Geography Reference
In-Depth Information
A thousandth is 111 meters.
A ten-thousandth is 11.1 meters.
A hundred-thousandth is 1.11 meters.
A millionth is 0.111 meters.
The precision of the recorded number (e.g., 38.047734), where the last digit is in the “millionths” position,
might suggest that the position is known to about the nearest tenth of a meter (i.e., 0.111 meters). Here is
where your knowledge of the difference between accuracy and precision comes in. TIGER data is rarely
good to 10 centimeters accuracy. Ten meters might be more like it. Sometimes the last two digits are zeros
in the files, which implies an accuracy of the nearest ten-thousandth of a degree, or about 10 meters—
roughly 33 feet.
Address Locators
The second piece required for converting a set of addresses to a set of latitude-longitude coordinates is
a mechanism for parsing the addresses so that queries to a TIGER-style database 18 will be recognized
by the database. With ArcGIS this takes the form of creating an “address locator.” You kick the process
off in ArcCatalog by finding Address Locators in the Catalog Tree, expanding it, and double-clicking on
Create New Address Locator. This opens a window that allows you to pick an address locator style. The
idea here is to provide information about the general format of addresses in the region of interest. You
have more than three dozen different styles to pick from. Your choice depends on the structure of your
reference data and what the form of address in the region of interest looks like.
Once the choice of address locator is made, you will encounter a dialog box that will let you specify a
number of parameters that will allow the form of your input addresses to be matched with the form of
the locations in the reference database. If you are only typing one or a few addresses into the address
locator, you can monitor whether or not you were successful. If you have a large file of addresses to find,
however, the problem gets a little stickier. Some addresses may pass muster and be represented on the
map; others may be rejected. Still others may find close, but not exact, matches. These latter ones are
assigned a score as to the closeness of the match, based on a number of factors. Successfully converting a
large file of addresses requires both some tweaking on your part and something coming close to artificial
intelligence on the part of the software. In the Step-by-Step section, you will go through the process of
converting addresses.
18 Many agencies and companies have taken the basic TIGER files and enhanced them.
 
 
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