1. Fundamentals of GIS Materials by Austin Troy © 2008 Lecture 18: Data Input: Geocoding and Digitizing By Austin Troy University of Vermont

2. Fundamentals of GIS Materials by Austin Troy © 2008 1.Geocoding

3. Fundamentals of GIS Materials by Austin Troy © 2008 Assign XY coordinates to entities having a different geographic reference system (e.g., street addresses) Convert lists/spreadsheets to features (usually points) Needs a mechanism to calculate the geographic coordinate for the address Address matching: uses street address database, created from a streets layer. 183 Williamson Road NE Roanoke, VA 24018 32.876374° N -78.228459° W What is Geocoding?

4. Fundamentals of GIS Materials by Austin Troy © 2008 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

5. Fundamentals of GIS Materials by Austin Troy © 2008 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. Computer knows topological left and right for each street segment

6. Fundamentals of GIS Materials by Austin Troy © 2008 How are addresses matched? Step 1: Computer looks for segment with correct name and address range Step 2: Computer interpolates the position of the address point on segment

7. Fundamentals of GIS Materials by Austin Troy © 2008 Geocoding example: 1060 Main Street Point is placed on even (upper) side of street Position of 1060 is interpolated Main St 1000 1100 10011101 L-F-ADDR L-T-ADDR R-F-ADDR R-T-ADDR It looks for Main street, then for the 1000-1100 block direction 1060 Main St

8. Fundamentals of GIS Materials by Austin Troy © 2008 First create an address locator in ArcCatalog Defines reference layer Also where you specify information about your reference layer that ArcGIS might not know, allowing for more efficient geocoding Many “styles” to choose from for address locators Address Matching in ArcGIS

9. Fundamentals of GIS Materials by Austin Troy © 2008 Why Geocoding Styles? Reference layers vary (e.g., right from address attribute might be fr_rt_add or add_rt_frm ) There are other general types of geocoding (e.g., geocoding points to the center of zip codes), and there are other types of address geocoding (e.g., using a property parcel layer as reference).

10. Fundamentals of GIS Materials by Austin Troy © 2008 Geocoding in ArcGIS In geocoding style interface: choose your reference file and then specify which attributes in the reference layer correspond with the inputs that ArcGIS needs to do geocoding. It also asks for some information about what to expect in your geocoding table (what the required attribute headings are called) and how sensitive to be to things like spelling differences

11. Fundamentals of GIS Materials by Austin Troy © 2008 Specify reference file Specify address range attributes Specify rules for address list Specify zone

12. Fundamentals of GIS Materials by Austin Troy © 2008 Geocoding in ArcGIS Now, in ArcMap we right- click on the address table and choose “Geocode Addresses” and we add the geocoding service we want to use This brings up the geocoding interface where we specify which field holds the address and which holds the zone Also specify an output shapefile or geodatabase and geocoding sensitivity

13. Fundamentals of GIS Materials by Austin Troy © 2008 Geocoding in ArcGIS After geocoding, it reports how many were successfully matched and how many were either totally unmatchable or potentially matchable We can interactively match the potential ones if we want

14. Fundamentals of GIS Materials by Austin Troy © 2008 Geocoding and Error Your Geocoding result is only as good as your reference data. If your streets layer is accurate only to 200 meters, then your geocoded points will be no better 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 others.

15. Fundamentals of GIS Materials by Austin Troy © 2008 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 100 m

16. Fundamentals of GIS Materials by Austin Troy © 2008 Geocoding and Error Here we see that many points were coded for Napa1 that were not coded for Napa2 possibly because the Napa1 street reference layer is newer, and has more streets

17. Fundamentals of GIS Materials by Austin Troy © 2008 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 100 m 300 m

18. Fundamentals of GIS Geocoding and Error A rural area with a long road segment: very imprecise An urban road segment: smaller, more precise Rural street segments are subject to greater error because longer street segments means more interpolation Materials by Austin Troy © 2008

19. Fundamentals of GIS Materials by Austin Troy © 2008 Geocoding in Action Mapping hazard zone properties in L.A. to see effects on property values

20. Fundamentals of GIS Materials by Austin Troy © 2008 XY Geocoding We can also create points from a simple table of latitude and longitude coords. Do this in ArcMap by clicking: CA Tricolored Blackbird breeding sites Then we specify the lat and long fields as well as the spatial reference system Lat and Long should be in decimal degrees

21. Fundamentals of GIS Materials by Austin Troy © 2008 2. Digitizing

22. Fundamentals of GIS Materials by Austin Troy © 2008 Digitizing This is generally the process of converting data from analog to digital with a device, such as a digitizing tablet or mouse, to create new vector features (or raster features in the case of scanning) User defines vector features by pointing and clicking.

23. Fundamentals of GIS Materials by Austin Troy © 2008 Digitizing Tablet digitizing involves use of a digitizing tablet or table A digitizing tablet is a big table with an electronic mesh that can sense the position of a digitizing cursor (puck) Transmits x and y coordinates of each mouse/cursor click to the computer and usually joins those with lines

24. Fundamentals of GIS Materials by Austin Troy © 2008 Digitizing Notice how map is attached with tape Source: http://ndis.nrel.colostate.edu/ndis/riparian/Tablet.jpg If the paper map moves, the digital map will be inaccurate, because the tablet is recording position relative to the tablet surface (mesh), not the map

25. Fundamentals of GIS Materials by Austin Troy © 2008 Digitizing Digitizing on a tablet requires defining “control points” which allow the conversion of the digitized map to real world coordinates.

26. Fundamentals of GIS Materials by Austin Troy © 2008 Digitizing Usually, a map corner point of known geographic location is digitized first and its coordinates are assigned in some sort of header file (e.g., quadrangle maps are registered to their corner “tics” stored in the quad index)

27. Fundamentals of GIS Materials by Austin Troy © 2008 Digitizing Snapping: Any unsnapped lines or polygons are snapped closed, and dangling lines are clipped off, based on user-defined tolerances Snap tolerance: won’t snap together Snap tolerance: will snap together Dangling arc Snapped to other arc

28. Fundamentals of GIS Materials by Austin Troy © 2008 Digitizing “Heads up” digitizing involves scanning a paper map to a digital file, or otherwise obtaining a digital raster map/image and digitizing “on top” of it on screen