Geocoding & Data Collection with GPS

1. Geocoding & Data Collection with GPS

2. Summary • Introduction to Geocoding • Geocoding: Concepts and Definitions • Relationship to other Census Processes • Approaches to Data Collection • NSO Benefits & Concluding Remarks • Introduction to GPS • How GPS Works • Sources of Error & Accuracy • Selecting a GPS • Advantages & Disadvantages

3. Introduction Many NSOs have a specialized coding scheme and understand geocoding as a dynamic process Clarification within the statistical community Expansion and discussion on components and methods within the process of geocoding The purpose of this section is to introduce geocoding concepts relevant for census mapping and the different approaches to related data collection.

4. Geocoding • Definitions: • Conceptual/Operational • Geocoding vs Georeferencing • Census Hierarchies • Coding Scheme • Data Collection Methods • Direct Collection • Matching Approach • Benefits for NSOs

5. “Geocoding can be broadly defined as the assignment of a code to a geographic location. Usually however, Geocoding refers to a more specific assignment of geographic coordinates (latitude, longitude) to an individual address.” • Reference: UN Report of the Expert Group Meeting on Contemporary Practices in Census Mapping and Use of Geographical Information Systems (2007)

6. Definition of Geocoding • Conceptual - 2 situations: • The more general process of assigning geographic codes to features in a digital database. • A GIS function that determines a point location based on an address. It could generally be expected that such point locations will be relatively precise (eg +/-2m) in accuracy and will be based upon use of GPS technology. • Operational • Geocoding is the computer oriented process which converts information about a unit from which statistical information is collected into a set of coordinates describing the geographic position of that unit

7. …cont. • Operational Elements • Collecting precise data at the level of point locations (or very low geographic level such as a city block) and assigning codes for use in dissemination. • Coding the centroid, building corners, or building point of entry coordinates for a unit such as a block of land, building or dwelling • Coordinates must contain latitude and longitude or standardized x and y points for gridded interpolation. A Z or Zed coordinate may represent altitude or elevation • Codes cover each geographic unit and have a combinational relationship to distinguish different units (Enumeration Areas/Blocks)

8. Georeferencing vs Geocoding • Georeferencing • Aligning geographic data to a known coordinate system so it can be analyzed, viewed, and queried with other geographic data • Geocoding • The process of assigning geographic codes to features in a digital database (including the GIS operation for converting street addresses into spatial data that can be displayed as features on a map)

9. Relationship to Other Census Processes Movement into a fully GIS based approach to census mapping Generation of high quality maps for use in the collection phase Reduction of work required for updating maps for future censuses Aggregation of records into customized units for satisfying users’ requirements

10. Census Enumeration & the Geocoding System • Delineation irrespective of the existence of address • Ability to apply a geocode to any geographic areal unit • Flexible Coding Scheme • Ability to incorporate future administrative divisions • Pre-enumeration geocoding critical • links between GIS boundaries and tabular census data

11. Census Hierarchies Define census geographic hierarchy Develop geographic coding scheme Development of an administrative and census units listing

12. Census Hierarchies: some principles Internal political Boundaries Areal unit aggregation Resolution suitable to NSO needs and user demands Considers available datasets for continuous development The smaller area defined by the geocode the more flexible the results for subsequent users

13. Example of Administrative Hierarchy country region province district sub-district rural locality urban locality Enumeration area ward Enumeration area

14. Illustration of a nestedAdmin. Hierarchy Provinces Districts Localities Enumeration areas

15. Hierarchical Coding Scheme: operational considerations • Geographic units are numbered at each level of the administrative hierarchy (gaps between the numbers to allow changes) • For example at the province level, units may be numbered 5, 10, 15 and so on. A similar scheme would be used for lower-level administrative units and for enumeration areas. • Since there are often, for example, more districts in a province than provinces in a country, more digits may be required at lower levels • The unique identifier for the EA (the smallest-level unit): concatenation of the identifiers of the Admin. Units into which it falls

16. Example of a Coding Scheme A small country could use the following coding scheme: Province 2 digits District 3 digits Locality 4 digits EA 4 digits An EA code of 10 025 0105 0073 means that enumeration area number 73 is located in province 10, district 25 and locality 105. The unique code is stored in the database as a long integer or as a 13-character string variable.

17. Example of a Coding Scheme (cont.) • The variable type needs to be the same in the census database and the geographic database. • The integer variable has the advantage that subsets of records can be selected easily (SQL) • Example of query: • SELECT ID > 1203501550000 AND ID < 1203501560000 • Will find all EAs within locality number 155 in the database or on the digital map-

18. (cont.) • Special coding conventions needed to be developed, in cases where admin. and reporting units are not hierarchical • In any case, consistency should be complete in defining and using the administrative unit identifiers, since they are the link between GIS boundaries and the tabular census data. • Maintenance: NSOs should maintain a Master List of EA and admin. units and their respective codes and report any changes made to the Master List to the GIS and census databases.

