Massachusetts Institute of Technology Department of Urban Studies and Planning

Transcription

1 Massachusetts Institute of Technology Department of Urban Studies and Planning A Workshop on Geographic Information Systems Urban Planning and Social Science Laboratory Fall 2002 Geocoding Introduction Sometimes you may have data that is not in a spatial format but does contain feature location information. You might have, for example, a database of customer addresses, a table of accident locations based on highway milepost locations, or a file of endangered species sitings from Global Positioning System (GPS) readings. Can you use this data in ArcView GIS? Yes. If you have files containing location information for geographic features or occurrences, you can add them to an ArcView project as event themes. This process is referred to as geocoding. In ArcView, the resulting point theme is called event theme. An event theme is constructed from an event table that contains geographic locations information. You will learn how to create new themes from tables of x,y coordinates or addresses. Once you have added the data to your map, the layer behaves just like any other feature layer. For instance, you can decide whether or not you want to disply it, symbolize it, or display a subset of feasutes that meet some criteria. 1. XY Geocoding Table of Contents 1.1 Science of XY Geocoding 1.2 XY Geocoding in ArcView (Lab #8 part #1*) 1.3 Issues with XY Geocoding 2. Address Geocoding 2.1 Science of Address Geocoding 2.2 Address Geocoding in ArcView (Lab #8 part #2*) 2.3 Issues with Address Geocoding * Refer back to the Lecture Notes. 0

2 3. Summary 4. References 1. XY Geocoding 1.1 Science of XY Geocoding X,y coordinates describe discrete locations on the earthís surface such as the location of fire hydrants in a city or the points where soil samples were collected. You can easily collect x,y coordinate data using a global positioning system (GPS) device. In order to add a table of x,y coordinates to your map, the table must contain two fields, one for the x-coordinate and one for the y-coordinate. The values in the fields may represent any coordinate system and units such as latitude and longitude or meters. In ArcView, locations stored in a tabular format are called event locations, or events, and the tables containing the locations are referred to as event tables. In the lab #8, the x,y coordinates for public airports in Massachusetts will be used to create an airport theme. The source tables for location information can be in any format that ArcView supports (INFO, dbase, SQL, or comma- and tab-delimited text files). You must first add the file to your project as a table. 0

3 The the locations can be in any coordinate system, not just latitude and longitude. 1.2 XY Geocoding in ArcView Event themes are created from a source table by using the Add Event Theme on the View menu. When the Add Event Theme dialog opens, you select the table containing the event information, click on the icon for x,y events, and select the fields containing the event information. ArcView creates the event theme (a point theme since it is based on x,y coordinates) and displays it in the view. ArcView also creates a theme attribute table for the new event theme. Event themes are temporary unless you convert them to shapefile themes using the Convert to Shapefile choice on the Theme menu. [Lab #8 part #2*] 1.3 Issues with XY Geocoding The features on a map reference the the actual locations of the objects they represent in the real world. ArcView stores features using x,y coordinates. These coordinates are linked to realworld locations by a coordinate system. The coordinate system specifies a datum and a map projection. Datum A datum is a mathematical representation of the shape of the earthís surface. A datum is defined by a spheroid, which approximates the shape of the earth and the spheroidís position relative to the center of the earth. There are many spheroids that represent the shape of the earth and many more datums based on them. A horizontal datum provides a frame of reference for measuring locations on the surface of the earth. It defines the origin and orientation of latitude and longitude lines. A local datum aligns its spheroid to closely fit the earth's surface in a particular area; its origin point is located on the surface of the earth. The coordinates of the origin point are fixed, and all other points are calculated from this control point. The coordinate system origin of a local datum is not at the center of the earth. NAD27 and the European Datum of 1950 are local datums. In the last 15 years, satellite data has provided geodesists - mathematicians concerned with the precise measurement of the shape and size of the earth - with new measurements to define the best earth-fitting ellipsoid, which relates coordinates to the earthís center of mass. Unlike a local datum, an earth-centered, or geocentric, datum does not have an initial point of origin. The earthís center of mass is, in a sense, the origin. The most recently developed and widely used datum is the World Geodetic System of 1984 (WGS84). It serves as the framework for supporting locational measurement worldwide. GPS measurements are based on the WGS84 datum. * Refer back to the Lecture Notes.

