7.1 How To Obtain the Products
It is the goal of the Gap Analysis Program and
the USGS Biological Resources Division (BRD) to make the data and
associated information as widely available as possible. Use of the
data requires specialized software called geographic information
systems (GIS) and substantial computing power. Additional information
on how to use the data or obtain GIS services is provided below
and on the GAP home page (URL below). While a CD-ROM of the data
will be the most convenient way to obtain the data, it may also
be downloaded via the Internet from the national GAP home page at:
The home page will also provide, over the long
term, the status of our state’s project, future updates, data
availability, and contacts. Within a few months of the project’s
completion, CD-ROMs of the final report and data should be available
at a nominal cost – the above home page will provide ordering
information. To find information on this state GAP project status
and data, follow the links to “project information”
and then to the particular state of interest.
In addition to availability of the Pennsylvania
data through the national GAP program, much of the data will also
be accessible through the PASDA
(Pennsylvania Spatial Data Access) website at the http://www.pasda.psu.edu
URL. PASDA is operated at Penn State University on behalf of the
Pennsylvania Department of Environmental Protection. PASDA offers
a wide array of other spatial data for Pennsylvania, including the
digital orthophoto quarter-quads (DOQQs) and compressed satellite
imagery used in conducting our gap analysis. Under the auspices
of PASDA, we have also produced a CD-ROM configured for convenient
analysis of our PAKAGE (PA Kilometer-Aggregated Gap Elements) database.
This database constitutes a geographic hyper-distribution of habitats
along with characteristics of Pennsylvania’s landscapes. It
has been the basis for analyses presented in Chapter 5, and will
also be the basis for more technical analyses that are planned as
a continuation of research reported here. A website located by links
from the National GAP home page will have postings of progress in
All of the Pennsylvania Gap Analysis data have
been structured for analysis via ArcView© GIS by ESRI®
and its companion software facilities (see website http://www.esri.com).
These databases are all large enough that many GIS operations require
several minutes to transpire, even on well-equipped computers. Time
required to complete such operations will increase considerably
if the computer has a slow processor or meager memory.
Following is the official Biological Resources
Division (BRD) disclaimer as of 29 January 1996, followed by additional
disclaimers from GAP. Prior to using the data, you should consult
the GAP home page (see How to Obtain the Data, above) for the current
Although these data have been processed successfully
on a computer system at the BRD, no warranty expressed or implied
is made regarding the accuracy or utility of the data on any other
system or for general or scientific purposes, nor shall the act
of distribution constitute any such warranty. This disclaimer applies
both to individual use of the data and aggregate use with other
data. It is strongly recommended that these data are directly acquired
from a BRD server [see above for approved data providers] and not
indirectly through other sources which may have changed the data
in some way. It is also strongly recommended that careful attention
be paid to the content of the metadata file associated with these
data. The Biological Resources Division shall not be held liable
for improper or incorrect use of the data described and/or contained
These data were compiled with regard to the following
standards. Please be aware of the limitations of the data. These
data are meant for use at a scale of 1:100,000 or smaller (such
as 1:250,000 or 1:500,000) for the purpose of assessing the conservation
status of animals and vegetation types over large geographic areas.
The data may or may not have been assessed for statistical accuracy.
Data evaluation and improvement may be ongoing. The Biological Resources
Division makes no claim as to the data’s suitability for other
purposes. This is writable data that may have been altered from
the original product if not obtained from a designated data distributor
Proper documentation of all information sources
used to assemble GAP data layers is central to the scientific defensibility
of GAP. The information used to describe GAP data is called metadata.
Metadata are information about data. Metadata contain information
about the source(s), lineage, content, structure, and availability
of a data set. Metadata also describes intentions, limitations,
and potential uses, allowing for the informed and appropriate application
of the data. Descriptions of metadata function have recently been
published by the Federal Geographic Data Committee (FGDC 1994, 1995).
The GAP metadata standards have been closely matched
to the FGDC standards to ensure current and future compatibility.
As the FGDC standards evolve beyond the current publication, we
anticipate corresponding refinements in GAP documentation. The format
of the GAP metadata consists of eight major documentation sections
(Table 7.1) containing one or more metadata elements. Each element
is named (e.g., Map Projection Name), and the “Type”
of entry (text, integer, date, time) and “Domain” of
the entry (i.e. x>0) are also defined.
Table 7.1 Metadata Data Element Categories.
I. Identification Information: What the data set is called, file
II. Data Quality Information: Accuracy, consistency, and data
III. Spatial Data Organization Information: Data structure –
raster, vector, point, etc.
IV. Spatial Reference Information: Coordinate units, map projection,
V. Entity and Attribute Information: Attribute codes and reference
VI. Distribution Information: How to order the data, on-line access,
VII. Metadata Reference Information: Date of the metadata, contact
for metadata updates.
VIII. Contact Information: General data contact, mail, voice,
fax, web, e-mail.
Demands for metadata will increase as electronic
networks expand across the national and international scene, and
more requests are made for distribution of information. As the number
of users and the diversity of disciplines and programs sharing the
data expand, the information carried by metadata will become increasingly
important. One of the goals in defining today’s metadata standards
is to anticipate these future needs.
