CLPPP databases contain numerous errors that must be corrected before they can be processed by geocoding software. If each record in a database is formatted properly, the software can correct for errors such as misspellings of street names and cities. A local street map is an excellent resource for verifying street names and number ranges.# Purpose of These Guidelines
The challenge for public health practitioners and policy makers is to prevent childhood lead poisoning, not just react to it (1). GIS technology is a powerful tool that can be used to effectively target lead poisoning preventive interventions. The addresses of old housing units can be geocoded (geographically located) to identify areas where children at risk for lead poisoning live. Interventions can then be directed to those areas and specifi c properties to address lead hazards. These guidelines will focus on mapping applications, although GIS also can be used for statistical modeling to predict risk for lead exposure (2). Examples are provided of how GIS mapping technology can use blood lead screening, tax assessor (property), and U.S. census data to develop and improve preventive interventions, especially primary prevention (before children are poisoned).

# Who is at Risk for Lead Exposure?
Lead poisoning is a preventable environmental disease in children (3).
Children under the age of 6 years commonly put things in their mouths that they fi nd around them. This hand-to-mouth behavior increases the young child's risk for ingesting lead-contaminated dust and soil.
CDC estimates that 434,000 children have BLLs >10 g/dL, CDC's level of concern (4).
Children at greatest risk for lead poisoning are those whose families are poor and live in substandard housing built before 1950.
These children tend to be African American or of Hispanic ethnicity.
Since the 1970s, policies have been implemented to limit the use of lead in products such as gasoline, food and drink cans, solder in pipes, and residential lead paint. Those polices have resulted in dramatic reductions in BLLs for children and adults (5,6,7). They have also reduced lead in our environment. Today, the most common high-dose source of lead exposure for young children in the United States is leaded house paint. That includes the dust and soil that becomes contaminated as the paint deteriorates (8,9). Although leadbased paint was banned for residential use in 1978, millions of properties built before that time are still lead hazards. House paint used before 1950 contained up to 50% lead by weight (10).
In the years between 1950 and 1977, manufacturers voluntarily reduced the concentration of lead in paint. Consequently, even though there is lead-based paint in nearly all houses built before 1977, houses built before 1950 place children at considerably higher risk (11).
At the end of the 20th century, an estimated 38 million housing units had lead-based paint, and 24 million of these had signifi cant lead-based paint hazards.
Low-income families (<$30,000/year) with children younger than 6 years of age occupied 1.2 million of those hazardous units (12).
A 1991-1994 national survey showed the prevalence of children with BLLs >10g/ dL varied by age of housing: 8.6% for children living in houses built before 1946, 4.6% for those living in houses built from 1946-1973, and 1.6% for those living in houses built after 1973 (13). Children who live in old housing units and are poor are at higher risk for having elevated BLLs than are children from higher income families. For example, the prevalence of elevated BLLs among children living in homes built before 1946 was 16.4% for those from low-income families compared with 4.1% and 0.95% among those from middle-and high-income families, respectively (14). Studies show that property valuation or the assessed values of houses (tax assessor data) can be used as a proxy measure of the structures' condition.
Children living in lesser-valued houses were found to be at greater risk of having elevated BLLs, even after controlling for the age of the house (15). Rental units also have been linked with higher prevalence of lead poisoning. That may refl ect a greater likelihood that paint in old rental housing is deteriorated and becomes accessible to children (16).

# How Can GIS Help?
With GIS, maps can be created that show the location and age of every housing unit in an area. These maps can include information on other risk factors for lead poisoning, including population distributions, housing conditions, and BLLs of resident children during a given period.
This information can be used to show the relationship between housing units and risk factors. GIS visually presents these geospatial and temporal relationships in data that have many uses:
- To identify where high-risk children live;
- To assess screening penetration among high risk groups;
- To obtain a better understanding of changes over time in areas where children at high risk live;
- To evaluate the impact of our targeted screening and other intervention efforts and improve them; and
- To identify those housing units responsible, over a period of years, for multiple cases of childhood lead poisoning (17).

