workshop on census cartography and management, lusaka, zambia, 08–12 october 2007 geocoding &...

Workshop on Census Cartography and Management, Lusaka, Zambia, 08–12 October 2007

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• 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


BlocksEnumeration Areas

Country

District

Province

Locality

Given Country

Building Dwelling

Census Hierarchies


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


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


Coding Scheme

Digits 1-2 = State code

250131402013

Digits 6-11 = Census Tract Code

Digits 3-5 = County Code

Digit 12 = Blockgroup code


Data Collection Methods

• Two main methods:

• Direct Collection Approach

• Matching Approach


Direct Collection Approach

• Digitizing from available topographic maps • Direct collection using field techniques

(ex.GPS)

Areas, Street, Dwelling

Global Positioning System (GPS)Digitizing from a topographic map


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

First A

venue

Second A

venue

Right of Street

Left of Street

#1 #99

#2 #100

address number

Main Street

First A

venue

Right of Street

Left of Street

#1 #99

#2 #100

#51

#32

Nodes

Second A

venue

Street SegmentStreet Network


Data Maintenance

• Cleaning AddressesCleaning Addresses• Retaining only the key address elementsRetaining only the key address elements

• Establish a Matchcode (indicator of which Establish a Matchcode (indicator of which address elements will determine the address elements will determine the geocode)geocode)

• Eliminating extraneous charactersEliminating extraneous characters• Standardizing Spelling Standardizing Spelling

Record Street Address City State ZIPcode Latitude Longitude Areakey MatchCode1 344 East 63rd New York NY 10023 40.47 73.58 3502508100 AS0


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


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

Prepared by UNSD. For Further Inquiries please contact [email protected]


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 (GPS)


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.


m e a s u r e dd is t a n c e

x

u

x

c

b

aHow GPS determines a location’s coordinates


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


DGPS ground station

DGPS mobile receiving station

GPS satellites

Correction signal

Space segment

Control segmentUser segment

Differential GPS


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.


Where’s your Datum

workshop on census cartography and management, lusaka, zambia, 08–12 october 2007 geocoding &...

Documents

census cartography

geocoding gps slide

process of geocoding

census mapping

geocoding concepts relevant

unit slide

geographic unit

census processes approaches