4 Map projection Map projections are systematic transformations of the spheroidal shape of the earth so that the curved, threedimensional shape of a geographic area on the earth can be represented in two dimensions, as x,y coordinates. Maps are flat, but the surfaces they represent are curved. Transforming three-dimensional space onto a twodimensional map is called "projection". Projection formulas are mathematical expressions that convert data from a geographical location (latitude and longitude) on a Earth s sphere or spheroid to a representative location on a flat surface. This process inevitably distorts at least one of these properties: shape, area, distance, or direction. For small areas, such as a city or county, the distortion will probably not be great enough to affect your map or measurements. If youíre working at the national, continental, or global level, youíll want to choose a map projection that minimizes the distortion based on the requirements of your specific project. Back to Top

5 2. Address Geocoding 2.1 Science of Address Geocoding Street address are the most common form of geographic information. Taking addresses from a table and turning them into a theme of point features is called address geocoding. Because so much data is referenced by addresses, address geocoding is a powerful tool for analyzing spatial distributions. With address geocoding you can, for example, see exactly where your customers live in relation to your business, where traffic accidents occur, where your your competition is, and where the highest incidences of crimes in a city are. In the lab #8, the addresses of public schools in Cambridge, MA will be used to create an public school theme. ORGANIZATION NAME,ORGANIZATION CODE,ADDRESS LINE 1,ADDRESS LINE 2,CITY/TOWN,STATE,ZIP CODE Baldwin, ,28 Sacramento Street,,Cambridge,MA, Amigos School, ,158 Spring Street,,Cambridge,MA,02141 Cambridgeport, ,89 Elm Street,,Cambridge,MA,02139 M E Fitzgerald, ,70 Rindge Avenue,,Cambridge,MA,02140 Haggerty, ,110 Cushing Street,,Cambridge,MA,02138 Charles G Harrington, ,850 Cambridge Street,,Cambridge,MA,02141 King, ,100 Putnam Avenue,,Cambridge,MA,02139 King Open, ,100 Putnam Avenue,,Cambridge,MA,02139 Longfellow, ,359 Broadway,,Cambridge,MA,02139 Morse, ,40 Granite Street,,Cambridge,MA,02139 Peabody, ,44 Linnaean Street,,Cambridge,MA,02138 Robert F Kennedy, ,158 Spring Street,,Cambridge,MA,02141 John M Tobin, ,197 Vassal Lane,,Cambridge,MA,02138 Graham and Parks, ,15 Upton Street,,Cambridge,MA,02139 Fletcher/Maynard Academy, ,225 Windsor Street,,Cambridge,MA,02141 Camb Rindge and Latin, ,459 Broadway,,Cambridge,MA,02138 Benjamin Banneker Charter School, ,21 Notre Dame Avenue,,Cambridge,MA,02142 Benjamin Banneker Charter School, ,21 Notre Dame Avenue,,Cambridge,MA,02142 Reference Theme To perform address geocoding, you must have a theme whose attribute table contains fields of address information. This theme is called the reference theme. Common reference themes are private and public utilities, highways, and city streets (also called street networks).

6 To use a theme for address geocoding, you need to know the style of the addresses its table contains. ArcView supports eight address styles.address styles you can use in ArcView are: US Streets Features are represented by two numeric ranges, one range for the left side of the street and one range for the right side of the street. This is usually referred to as parity (even and odd); that is, one side of the street has a range of odd numbers and the other side has a range of even numbers. US Streets with Zones Same as US Streets, but includes left and right zones (i.e., postal zones, city names, or US ZIP Codes). US Single Range Features are represented by a single range of numbers. No parity (left/right and odd/even) of the numbers is required. US Single Range with Zone Same as US Single Range, but zones are available for matching.

