Environmental Engineering Reference
In-Depth Information
200 m scale. Selectable areas at 200 m resolution (PDF or GIS data set) can be purchased. Satellite,
hybrid, and terrain views are available for the world.
A similar interactive wind resource map (map, satellite, hybrid, and terrain views) and data for
the world, FirstLook, has wind speeds at 20, 50, and 80 m [23], and presently wind data for the
United States, Alaska, Canada, and Mexico are online. With FullView Assessment, resolution is at
90 m. In addition, a solar resource map is available. Remember, wind speed maps are useful for an
indication of wind energy, but wind power maps are the next step.
9.4 GEOGRAPHIC INFORMATION SYSTEMS
A geographic information system is a computer system capable of holding and using data, which is
spatially oriented. A GIS typically links different data sets, or a base set is displayed and overlays of
other data sets are placed on the base set. Information is linked as it relates to the same geographical
area. A GIS is an analysis tool, not simply a computer system for making maps.
There are two general methods of representing the data, raster and vector. Raster based means
every pixel has a value, and vector based means that the data are represented mathematically—
endpoints for lines and lines for polygons. Each pixel can represent an attribute, and the number
of attributes depends on the number of bits: 16 to 256 colors or shades of gray. Therefore, pixels
or vectors can have different attributes and are linked to a database, which can be queried. A GIS
gives you the ability to associate information with a feature on a map and to create relationships that
can determine the feasibility of various locations, for example, a hierarchical system for locating
anemometer stations for wind prospecting.
An overlay is a new map with specific features, which is overlaid on the base map. Overlays are
one form of database query functions. The overlay can be a raster or vector image, with the base
map being a raster or vector image. The number of overlays is generally limited only by the amount
of information that can be presented with clarity.
The main types of terrain data are the Digital Elevation Model (DEM) data and the Digital Line
Graph (DLG) data. These are available at different scales, for example, the DLG at 1:2,000,000,
1:100,000, and 1:24,000. Depending on the scale, the DLG data contain highways, roads (even down to
trails), lakes and streams, transmission lines (utility and gas), etc. The problem is that the data may be
taken from fairly old maps and therefore be incomplete. The DEM data give the terrain height to 1 m on
a latitude-longitude grid with a resolution of 3 arc seconds [pixel around 90 m by 90*cos(latitude) m].
NREL coupled the DEM database with software to produce shaded relief maps of 1° by 1°.
A technique of terrain enhancement [24] was used to identify windy areas in the Midwest. In
the flat or rolling terrain found in most of the Midwest, the two most important factors influenc-
ing wind speed are terrain elevation and surface roughness. The wind map (normalized wind map
from PNL digital map) was adjusted to an average elevation and average surface roughness in a
circle (12 km radius) around that point. The U.S. Geological Service Terrain Elevation Data was
the base map, which consisted of average elevations in 1 km
2
grid cells rounded to the nearest 6 m.
Terrain exposure was determined by subtracting actual elevation from the average elevation for each
1 r 1 km grid cell. Then a power correction factor was calculated by
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hub height, 50 m
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