For additional information via Internet:
GAP home page address: http://www.gap.uidaho.edu/default.htm
Cogan, C. B., and T. C. Edwards. 1994. Metadata
standards for GAP. Gap Analysis Technical Bulletin 3. Idaho Cooperative
Fish and Wildlife Research Unit, University of Idaho, Moscow, Idaho.
28 pp. (A postscript file is available from the GAP web page listed
For a postscript version of the current FGDC Metadata
Standards (8 June 1994):
Waisqvarsa.er.usgs.gov (anonymous ftp, cd to wais/docs, get FGDCmeta6894.ps)
Federal Geographic Data Committee. 1995. Content
Standards for Digital Geospatial Metadata workbook (March 24). FGDC.
Washington D.C. (Describes the FGDC metadata standards.)
7.4 Appropriate and Inappropriate
Use of These Data
All information is created with a specific end
use or uses in mind. This is especially true for GIS data, which
is expensive to produce and must be directed to meet the immediate
program needs. For GAP, nominal standards were set (see A Handbook
for Gap Analysis, Scott et al. 1993) to meet program objectives.
These standards include: scale or resolution (1:100,000 or 100 hectare
minimum mapping unit), accuracy (80% accurate at 95% confidence),
and format (ARC/INFO coverage tiled to the 30’ x 60’
USGS guadrangle). Since those standards where determined, however,
normal computer capacity has increased by several orders. Likewise,
ArcView-compatible shapefiles and grids have become more generally
accessible and more readily portable than ARC/INFO coverages. Therefore,
Pennsylvania data layers are integral rather than tiled, and shapefiles
have been used in preference to coverages for vector data. Any desired
geographic subset can thus be extracted directly without first having
to mosaic tiles. ARC/INFO has capability to convert shapefiles to
coverages if this should become advantageous for particular purposes.
Recognizing, however, that GAP would be the first,
and for many years likely the only, source of statewide biological
GIS maps, the data were created with the expectation that they would
be used for other applications. Therefore, we list below both appropriate
and inappropriate uses. This list is in no way exhaustive, but should
serve as a guide to assess whether a proposed use can or cannot
be supported by GAP data. For most uses, it is unlikely that GAP
will provide the only data needed, and for uses with a regulatory
outcome, field surveys should verify the result. In the end, it
will be the responsibility of each data user to determine if GAP
data can answer the question being asked, and if they are the best
tools to answer that question.
Scale: First we must address the issue of appropriate
scale to which these data may be applied. The data were produced
with an intended application at the ecoregion level, that is, geographic
areas from several hundred thousand to millions of hectares in size.
The data provide a coarse-filter approach to analysis, meaning that
not every occurrence of every plant community or animal species
habitat is mapped, only larger, more generalized distributions.
The data are also based on the USGS 1:100,000 scale of mapping in
both detail and precision. When determining whether to apply GAP
data to a particular use, there are two primary questions: do you
want to use the data as a map for the particular geographic area,
or do you wish to use the data to provide context for a particular
area? The distinction can be made with the following example: You
could use GAP land cover to determine the approximate amount of
a cover type occurring in a county, or you could map the cover type
with aerial photography to determine the exact amount. You then
could use GAP data to determine the approximate percentage of the
cover type in the region or state that contains the county, and
thus gain a sense of how important the county’s distribution
is to maintaining that cover type.
Appropriate Uses: The above example illustrates
two appropriate uses of the data; as a coarse map for a large area
such as a county, and to provide context for finer-level maps. Following
is a general list of applications:
• Statewide biodiversity planning.
• Regional (Councils of Government) planning.
• Regional habitat conservation planning.
• County comprehensive planning.
• Large-area resource management planning.
• Coarse-filter evaluation of potential impacts or benefits
of major projects or plan initiatives on biodiversity, such as
utility or transportation corridors, wilderness proposals, regional
open space and recreation proposals, etc.
• Determining relative amounts of management responsibility
for specific biological resources among land stewards to facilitate
cooperative management and planning.
• Basic research on regional distributions of plants and
animals and to help target both specific species and geographic
areas for needed research.
• Environmental impact assessment for large projects or
• Estimation of potential economic impacts from loss of
biological resource based activities.
• Education at all levels and for both students and citizens.
Inappropriate Uses: It is far easier to identify
appropriate uses than inappropriate ones, however, there is a “fuzzy
line” that is eventually crossed when the differences in resolution
of the data, size of geographic area being analyzed, and precision
of the answer required for the question are no longer compatible.
• Use of the data to map small areas,
typically requiring mapping resolution at 1:24,000 scale and using
aerial photographs or ground surveys.
• Combining GAP data with other data finer than 1:100,000
scale to produce new hybrid maps or answer queries.
• Generating specific areal measurements from the data finer
than the minimum mapping unit.
• Establishing exact boundaries for regulation or acquisition.
• Establishing definite occurrence or non-occurrence of
any feature for an exact geographic area.
• Determining abundance, health, or condition of any specific
• Establishing a measure of accuracy of any other data by
comparison with GAP data.
• Altering the data in any way and redistributing them as
a GAP data product.
• Using the data without acquiring and reviewing the metadata
and this report.