GIS Data
There are two basic types of data used in GIS work; raster and vector. Definitions and
a general discussion of those data categories can be found below. There are also examples
of the most common types of data found on the Internet and pointers to more detailed
discussions of those data types. In addition to these general data types, you need to be
aware if the data has been formatted for the PC or UNIX environment. Because the UNIX
environment is more powerful, data sets used there can be larger and more detailed (eg
double precision vs single precision in Arc/Info).
Vector Data
Vector data is what most people think of when they picture a map. The lines on a road
map, the points that indicate cities, and the polygons that enclose a state are good
examples of a vector map and its components (points, lines, polygons). This type of data
looks the most realistic to people, and when used in GIS applications can generate the
most accurate data. Much of the data that comes from USGS (topographic data) and NWI
(wetland data) is in some type of vector format. Vector is a generic term; there are many
vector formats such as DLG, SDTS, DXF, ARC/INFO, etc. Some of these are software specific
and some are not. Vector format is recommended when accuracy is needed (eg boundary
information), or very realistic looking maps are desired. It is generally not the best
format for doing analytical models, or combining many layers of information.
GPS Data
GPS data is neither vector nor raster, but is usually converted into vector data. It
consists of point locations, and thus would be difficult to fill in a block of raster data
unless many readings were taken close together.
Additional Vector Data Information (not an all inclusive list)
- DLG - Digital Line Graph
DLG was the standard vector format used by USGS for many years in creating
and distributing electronic versions of their quad maps. Types of data USGS
created in this format included transportation, boundaries, PLSS, hypsography,
and man-made features. Scales ranged from 1:24,000 to 1:2,000,000. Much of
this data is in the public domain, and can be acquired free or for replication
costs. See the GIS Data Sources for locations to
obtain the data. For complete documentation on 1:100,000 DLG data, visit the
USGS EROS data center
GLIS list. For information on the 1:2,000,000 (organized slightly differently),
visit this EROS
location.
If you are familiar with the WWW, simply type in the address
http://edcwww.cr.usgs.gov/glis/hyper/guide/ to get a complete list of the types of data
available at the EROS site. Any item with a notation of size such as 15k after it is
documentation for that type of data. Many types of data (landsat, Spot, DLG, LULC, AVHRR,
DEM, DOQ, etc) are documented here. This is a good place to start.
- SDTS - Spatial Data Transfer Standard
SDTS is the data format that the USGS is in the process of converting everything to. This
new standard was mandated by law, and USGS was delegated the responsibility of leading the
transition. The assumption was made, based on statements from software firms, that
translators would be written into commercial programs to allow importation of SDTS format
data. This has not happened on as wide a scale as might be desirable. The conversion is
moving forward, nonetheless. For complete information check the SDTS home page.
- NWI - National Wetlands Inventory Data
NWI data is actually in DLG format, but at 1:24,000 scale. There are sample
datasets on the NWI homepage in other formats, including SDTS, ARC/INFO, and
DXF (AutoCad) , but all the digital data is available in DLG. There are no
plans at this time to do a mass conversion to another format. Visit the NWI
homepage think tank for further information and discussion. You can get
to the main NWI page also from the think tank.
- GIRAS - Geographic Information Retrieval Analysis System
The data files found in the GIRAS format that are most likely to be used by
FWS staff are the USGS Land Use and Land Cover Data. Most of them are at the
1:250,000 scale, but a few are also at 1:100,000. The US EPA has converted
much of this data into the ARC/INFO format as well. Good detailed documentation
on the Land Use and Land Cover files can be found at EROS data center.
There is also good metadata (following the FGDC Standard) at the EPA EMIS site
Raster Data
Most of the raster data available either in the public domain or commercially is of
satellite or high altitude origin. USGS is currently creating DRG data (digital raster
graphics) which are scanned images of quad maps, but that is a very new product, not
widely used yet. Raster data is actually a grid of pixels, each with an assigned value.
The sharpness of the maps created from this type of data depends on the size of the pixel
relative to the size of the area being mapped. One meter, or even 3 meter pixels of any
entire city can give a very detailed picture; 10 meter pixels looks very blocky of the
same area, but may be fine for an entire state. One of the major uses of this type of data
is analytical modeling. Modeling usually involved combining or overlaying many layers of
data. GAP is an example of a project that combines data in this way.Depending on the
originating data sets it may involved mathematically combining or subtracting data layers.
With raster data, if the pixels are the same size, it is relatively easy to precisely line
up the data layers, matching pixels and corners of the pixels. With the meandering lines
of vector data lining up multiple layers often results in multitudes of slivers rather
than clean matches.
You should keep this in mind when designing a project and purchasing both data and
software. While you can create raster data from vector data, it is more difficult to go
the other way. Also, some programs will let you overlay vector data on raster data for
cleaner maps, others will only handle one or the other. Finally, for coverage of large
areas such as ecosystems, states, or some of the larger parks or refuges, satellite data
may be the only feasible way to get data. You need to consider all aspects of a project
before making final decisions.
Addition Information on Raster Data (sample list)
- Landsat
Landsat, or thematic mapped data, was one of the first types of satellite
data available to the public. It gives, at best, 10 meter resolution although
there is discussion that the new satellites to be launched soon will improve
on that.. It is very cost effective for large areas, particularly when you
wish to get older scenes for an historic perspective. Documentation on the
Landsat program is available from NASA. Data and documentation are
available through EROS.
The NASA site has more background, but the EROS site is faster.
- SPOT
Spot data is produced by a French company. It is a later form of the technology,
and will give 3 meter resolution. It also costs more! There is a federal program
where scenes purchased once can be stored at EROS, and reused by other Department
of the Interior agencies (up to 10 times) for a minimal cost. Large, coordinated
purchases of multiple scenes can also assist in reducing the cost. Information
on Spot is available from EROS as well as
from Spot Corporation (this link does not always
work well - try back if you cannot get in the first time) . Check with EROS
(or your local GIS coordinator) before purchasing new scenes on the open market.
- DRG (Digital Raster Graphics)
DRG data is generated from scanned quad maps. They can be very useful as backdrops to
other data, for guiding digitizing, assisting in classification, or other operations where
broad background coverage is needed. They are available on CD-ROM, from the USGS. Complete
national coverage is planned, but not completed. For more information visit the Mid-continent Mapping Center site.
For additional information regarding this Web page, contact Chris Lett, in the Division of Information Resources and Technology Management, at Deb_Green@fws.gov
Return to the U.S. Fish and Wildlife Service GIS Home Page
Visit the U.S. Fish and Wildlife Service Home Page
Keywords=GIS, data, metadata
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