# What is GIS?
GIS is a computer-assisted system for the acquisition, storage, analysis, and display of geographic data (18). GIS software allows the user to create maps that display spatially related or geographically based data. Data representing geographic features (landscape elements) can be visually displayed as points, lines, and polygons (19). Each type of feature element in a GIS is contained in its own feature layer. A layer can only contain one type of feature. For example, a map displaying a city with streets and locations of individual residences, is composed of three feature layers:
- One polygon layer denoting the area boundaries of a city,
- One line layer to denote streets,
- One point layer denoting the location of individual residences.
In a GIS, every layer's geometric relationship to all other layers is prescribed through a process called topology (not to be confused with topography). Topology represents angular relationships (order, adjacency, etc.) that remain constant regardless of map distortion.

# Polygon

# Lines

# Points
Any record containing geographic information can be mapped. The geographic data may represent any size geographic area (e.g., a single housing unit, a building, a census block or tract, municipality, ZIP code, county, state, etc.).
Address Geocoding is the process whereby specialized software matches an address against a database of standardized addresses and assigns unique map coordinates for location (i.e., latitude and longitude). Some software contain "tables of alternate street names" that allow correction for streets having different names over time. Once an address is geocoded, it can be added to a GIS for spatial analysis (19). Geocoding allows us to establish relationships to other geographic identifi ers (ZIP code, census area, municipality, block group) and query the data (20).

# Dot-Density

# Choropleth
The symbology used to picture the data displayed refl ects the classifi cation of the Populations for census tracts are usually between 2, 500 and 8,000 and block groups between 600 and 3,000 (detailed descriptions of census levels can be found at ). Census data related to childhood lead poisoning is available from CDC's Lead Poisoning Prevention Branch (see the How CDC Can Help CLPPPs section below for further details).

# Tax Assessor
Tax assessor data (real estate property data) is a source of detailed housing information (22). County and municipal government offi ces, known for our purpose as the "tax assessor offi ce," collect detailed information on each parcel of land (property) as a record of real estate transactions and to valuate property for taxes. Each parcel of property in a county has its own unique number-the "assessor's parcel number" (APN)-and every county has a unique numbering method. A tax assessor database can be linked to a geographical information system to reveal the actual street location of housing. Tax assessor data is public information, meaning every member of the public has a right to view these data.
There can be more than 400 variables in the real estate database, including whether the property is residential or commercial.
Residential property type is further divided into categories such as single-family, apartment, or condominium. Each record can contain hundreds of variables, including sales price, date of sale, property address, and owner address (street number, street name, street type, ZIP code, and county). Additional information usually includes subdivision name, number of housing units (apartments), number of rooms, number of stories, whether owner or renter occupied, and most important, the year the structure was built. The database may even include information on renovation history. Address information is listed in two categories: owner information and property information.
Owner information is the mailing address provided by the owner to the assessor for tax billing purposes. Property information is the actual property location. If the owner lives at the property, the mailing and property address should be the same.
Real estate data is available commercially but is costly. Tax assessor data is available at a little cost and is relatively accurate and updated frequently. As a result, it is recommended that local tax assessor data be used, when it is available.

# Getting Started

# GIS Offi ce
Many counties and municipalities have a GIS offi ce (usually in the planning department). CLPPPs are encouraged to seek out their help, especially through a personal visit. Even if the tax assessor does not have a GIS property map nor is aware of the existence of a map, a visit to the county or local GIS offi ce may reveal the existence of one. The GIS staff also know the "lingo" of the GIS world and will probably be happy to cooperate. GIS offi ces can provide additional information that the tax assessor may not be knowledgeable about (resources, persons to contact, current GIS fi les, etc.). While in the area, visit the local county and city GIS offi ces, the library (main branch), and the local health offi cer. It is a good idea to set up appointments with a planned agenda for each stop. In all, expect at least a 1-day visit.

