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Benthic Habitat Mapping

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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.

Vector Data Examples

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.

Aerial Photograph

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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