" point and compute an interpolated value for the address latitude-longitude. The software then outputs the geocodes and latitude-longitude for the 105 Oak Avenue address record. Using the Geocodes and Latitude-Longitude Values. The existence of the latitude-longitude for an address oriented record enables GIS and mapping software to display the location of the point on a map in the correct relative position. The latitude-longitude is also useful for routing (such as bus routing) and computational applications that require shortest distance or nearest point type computations. The geocodes enable software to summarize data for a geographic area based on the geocode assigned to a set of address records. For example, the total number of housing units in an attendance area (that is comprised of a set of whole census blocks) can be determined by summing the housing unit records with blocks assigned to that attendance area. |
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