Geocoding

1
Geocoding 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

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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

4
Geocoding Concepts and Definitions

• Definitions
• Conceptual
• Operational
• Geocoding vs Georeferencing

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.
  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)

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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

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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)

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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

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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

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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

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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

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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

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Census Hierarchies
Country
Given Country
Province
District
Locality
Enumeration Areas
Blocks
Building
Dwelling
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Geocoding Classifications

• Disaggregation into Spatial Entities or Civil
  Divisions and Compatibility

1st Region Province
2nd District Municipality
3rd Town/Village
4th Dwelling

• Resultant geocoded units placed within a set of
  Latitude and Longitudinal boundaries

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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

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Coding Scheme
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Data Collection Methods

• Two main methods
• Direct Collection Approach
• Matching Approach

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Direct Collection Approach

• Digitizing from available topographic maps
• Direct collection using field techniques (ex.GPS)

Global Positioning System (GPS)
Digitizing from a topographic map
Areas, Street, Dwelling
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Matching approach

• 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

Street Network
Street Segment
Left of Street
First Avenue
Left of Street
First Avenue
1
99
51
address number
1
99
Main Street
2
100
32
2
100
Right of Street
Right of Street
Second Avenue
Second Avenue
Nodes
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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

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Staff Expertise Recommendations
Task/condition Direct collection Matching
Existence of digital base map for country Highly desirable Highly desirable
Statistical staff with expertise in use of GPS Essential Not Essential
Acquisition of large numbers of GPS receivers Essential Not Essential
Geo-referenced list of addresses or equivalent Not Essential Essential
Excellent address matching algorithms Not Essential Essential
Existence of a rational, consistent, and locally-recognized addressing system for housing units Highly desirable Essential
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Geocoding Benefits for NSOs

• 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

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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

Prepared by UNSD. For Further Inquiries please
contact laaribi_at_un.org
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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

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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

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How GPS Works

• GPS receivers collect the signals transmitted
  from more than 24 satellites21 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 earths surface
• The position on the earths surface is determined
  by measuring the distance from several satellites

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The global positioning system (GPS)
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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.

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How GPS determines a locations coordinates
a
b
c
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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

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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.

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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

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Differential GPS
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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

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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
  applicationshigh accuracy achievable with
  differential correction

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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.

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Wheres your Datum