Analyzing Benthic Data:
GIS Basics
Vector Data
The vector data model is used to represent discrete features that
are defined as points, lines, and polygons in a geographic information
system (GIS). Vector data represent features as pairs of x,y coordinates.
A point is defined as a single x,y coordinate pair. Lines and polygons
are defined by a set or series of coordinate pairs. The GIS connects
the dots to draw lines or areas (polygons). All three of these types
of vector data are commonly used and suitable for benthic habitat mapping
applications.
 |
Examples of vector data model used for representing benthic habitat
features in a GIS.
Courtesy: Science Applications International
Corporation |
Points
Point data are the simplest type of spatial object. A point is defined
as a discrete location (x,y) with one or more attributes. Single-beam
sonar produces point data because each "ping" of
the echosounder records a discrete point with longitude (x coordinate),
latitude (y coordinate), and usually depth (z) values along survey
transects. The number of soundings (points) along the survey line is
determined by the sampling rate of the sonar and vessel speed.
Some types of benthic survey data are converted to point data during
data processing, prior to input to a GIS. For example, sub-bottom sonar
data are collected along survey transects, producing a profile view
of seafloor composition. The layers below the seafloor are then digitized
(that is, traced) and converted to points, with each point containing
a discrete x, y, and z value. Discrete sampling locations, such as sediment grabs and underwater
photography stations, are another example of point data utilized
in benthic mapping applications.
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Lines and polygons are used to delineate coral patches seen in
an aerial photograph.
Courtesy: NOAA Coastal Services Center |
Lines
Line data are created using an ordered set of coordinates that represent
the shape and length of a linear feature. A line can be defined by two
end points (for example, a planned survey line) or a series of smaller straight
line segments (for example, coastline). Examples of line vector data models
used in benthic habitat analyses include survey track lines and digitized
habitat boundaries derived from imagery analysis of aerial and marine
remote sensing data.
Polygons
The third type of vector data are polygons – areas completely bounded
by one or more polylines. The entire area within the polygon is defined
by an attribute, such as bottom type. Polygons are often used to represent
two-dimensional areas of homogeneous benthic habitat. For example, the
interpretation of aerial remote
sensing imagery generally entails digitizing polygons around areas that
visually appear similar such as a coral reef assemblage, and then assigning
an attribute such as "elkhorn coral" to the area.
Assigning attributes, such as habitat
classification, to the polygon facilitates geospatial queries of
the final digitized habitat data in a GIS. For example, the total acreage
of elkhorn coral in a study area could be calculated in the GIS based
on the attribute information.
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