PLANT DESIGN AND ENERGY PRODUCTION ESTIMATION - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 234

• Raster layers consist of data values on an evenly spaced grid, usually with one
value for each grid cell or node (though interleaved and other multidimensional
formats exist). The files can contain digital terrain models (DTMs) or digital
elevation models (DEMs), land cover classes, surface roughness values, wind
speeds, definitions of allowed or excluded development areas, and many other
types of information. Aside from providing a visual reference, such information
can be used directly in the analysis. For example, if the software includes a wind
flow model, it can make use of the elevation model and roughness map in the
simulation. Elevation models can be used directly or to derive other information,
such as slopes, to determine where turbines can be placed. Land cover classi-
fication maps can be used to define areas, such as water bodies or wetlands,
excluded from development.
• Vector layers are made up of points, lines, or polygons. They are most often
used to define locations and extents of important features such as buildings
and water bodies, as well as the boundaries of administrative areas (e.g., cities
and counties), land parcels, and the project area itself. Another common use is
for defining topographic contours, which represent lines of constant elevation.
Although almost anything that can be communicated in a vector layer can be
transformed into a raster layer and vice versa, vector layers are not limited
by spatial resolution in the same way that raster layers, with their fixed grid
spacing, are. This makes them well suited for defining boundaries and locations
very precisely. They can also contain much additional information in associated
descriptive database files, such as the names and addresses of landowners, the
names of water bodies, or the voltages of transmission lines.
During the project setup, the user must define the boundaries of where turbines can
be placed. This is usually the land that is controlled by the developer or by others
who have reached an agreement to lease the land to the developer. One way this
could be accomplished is by selecting parcels of land already defined in a vector
layer. From the selected parcels, a new vector layer can be created. Another way is to
draw the boundaries manually. Depending on the software, these methods may require
aGIS.
16.3 WIND RESOURCE DATA
Once the project has been set up, the next step is usually to import or create the wind
resource information required for the simulation. This is where everything covered
in this topic until now comes into play. Two main types of wind resource infor-
mation are used in wind plant design software: measurements from a point location
such as a wind-monitoring mast, and the output of a numerical wind flow model
describing the variation of the wind resource across the project area. In most cases,
both types of information are required for an accurate simulation, and they are used
together.