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Geocoding & Data Collection with 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

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summary

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

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.

geocoding

Geocoding

  • Definitions:
    • Conceptual/Operational
  • Geocoding vs Georeferencing
  • Census Hierarchies
    • Coding Scheme
  • Data Collection Methods
    • Direct Collection
    • Matching Approach
  • Benefits for NSOs
slide5

“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)
definition of geocoding

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
slide7

…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)
georeferencing vs geocoding

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)
relationship to other census processes

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

census enumeration the geocoding system

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

Census Hierarchies

Define census geographic hierarchy

Develop geographic coding scheme

Development of an administrative and census units listing

census hierarchies some principles

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

example of administrative hierarchy

Example of Administrative Hierarchy

country

region

province

district

sub-district

rural locality

urban locality

Enumeration area

ward

Enumeration area

illustration of a nested admin hierarchy

Illustration of a nestedAdmin. Hierarchy

Provinces

Districts

Localities

Enumeration

areas

hierarchical coding scheme operational considerations

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
example of a coding scheme

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.

example of a coding scheme cont

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-
slide18

(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.
census hierarchies19

Census Hierarchies

Given Country

Country

Province

District

Locality

Enumeration Areas

Blocks

Building

Dwelling

geocoding classifications
Disaggregation into Spatial Entities or Civil Divisions and Compatibility

Geocoding Classifications

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

Data Collection Methods

  • Two main methods:
    • Direct Collection Approach
    • Matching Approach
direct collection 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

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

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 national statistical offices

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

concluding remarks

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
global positioning systems gps

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

global positioning systems c ont

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
how gps works

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

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

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

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

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

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

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 gps41

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.
geocoding classifications cont

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