19. Census Hierarchies Given Country Country Province District Locality Enumeration Areas Blocks Building Dwelling

20. Coding Scheme

21. Disaggregation into Spatial Entities or Civil Divisions and Compatibility Geocoding Classifications • Resultant geocoded units placed within a set of Latitude and Longitudinal boundaries

22. Data Collection Methods • Two main methods: • Direct Collection Approach • Matching Approach

23. Direct Collection Approach Digitizing from a topographic map Global Positioning System (GPS) Areas, Street, Dwelling • Digitizing from available topographic maps • Direct collection using field techniques (ex.GPS)

24. Matching approach Street Network Street Segment Left of Street First Avenue Left of Street First Avenue #1 #51 #99 address number #1 #99 Main Street #2 #32 #100 #2 #100 Right of Street Right of Street Second Avenue Second Avenue Nodes Using an Address locator database and street network database in a GIS Joining an address database to an existing spatial database for the area of interest

25. Data Maintenance • Cleaning Addresses • Retaining only the key address elements • Establish a Matchcode (indicator of which address elements will determine the geocode) • Eliminating extraneous characters • Standardizing Spelling

26. Staff Expertise Recommendations

27. Geocoding: Benefits for National Statistical Offices Improved map creation for the field Customizable map outputs for specified regional activities Coding techniques are transparent and transferable Fixates the groundwork for future statistical activities and coding schemes

28. Concluding Remarks • Technologies are accessible and allow delineation irrespective of the existence of address • Many available methods and technologies exist to support accurate geocoding frameworks • Geocoding system is value-added for GIS based Spatial Analysis of Statistical Data

29. Global Positioning Systems (GPS) Technology has revolutionized field mapping in recent years Prices of GPS receivers have dropped GPS methods have been integrated in many applications User groups are widespread (utilities management, surveying and navigation). GPS has contributed and advanced to improve field research in areas such as biology, forestry, geology, epidemiology and population studies

30. Global Positioning Systems (cont.) • GPS has become a major tool in census cartographic applications • Preparation and updating of enumerator (EA) maps for census activities • Location of point features such as service facilities or village centers • Coordinates can be downloaded or entered manually into a digital mapping system or GIS, and can be combined with existing, georeferenced information

31. How GPS Works GPS receivers collect the signals transmitted from more than 24 satellites—21 active satellites and three spares. The system is called NAVSTAR, and is maintained by the U.S. Department of Defense The satellites are circling the earth in six orbital planes at an altitude of approximately 20,000 km. At any given time five to eight GPS satellites are within the “field of view” of a user on the earth’s surface The position on the earth’s surface is determined by measuring the distance from several satellites

32. The global positioning system (GPS)

33. The global positioning system (cont.) • GPS satellites circle the Earth twice a day … • The satellite signal: • Three kinds of coded information essential for determining a position; • The receiver: • 1. Calculates the distance to the first satellite user is able to catch. • 2. Calculates the distance to a second satellite for which it is able to catch a signal. • 3. Repeats the operation mentioned under point 2 with a third satellite.

34. How GPS determines a location’s coordinates a b c

35. Sources of GPS signal errors Good visibility and bad visibility of satellites due to obstacles signal multipath Uncontrollable sources of error over which the user does not have control Atmosphere delays Receiver clock errors Orbital errors

36. Differential GPS

37. GPS Accuracy Inexpensive GPS receivers Within 15 to 100 meters for civilian applications. Differential GPS reduces error further Accuracy of about 3-10m can be achieved with quite affordable hardware and shorter observation times. More expensive systems and longer data collection for each coordinate reading can yield sub-meter accuracy.

38. Problems with GPS • In dense urban settings, the possible error of standard GPS (standard ~15m up to 100 meters) may not be sufficient • Differential GPS can be used for cross-checking GPS readings with other data sources • published maps • aerial photographs • sketch maps produced during fieldwork

39. Selecting a GPS Unit • Commercially available GPS receivers vary in price and capabilities • Technical specifications determine the accuracy by which positions can be achieved • The more powerful a receiver, the more expensive it will be • In many mapping applications, the accuracy of standard systems is quite sufficient • Receivers also vary in terms of user-friendliness, tracking capabilities which are useful in navigation

40. Summary: Advantages and Disadvantages of GPS Advantages • Fairly inexpensive, easy-to-use field data collection • Modern units require very little training for proper use • Collected data can be read directly into GIS databases minimizing intermediate data entry or data conversion steps • Worldwide availability • Sufficient accuracy for many census mapping applications—high accuracy achievable with differential correction

41. Summary: Advantages and Disadvantages of GPS Disadvantages • Signal may be obstructed in dense urban or wooded areas • Standard GPS accuracy may require differential techniques • Differential GPS is more expensive, requires more time in field data collection and more complex post-processing to obtain more accurate information • A very large number of GPS units may be required for only a short period of data collection.

42. Where’s your Datum

43. Geocoding Classifications (cont.) Initial creation of Civil Divisions through digitizing or segmentation/pixel based-approaches Low to Zero levels of sampling through the accurate placing of coded units, but flexible enough to include changes Appropriate detail that fits with the boundaries of a geographic area for a given country