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Geocoding Public Health Data. Lecture 5 Locating Street Addresses and Global Positioning System GIS and RS in Public Health Edmund Seto, Ph.D. School of Public Health University of California, Berkeley. Spatial Data.

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geocoding public health data

Geocoding Public Health Data

Lecture 5

Locating Street Addresses

and

Global Positioning System

GIS and RS in Public HealthEdmund Seto, Ph.D.School of Public HealthUniversity of California, Berkeley

spatial data
Spatial Data

In previous lectures we talked about the wide availability of spatial data.

Public Health data are often inherently spatial:

Vital stats have residential street addresses

A cohort study of exposure to air pollution might

consider residence and work addresses

The problem is how to get these locations on a map. (ie. in a format that is readily usable within a GIS)

The process of getting such data placed onto a map or within a GIS is known as Geocoding.

types of geocoding
Types of Geocoding
  • Relational Joins for Spatially Aggregated Data
  • Address Matching
  • Global Positioning System
  • Other Alternatives
aggregated data
Aggregated Data

For example: A table of data that is grouped at the county level… How do we match this up with GIS map of counties?

relational join
Relational Join

A GIS is based on the concept of relational databases, which allow us match geographic features with the corresponding attribute data.

In exercises 1 and 2, we saw that a table of attributes can be “joined”with a table of geographic features based on a common identifier in GIS.

Where common identifiers might be:

country name, county name, postal code, etc.

slide6

cdc wonder 1999 disease of circulatory system

age-adj to yr 2000 pop

ICD codes I00-I99

geocoding limitations
Geocoding Limitations

Beware! Your choice, or lack of choice in terms of the scale, or choice of area-based measure (individual address vs census tract vs block vs zip, etc) can affect the results of your study.

Modifiable Area Unit Problem

Openshaw, S., and P. Taylor, 1979: A million or so correlation coefficients: Three experiments

on the modifiable area unit problem, in Statistical Applications in the Spatial Sciences, ed.

N. Wrigley, (London: Pion), 127-144.

slide13

Nancy Krieger, Jarvis T. Chen, Pamela D. Waterman, Mah-Jabeen Soobader, S. V. Subramanian and Rosa Carson

Geocoding and Monitoring of US Socioeconomic Inequalities in Mortality and Cancer Incidence: Does the Choice of Area-based Measure and Geographic Level Matter? The Public Health Disparities Geocoding Project Am J Epidemiol 2002; 156:471-482

street addresses
Street Addresses

For example: A table of individual street addresses… How do we match this up with a GIS map of streets?

address matching in gis
Address Matching in GIS

This is known as

Address

Matching

Street geography layer:

Street: name, starting & ending address

1234 University Ave

1. matching

2. interpolation

Coordinates for the address

geocoding tiger
Geocoding TIGER

The US Census Bureau’s TIGER files include street address information.

slide17

FRADDL TOADDL

University Ave

FRADDR TOADDR

geocoding services in arcgis
Geocoding Services in ArcGIS

Arcview provides a tool known as Geocoding Services that allows us to geocode, in particular, street addresses.

For address matching, Geocoding Services works along the same principle as we have just discussed, relying on street geography, and interpolating the address numbers.

Arcview comes with a license for StreetMap USA.

For the following example, however, we will rely on TIGER files for our Geocoding Service.

geocoding berkeley clinics
Geocoding Berkeley Clinics

From the Yellow Pages, I created a table of Berkeley Clinics and their addresses.

We will create a Geocoding Service in Arcview for geocoding these addresses. The Geocoding Service will be based on the Berkeley streets file that we clipped out from TIGER data in exercise 2.

slide22

Addresses can be formated in a number of different ways, and here you can choose the style that fits the data that you’re using. For TIGER data we will use:US Streets (File-based)

address matching difficulties
Address Matching Difficulties

Address Matching isn’t as easy as it seems. Even in our little example, we only had good matches for around 50% of our addresses. And we only tried 18 addresses in Berkeley!

Problems:

Not all mailing addresses correspond to street addresses:

PO Box

140 Warren Hall

Trailer Parks

Newly developed areas lack street maps for geocoding

Quality of data, which could be poorly formatted address data and/or errors in street geography data.

address matching difficulties31
Address Matching Difficulties

Texas DOH Guideline for Geocoding

http://www.tdh.state.tx.us/gis/Images/Docs/GUIDELINE_FOR_GEOCODING.pdf

New Jersey Geocoding problems

http://www.state.nj.us/health/chs/releasable.htm

Jane McElroy’s talk - Univ of Wisc.

Geocoding addresses from a large population-based study: Lessons learned and applied

http://www.pophealth.wisc.edu/lecture/pm803-02/pm803-25slides.ppt

no geographic data
No Geographic Data

For example: Mapping data that cannot be easily located on existing maps.