# Obtaining Tax Assessor Data
Tax

# Preparing to Visit the Tax Assessor Offi ce
To prepare for the visit, learn about the differences between census data and tax assessor data. For example, "owner address" is where the tax bill is sent, "situs address" is the property location, and "assessor parcel number (APN)" is the unique number assigned to each property.

# Preparing for the Tax Assessor Meeting
Tax assessors are usually very receptive when their data can be used for positive purposes. Explain the reason for the visit, describe the project, and show what information has been collected so far. If commercial data has been acquired, share that with the tax assessor and show where there is missing information (i.e., "year built" may be missing on some early properties).
Present a list of the data one would like to have and be prepared to leave a copy of this request with the tax assessor. A list of fi elds one might fi nd useful may be found in Appendix D. Most tax assessors will have the bulk of this information. At a minimum, request the following:
- owner information (street number, name, and type; city; state; ZIP code);
- situs information (street number, name, and type; city; state; ZIP code);
- number of units;
- number of stories;
- type of housing (single family residence, apartment, condominium, duplex, triplex);
- year built (the year construction was begun); and
- effective year built (the year construction was completed).
Stress that it is important to obtain information on properties with "year built" data, as well as information on properties that are missing "year built" data entries. - Many properties contain more than one house, but the properties are only assigned one APN.
- APNs might not be assigned consecutively and therefore cannot be used as a method to ascertain the "year built" when "year built" was missing from a record. (In some counties, however, APNs might be consecutive.)
- There are more municipalities (census places) in the county than listed in the U.S. census, which excludes census places that do not meet criteria, such as minimum population size.
- Most municipalities are represented by a single "tax district" and each property record includes the tax district.
Properties in the same tax district will probably share the same "ZIP code."
This can be useful when fi lling in missing information, such as ZIP codes.
- City boundaries in the assessor database differ from those in the U.S.
Census Bureau's database (larger taxable areas).
- The "offi cial" county street name fi le can be used to verify spellings, directions, and street number ranges.
- ZIP code information may be incomplete on property data, but almost never on owner data (because the tax assessor needs the "ZIP code" to send the owner of the property the tax bill!).
- Residential properties do not include apartments; apartments are listed as commercial property.
- The county may not have collected "year built" information until a few years ago. However, whether a unit was built before 1950 could be determined through the tax category "percent good," which is a method used to fairly tax older housing. Each time property was reassessed, the assessment is reduced by a few percentage points to compensate for infl ation (increased value). Therefore, on the basis of accumulating percentage reductions, a property assessed at "percent good" below a certain level (75% in this case)
was determined to be a pre-1950 building.

# Analysis

# Questions to Consider

# Housing
- How can pre-1950 housing be located?
- How to know which housing has been remediated?
- Will remediation of contaminated housing before habitation prevent a child from being lead poisoned?
- How can the age and condition of housing in which children reside be determined?
- How can housing responsible for multiple poisonings be identifi ed?

# Screening
- Have all of the children at risk been screened?
Have some groups of high-risk children been missed?
- Are all children living in poverty at risk?  This is a useful tracking tool for health departments.
- Indicate which houses have been remediated through renovation and are presumably no longer hazardous.
- Monitor other children who later live in the homes to confi rm that remediation was done properly and test whether this is an effective method of primary prevention.
- Map addresses where owners have applied for renovation licenses.
Improperly performed renovation and remodeling can create lead hazards when old paint is disturbed. The health department may wish to target special educational programs to residents of neighborhoods where these activities are common.
- Identify homes that are associated with more than one case of lead poisoning.
These data can help a health department focus on neighborhoods of very high risk. It also justifi es the case to Medicaid for reimbursement of environmental investigation services to prevent future lead poisonings in the same housing unit.

# Limitations

# Census
There are limitations to using census data:
- Although certain basic demographic and housing questions were asked for every person in the United States, detailed information on housing (including the year a structure was built)
was collected on a subset sampled at a 1-in-6 rate (long form).
- Data on the year the structure was built are susceptible to errors of response and nonreporting because respondents must rely on their memories or on estimates by persons who have lived in the neighborhood a long time (26).
Available evidence indicates there is underreporting in the older yearstructure-built categories, especially "built in 1939 or earlier" (27).
- Data are only available every 10 years. In the time from one census to the next, some units are lost through attrition, and the demographics of neighborhoods changes.
- Age of housing below the block group level is not available.