7 US Single House Features are represented by a single house number; no ranges are used. US Single House with Zone Same as US Single House Address but with zone information. Single Field One field, the key field, in the reference theme's attribute table contains the geocoding information. Features are matched using only values from this field. ZIP+4 Values in two fields, containing US ZIP Code and +4 values, are used for matching. (e.g., ). Features are matched against values in these two fields. ZIP+4 Range Similar to ZIP+4 except that this style allows the addresses to be matched against a +4 range by defining a low +4 value and a high +4 value (e.g., and ). 5-digit ZIP Addresses are matched using a 5-digit ZIP Code (e.g., 92397). Interpolation During the address geocoding process, ArcView uses the address ranges of the reference theme's line segments in the index file, estimates (interpolates) the location of the address to be matched, and then assigns the coordinates for the event. For example, if an address of 33 is matched against a line with a length of 450 feet and addresses of 2-98 on the right and 1-99 on the left (even and odd parity), the location of the address (point feature) would be on the left side of the line segment 33%, or 150 feet, from the line's beginning point. 2.2 Address Geocoding in ArcView Making a theme matchable Before you can match a list of addresses to a reference theme, you must make the reference theme matchable using the Theme Properties dialog. In the Theme Properties dialog, you select the style of the theme's addresses and the fields in the theme attribute table containing the address information. The information you define in the Theme Properties dialog is used to create the reference theme's geocoding index files, which are stored in ArcView's working directory with.ixs and.mxs extensions. It is the index files that make the theme matchable Matching a list of addresses To match a list of addresses, you choose Geocode Addresses from the View menu to open

8 the Geocode Addresses dialog. Next, select the reference theme, the name of the table containing the addresses, the field in the table containing address information. You can also name the new point theme that will be created and specify where it will be saved. When you are ready to have ArcView begin geocoding, click Batch Match. ArcView looks for matches between addresses in the table and address information in the reference theme. For each match, ArcView creates a point feature in a new shapefile theme, using the geographic coordinates from the matched street features and assigning them to the point. [Lab #8 part #2*] 2.3 Issues with Address Geocoding Base Map Problems Missing address ranges Example: Cambridge TIGER file Gaps in address ranges Missing ZIP codes Incorrect ZIP codes Some ZIP codes refer to points, e.g., a single building Inaccurate geometry All this data ages over time New streets Street changes ZIP code changes Under the control of the US Postal Service Primary purpose is for delivering mail, not demographic analysis or geocoding Address Errors Some towns in Eastern Massachusetts got new ZIP codes a few years ago! Misspelled street and town names Missing or inaccurate address numbers Using a spelled-out address number: "One Main St." instead of "1 Main St." Addresses that are not traditional street addresses: 1 Cambridge Center 2 Kendall Square 10 Post Office Square More than one name for a street: "Mass. Ave." or "Massachusetts Ave." or "State Hwy. 2A" "I-90" or "Massachusetts Turnpike" or "Mass. Pike" "O'Brien Hwy." or "Msgr. O'Brien Hwy" or "Monsignor O'Brien Hwy." "Glynn Way" or "Theo A. Glynn Way" "3 St." or "3rd St." or "Third St." If ZIP codes have changed recently, do the addresses use the old or new ZIP codes? Back to Top

9 3. Summary You don't always have to have a data source, such as a shapefile, to add data to your map. If you have some tabular data that contains geographic locations in the form of x,y coordinates, you can add this to a map as well. Back to Top 4. References Booth, Bob, and Andy Mitchell Getting Started with ArcGIS, GIS by ESRI. Redlands, CA: ESRI Press. ESRI Introduction to ArcView 3.x. ESRI 2002 [cited November ]. Available from Hutchinson, Scott, and Larry Daniel Inside ArcView GIS. 2nd ed. Santa Fe, NM: OnWord Press. Karman, Michael, and Gary Amdahl, eds The ESRI Press Dictionary of GIS Terminology. Redlands, CA: ESRI Press. Minami, Michael Using ArcMap, GIS by ESRI. Redlands, CA: ESRI Press. Worboys, Michael F GIS: A Computing Perspective. Bristol, PA: Taylor & Francis Inc. Back to Top 0