1. Geocoding Chapter 16 GISV431 &GEN405 Dr W Britz

2. Georeferencing, Transformations and Geocoding Georeferencing is the aligning of geographic data to a known coordinate system so it can be viewed, queried, and analyzed with other geographic data. Georeferencing may involve shifting, rotating, scaling, skewing,and in some cases warping, rubber sheeting, or orthorectifying the data. (It has to do with real world coordinates) Transformations are used when Georeferencing

3. Transformation The process of converting the coordinates of a map or an image from one system to another, typically by shifting, rotating, scaling, skewing, or projecting them. transformation Illustration [data conversion]

4. Geocoding A GIS operation for converting street addresses into spatial data that can be displayed as features on a map, usually by referencing address information from a street segment data layer.

5. 5 What does Chang say about Geocoding? Geocoding refers to the process of assigning spatial locations to data that are in tabular format but have fields that describe their locations. Address geocoding is the most common type of geocoding, which plots street addresses as point features on a map. Address geocoding requires two sets of data. The first data set contains individual street addresses in a table, one record per address. The second is a reference database that consists of a street map and attributes for each street segment such as the street name, address ranges, and ZIP/postal codes. Address geocoding interpolates the location of a street address by comparing it with data in the reference database.

6. 6 The Geocoding Process The geocoding process consists of two phases: preprocessing and matching. The preprocessing phase involves parsing and address standardization. The next phase matches an address against a reference database and plots the address as a point feature.

7. 7 Address Matching Options Typically, a geocoding engine has provisions for relaxing the matching conditions but uses a scoring system to quantify the matches at the same time. Because of the various matching options, we can expect to run the geocoding process more than once. GIS packages have the side offset and end offset options for plotting geocoded points.

8. 8 The end offset moves a geocoded point away from the end point of a street segment, and the side offset places a geocoded point away from the side of a street segment.

9. 9 Other Types of Geocoding Intersection matching matches address data with street intersections on a map. ZIP code geocoding refers to the process of matching a ZIP code to its centroid location. Reverse geocoding is a process of converting latitude and longitude coordinate data of locations into descriptive addresses.

10. 10 Applications of Geocoding 1.Location-based services 2.Business applications 3.Wireless emergency services 4.Crime mapping and analysis 5.Public health

11. Let’s focus on Address Matching Geocoding Two inputs: – 1) a DBF or text table with the address records to be geocoded and – 2)a geographic reference layer, like streets Output: a point file, where each point represents an address record

12. How are addresses matched? Common method: matching address to street ranges. Urban areas: usually each street segment (arc) corresponds to a block. Each segment has attributes for the left from and to and right from and to addresses. Hence computer knows the left address and right address and the beginning and end of the block. (topology) Computer matches street name, address range Interpolates the position of the address point on segment

13. Address Matching in ArcGIS First create a address locator Defines reference layer Also where you specify information about your reference layer that ArcGIS might not know, allowing for more efficient geocoding

14. Geocoding in Arc GIS We create our address locator in Arc Catalog. This will bring up a dialogue asking for the geocoding style for that geocoding service

15. Geocoding and Error Your Geocoding result is only as good as your reference data. If your streets layer is accurate only to 200 meters of accuracy, so will your geocoded points be. If your streets are consistently 100 meters to the north, then your points will be the same too Some roads layers may have better attributes than other too.

16. Geocoding and Error Here’s an example where the same address list was geocoded with two different street layers. Note here how the same house is 100 m off between the two geocoding attempts

17. Geocoding and Error Here we see that many points were coded for Napa1 that were not coded for Napa2 possibly because Napa1’s street reference layer is newer, and has more streets

18. Geocoding and Error This error is due to an attribute error in one of the layers which puts that address in the wrong street segment

19. ©2007 Austin Troy Geocoding and Error A rural area with a long road segment: very imprecise An urban road segment: smaller, more precise Rural street segments are also more subject to more error because street segments longer, so relies more on interpolation

20. XY Geocoding We can also create points from a table by their latitude and longitude Do this by clicking: CA haz. waste sites Then we specify the lat and long fields as well as the spatial reference system Lat and Long should be in decimal degrees