"Let's Look at the Map [Digital Data]"
The following text is Gil Castle's final draft of the real estate
column appearing in Business Geographics, March/April 1995
Copyright © 1995 GIS
This is the second in a series of columns on real estate information
sources. The topic this time is digital data. I will define the topic,
delineate the many types of digital data, and conclude with data sources.
As used here, digital data refers to maps-e.g., zip code zones, census
tracts, streets, individual buildings-displayed on a computer monitor or
printer. Everything on each map is defined by one or more longitude-latitude
coordinates, state plane coordinates, or cartographic coordinate system.
Without becoming too technical, such maps are vector files, in contrast
to aerial photographs, satellite images, and other raster files (which
will be discussed in a future column).
Innumerable types of digital data exist. Anything that can appear on
a map can be digitized. In real estate, the most frequently used digital
files are these:
- Zip code zones, census tracts and census block groups-for thematic
maps of socioeconomic variables that correlate to the demand for real estate,
including population growth, household income, unemployment rates, and
- Street networks-for reference purposes (to help viewers identify the
area that the map covers), address matching (software that automatically
identifies the location of a property based on its street address), and
analysis of traffic volumes; and
- Individual properties-sometimes covering a large area, such as a subdivision,
and other times a single coordinate point, representing a single residential
or commercial building.
Other files that are employed less frequently (because they are not
conveniently available, are too expensive, and/or are not easily interpreted
by users) include the following:
- State, metropolitan statistical area (MSA), county and city boundaries-for
regional market research, market ranking, and trade area analysis;
- Submarket boundaries-again for market research et al., but at a more
focused geographic level;
- Assessor parcels, general plan, zoning, land use, water and sewer network,
capital improvements program, tax increment financing districts, etc.-to
identify regulatory and infrastructural factors governing a property's
development and resale;
- Flood zones, earthquake fault lines, toxic waste sites, rare and endangered
species, archaeology sites, soils, and so forth-to identify any environmental
constraints that could affect the use and value of a property; and
- Elevation contours-for reference purposes, and for conversion in more
sophisticated GIS packages to three-dimensional renderings of a site, slope
and aspect maps, cut-fill calculations, view shed analyses, etc.
Digital files that are only now beginning to become available but which
will be extensively used by real estate professionals are the locational
coordinates of individual properties plus comprehensive attribute information
on square footage, acreage, number of stories, year built, type and quality
of construction, ownership, assessed valuation of land and improvements,
asking price (if any), selling price in previous transactions, and similar
information published by select government agencies and other real estate
Sources of data can be as diverse as the types of data. Broadly speaking,
real estate professionals acquire their digital files in these ways:
- Purchase through a software vendor-Major vendors of GIS software also
sell digital and attribute data so that their clients can become operational
as quickly and easily as possible. The largest vendors publish comprehensive
catalogs on the data they offer, e.g., "Conquest MarketData Catalog"
from Strategic Mapping and "ArcData" from ESRI. Among the advantages
of purchasing digital data from the GIS vendors are that the data is assured
to be compatible with the vendor's GIS software, and maps will all be using
the same cartographic projection system and coordinate system, and convenience.
Disadvantages include GIS vendors not offering sources that a given user
would prefer, and GIS vendors sometimes charging premium prices (not unlike
a doctor prescribing an expensive "name" drug when an inexpensive
"generic" drug might be equally effective).
- Purchase from a private sector data vendor-Private sector vendors usually
bundle digital data with attribute data, e.g., the user receives census
tract boundaries with the purchase of a sufficient number of variables
on those census tracts (current and projected population, households, per
capita income, number of houses in various price ranges, etc.). A compelling
reason for this is that digital data does not change very often, but attribute
data changes frequently, and so the vendor is mainly interested in repeat
sales of attribute data. Most of the principal providers of digital data
offer their products both directly to the public and indirectly through
one or more of the above-described GIS vendor data catalogs.
- Purchase from a public agency-The leading sources of digital data are
federal agencies, notably the United States Geologic Survey (USGS) and
the Department of Commerce. Many if not most of the digital files sold
by private sector vendors consist of repackaged and enhanced federal data
bases; notable examples include the census tracts, census block groups,
and street networks for all cities in the nation. At the state and local
levels, government agencies vary widely in their willingness and ability
to sell digital files of, say, all the assessor parcels in a community.
Technical issues are secondary to political and legal issues, including
freedom of information, warranties and liabilities, confidentiality and
right to privacy, government entities competing with private companies,
and so on. Though the number of local government agencies offering digital
data to the public is increasing daily, the debate over policies, procedures
and prices will continue to rage for some time to come in state houses,
city council chambers, and court rooms throughout the nation.
- Create the data base in-house-Sophisticated GIS packages encompass
digitizing capabilities so that users can convert hard copy maps into proprietary
electronic files. Digitizing is labor-intensive and expertise-intensive,
and therefore expensive. The vast majority of real estate professionals
will not and probably should not learn how to digitize. If custom maps
are needed (e.g., of submarket boundaries unique to a brokerage firm),
outsourcing the digitizing to a service bureau is usually the better alternative.
In the near term, real estate professionals will acquire data in the
above order; that is, buying data from the GIS software vendor will be
the most frequent approach, followed by purchases directly from data vendors,
and so on. Within a year or so "data stores" will begin popping
up in cities around the nation for one-stop shopping of whatever data one
needs in a given geographic area. Shortly thereafter, as data becomes more
and more of a commodity, data catalogs will begin arriving in the mail
much like the catalogs of MacWarehouse®, Egghead Software®, and
other discount computer supply houses.