Arcview GIS Quick Tips

ArcView GIS Quick Tips

Geocoding

To geocode addresses, you need a reference theme (street theme) to serve as a base map. The reference theme is usually a street theme with attributes that specify the street name, street type, and a range of addresses that occur along each street. In this case we will be using the roads_42 file as a reference theme.

You also need and address table (.dbf) that contains the addresses you want to locate as points on the map.

The geocoding process consists of three steps:

1. Preparing the reference theme for geocoding (this is called making the reference theme matchable).
2. Batch matching the addresses. During this step, ArcView compares the table of addresses to the street information in the reference theme and looks for matches. Each matched address is added as a point in a new geocoded theme.
3. Rematching the addresses that couldn’t be batch matched. ArcView also adds these points to the geocoded theme.

The step first in geocoding is to make the reference theme matchable by selecting an address style. An address style determines which address components (e.g. street name, street type, direction, zip code) will be required in the address-matching process. The address style you choose will depend mainly on the fields that are available in the reference theme table.

1. Open ArcView. Create a new project with a new view. Add the roads_42.shp file as a new theme. This is the references theme to your base map.
   
2. Turn on this theme so it is visible in the view then click the table button from the toolbar at top to view the properties of the roads theme.
   NOTICE: The addresses in this table are divided into several attributes. For example, some field attributes include L_add_from, and L_add_to. These contain the lowest and highest address numbers on the left had side of the street. There are also attributes like the zip code and even street name. When you geocode, you’ll select and address style that uses one or more of these fields for matching.

1. Now click the table icon in the table of contents in project windows. Click “add table” and locate the abandoned_42.dbf file from proper directory.

NOTICE: This table is not a theme attribute table because it is not associated with any theme. It is a database file of addresses for known abandoned homes. Geocoding allows the analyst to pinpoint these know addresses on an existing street map.

1. Make the view window active. From the Theme menu at top, choose Properties to display the Theme Properties dialog box.
   
2. Click on the Geocoding icon on the left to display the geocoding theme properties.

NOTICE: ArcView selects “US streets with Zone” in the address field. It chooses this address style because there are fields in the reference theme table that correspond to each of the required address components. Because the database file of abandoned homes does not contain any fields for zip codes, change the address style to “US Streets.” The fewer address components or attributes ArcView has to match, the faster the geocoding.

1. Scroll through the rest of the list of attributes/components so you can see the requirements for this address style. Notices that the US Streets style requires address attributes such as LeftFrom and RightFrom. These are already set correctly so you do not need to change them.
   
2. Unless it is already set, change the optional PreDir (prefix direction) to none because the abandoned house .dbf table does not have prefix directions before the street names. ArcView will ignore this attribute during the matching process.
   
3. Click OK to set the geocoding theme properties. The Build Geocoding Index dialog box displays.
   
4. Click Yes. ArcView builds geocoding indexes for the address fields in the attribute table to make the address geocoding go faster.

The Roads_42 reference theme is now matchable.

The Second step in address geocoding is to batch match addresses in a table (.dbf) to the reference theme roads_42. ArcView completes two processes during batch matching: the first is address matching where addresses in the address table are compared to address ranges in the reference theme. The second is geocoding where each matched address in the address table is assigned coordinates and made a point feature in a new theme.

1. Make the view the active window. Choose Geocode Address from the View menu. The Geocode Addresses dialog box displays.

NOTICE: The reference theme drop-down list shows the reference theme Roads_42. The “Using Address Style” box shows the address style you selected for the reference theme. The address table list lets you select the address table (.dbf) you want to geocode.

1. From the Address Field drop-down box choose St_name because this is the .dbf field that you are trying to match.
   
2. From the Address Table drop-down list, choose Abandon_42 because these are the addresses you want to geocode. To match every address in the table automatically, click Batch Match.
   
3. For all addresses that were not matched. Click interactive match. By doing this you can selectively geocode the remaing addresses.