# Tax Assessor
There are some limitations to using tax assessor data, too. Not all counties have tax assessor information in an electronic form.  If the structure was renovated properly, other children who eventually live in these renovated housing units will not be lead poisoned (28). Children who are lead poisoned and move into these renovated units should initially demonstrate decreases in their BLLs, followed by lead values below 10 g/dL with time.
The capacity to achieve the 2010 elimination goal is directly related to the ability to target strategies to geographic areas (29).
Geocoding (street address matching or assignment of map coordinates) will be the basis for data linkage and analysis in the 21st century.
The versatility of GIS supports the exploration of spatial relationships, patterns, and trends that may otherwise go unnoticed (30). This technology also allows for the linking of nongeographic data, such as blood lead levels, to geographic locations. GIS allows for the analysis of all data related to geographic location data.
Traditional biostatistical and spatially based data analytic methods can be used to estimate risk for lead exposure (31).
GIS is a powerful tool that can precisely locate the home of a child at risk from exposure to lead. This level of information is necessary for public health professionals to accurately assess the extent of childhood lead poisoning, to identify new cases, and to evaluate the effectiveness of prevention activities. However, public presentation or release of maps at this level is discouraged.
Public access to data below the county level is prohibited or severely restricted because of confi dentiality and privacy issues. A major challenge in the coming decade will be to increase public access to GIS information without compromising confi dentiality (32).
New methods must be developed to identify these high-risk children, whose homes may be dispersed over a large geographic area.  - Eliminate blood leads levels in children >10 g/dL (section 8-11).
- Increase the proportion of persons living in pre-1950s housing that has been tested for the presence of leadbased paint (section 8-22).
- Increase the proportion of all major national, State, and local health data systems that use geocoding to promote nationwide use of geographic information systems (GIS) at all levels.
Public health rests on information.
Increased geocoding in health data systems will provide the basis for more cost effective disease surveillance and intervention (section 23-3).

# Internet Resources
Listed below are some websites that provide information about using geographical information systems (GIS) for readers who want to learn more about GIS.

# Glossary of Terms
Attribute-Data about a map feature.
Attributes of a property include address, year-built, value, and number of apartments - Multiple spaces-caused by pressing the space bar more than necessary.
- Punctuation -,, ;,., /, \, , ', =, -.
- Shift key errors -!, @, #, $, %, ^, &, *, (, ), _, +, ", :, , ?. Most entries of this type probably represent attempts to enter the numbers 1 through 10.

# Street Numbering Errors
- Street numbers present numerous obstacles (see Table A.1 for summary):
- Address appears as single fi eld-lack of space between number, name, type, and direction.
- Direction is placed in front, the middle, or at the end of a number, such as: W1224 or 1224W or 1W224.
- Punctuation inserted improperly, as noted above.

# Street Name Errors
- Street names can present the most diffi culties (see Table A.1 for summary):
- Misspellings are frequently encountered where names have been merged or separated, transposed, or have letters added or dropped. These errors are especially diffi cult to fi nd and fi x when the fi rst letter is wrong (e.g. Butenberg, rather than Gutenberg).
- Inconsistent Abbreviations-Two common errors involve the words "Mount" and "Saint" in street names.

# APPENDIX A
Preparing Data for GIS Use-Problems to Avoid Geocoding is the process whereby specialized software assigns unique map coordinates (i.e., latitude and longitude) to each address in a database. Once an address is geocoded, it can be added to a geographical information system (GIS) for spatial analysis. Geocoding software can also correct errors in address, but addresses must be set up in a proper format for this to occur.
Many problems are encountered in preparing Childhood Lead Poisoning Prevention Program (CLPPP) databases for GIS use. Table I