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Geocoding & GPS. 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
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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
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
Geocoding: Concepts and Definitions • Definitions: • Conceptual • Operational • Geocoding vs Georeferencing
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. The purpose of this section is to introduce geocoding concepts relevant for census mapping and the different approaches to related data collection. • Reference: UN Report of the Expert Group Meeting on Contemporary Practices in Census Mapping and Use of Geographical Information Systems (2007)
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
Definition of Geocoding (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)
Geocoding vs. Georeferencing • Geocoding • A GIS operation for converting street addresses into spatial data that can be displayed as features on a map • Georeferencing • Aligning geographic data to a known coordinate system so it can be analyzed, viewed, and queried with other geographic data
Relationship to Other Census Processes • N.S.O Senior Managers must evaluate when the use of geocoding should be considered and consideration must be given due to: - range of hardware and software - availability of resources (street network reference layers, address database) - staff skill set - funding - user demands
Relationship to Other Census Processes (cont.) 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
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
Census Hierarchies 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
Census Hierarchies Country Given Country Province District Locality Enumeration Areas Blocks Building Dwelling
Disaggregation into Spatial Entities or Civil Divisions and Compatibility Geocoding Classifications • Resultant geocoded units placed within a set of Latitude and Longitudinal boundaries
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
Data Collection Methods • Two main methods: • Direct Collection Approach • Matching Approach
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)
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
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
Geocoding: Benefits for NSO’s 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
Concluding Remarks • Technologies are accessible and allow delineation irrespective of the existence of address • There is a need for an agreement on a definition of geocoding for census purposes • Many available methods and technologies exist to support accurate geocoding frameworks • Geocoding system is value-added for GIS based Spatial Analysis of Statistical Data
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 GPS has become a major tool in census cartographic applications
Global Positioning Systems (cont.) • 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
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
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.
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
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.
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
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
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
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.
