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Geocoding & GPS

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Title: Geocoding & GPS


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

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

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)

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
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)

8
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

9
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

10
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

11
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

12
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

13
Census Hierarchies
Country
Given Country
Province
District
Locality
Enumeration Areas
Blocks
Building
Dwelling
14
Geocoding Classifications
  • Disaggregation into Spatial Entities or Civil
    Divisions and Compatibility
  • Resultant geocoded units placed within a set of
    Latitude and Longitudinal boundaries

15
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

16
Coding Scheme
17
Data Collection Methods
  • Two main methods
  • Direct Collection Approach
  • Matching Approach

18
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
19
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
20
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

21
Staff Expertise Recommendations
22
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

23
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

24
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

25
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

26
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

27
The global positioning system (GPS)
28
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.

29
How GPS determines a locations coordinates
a
b
c
30
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

31
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.

32
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

33
Differential GPS
34
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

35
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

36
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.

37
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