Residential locations in rural villages

Environmental sampling sites

Infectious disease vector breeding & control sites

global positioning system
Global Positioning System
  • What is the Global Positioning System (GPS)?
    • A global navigation system
    • Answers the questions:
      • Where am I now?
      • How far is my destination?
      • How do I reach my destination?
gps background
GPS Background
  • A satellite-based navigation system
  • 24 very high-altitude orbiting satellites
  • Launched by U.S. Department of Defense
  • 24-hour, worldwide coverage
  • Free and reliable
  • Capable of very high accuracy location measurements
how does gps work
How Does GPS Work?
  • Uses radio signals transmitted from satellites to triangulate a position on the earth
  • 4 unknowns: x, y, z, time
  • Hence 4 satellites are required for triangulation
triangulating position

1

2

d2

d1

3

d3

Triangulating Position
  • 3 Satellites to locate position down to one of either 2 points.
  • One of those 2 points is off in space or is changing very rapidly. So theoretically if we calculate the range to each satellite exactly, then only 3 satellites would be necessary.
distance from each satellite

1

2

d2

d1

3

d3

Distance from each satellite?
  • Satellites are all coordinated to send the same psuedo-random code
  • The receiver in the field also produces the same psuedo-random code and determines the delay or offset in the code due to transmission time from each satellite. The farther away a satellite is, the larger the delay in its signal.
the fourth satellite

1

2

d2

d1

3

d3

The fourth satellite
  • A fourth satellite signal is needed to triangulate the position because the clocks on field receivers are not as accurate as those onboard the satellites. Hence, the fourth satellite is used to solve the position even when there is imperfect timing.

d4

sources of inaccuracy
Sources of Inaccuracy
  • Multipath reception
  • Timing offset
  • Signal delays due to Earth’s ionosphere and atmosphere
  • Poor satellite geometry
  • Selective Availability (turned off May 1, 2000)
differential correction
Differential Correction
  • Eliminates systematic errors:
    • S/A, receiver clock, satellite clocks, satellite position, ionosphere and atmosphere delays
  • Uses GPS receiver at a static known reference point to determine error in the signal
  • This error is similar for nearby GPS receivers at unknown positions
  • Error correction signal from the reference receiver can correct positions for the nearby receivers
differential correction41
Differential Correction

Radio link sends

correction information

or post-processed

in office

Moving ROVERs

at unknown locations

BASE

Reference station

at known location

gps accuracy43
GPS Accuracy
  • GPS accuracy depends on other variables too:
    • Time spent on measurements
      • Averaging a bunch of measurements
    • Design of the receiver and antenna
gps for public health
GPS for Public Health
  • Disease case or incident sites
  • Sites of major exposure
  • Hazardous sites
  • Vector breeding sites
  • Intervention or control sites
creating an appropriate gps gis database
Creating an Appropriate GPS/GIS Database
  • What spatial/temporal factors are relevant?
  • Spatial component:
    • Point Features
    • Line Features
    • Area Features
  • Attribute Data component:
    • What sorts of data are relevant for each particular type of spatial feature?
  • Spatial Resolution
creating an appropriate gps gis database46
Creating an Appropriate GPS/GIS Database
  • Fieldwork logistics are a real issue because you have to physically be at the site you want to map!
    • Cost of receivers vs efficiency
    • Battery power
    • Time needed for each feature
    • Difficulty getting to the sites
schistosome lifecycle
Schistosome Lifecycle

Humans

worms

eggs

Irrigation ditch exposure

Fertilization

miracidia

cercaria

Irrigation ditch habitat

Snails

geocoding snail density
Geocoding Snail Density
  • Intermediate host for the disease is a snail that lives in irrigation ditches
  • Preexisting methods for estimating snail density based on sampling frames
  • How can we geocode these frames?
    • Rural area
    • No maps available
    • Roughly 500 frames within a village
    • Money and time are limited
one solution
One solution
  • Map the ditches with GPS
    • Line feature
    • Attributes
      • Ditch ID
      • Ditch properties: width, flow, construction
then code sites with ditch map
Then code sites with ditch map
  • To avoid having to GPS each snail site individually, we provided teams of snail surveyors with ditchmaps.
  • They used the Ditch IDs to label their snail survey results. ie. frame “D31-10” means the frame is located in ditch ID 31, 10m from the head of ditch.
answers to questions
Answers to Questions
  • Where are most of the snails found?
  • What type of ditch construction has the highest snail density?
  • How does ditch flow influence snail density and location?
  • What’s the spatial relationship between snail density and water contact exposure?
  • If we take snail surveys over time, we can determine the locations of particularly stable snail dense ditches, which might lead to a control strategy
what about humans
What about humans?

Humans

worms

eggs

Irrigation ditch exposure

Fertilization

miracidia

cercaria

Irrigation ditch habitat

Snails

exposure assessment using gps
Exposure Assessment using GPS

Kai Elgethun, Richard A. Fenske, Michael G. Yost, and Gary J. Palcisko

Time-Location Analysis for Exposure Assessment Studies of Children Using a Novel Global Positioning System Instrument

Environmental Health Perspectives Volume 111, Number 1, January 2003

Department of Environmental Health, School of Public Health and Community Medicine, University of Washington

exposure assessment using gps59
Exposure Assessment using GPS

Modelling Concentrations of and Human Exposure to Air Pollution in Danish Cities

Contribution to subproject SATURN

O. Hertel, S. S. Jensen, R. Berkowicz, J. Brandt and J. Christensen

National Environmental Research Institute (NERI), Frederiksborgvej 399, P. O. Box 358, DK-4000 Denmark

alternatives
Alternatives
  • Remote Sensing and Graticules
alternatives61
Alternatives
  • Future: Location Based Services using mobile phones?