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Analyzing Benthic Data: GIS Basics

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Summary Vector Data

Raster Data

Summary

Example of data layers that can be combined and analyzed with a GIS to calculate and visualize spatial relationships. Courtesy: U.S. Geological Survey

A Geographic Information System (GIS) is an information management system of geographically referenced data. The system provides tools for displaying, manipulating, analyzing, and visualizing spatial data. GIS allows data from many sources to be integrated into interactive map displays. This type of mapping and visualization helps identify patterns in the data and the underlying environmental processes that created those patterns.

GIS is a powerful analytical tool that can be used to evaluate the relationship between map features and their related database information. It integrates spatial (positional) and attribute (descriptive) databases. The spatial data contains location information and the attribute data contains information about the characteristics or qualities of the spatial features. GIS analysis helps identify relationships between separate, but overlapping layers of information, such as seafloor depth, sediment type, and eelgrass distribution.

Comparison of Vector and Raster Data Models

Geographic features in a GIS can be created in raster (left) data form or vector (right) data form. Courtesy: NOAA Ocean Explorer

Spatial features in a GIS database are stored in two forms: vector and raster. Vector data are linear data consisting of points, lines, and polygons. Raster data are gridded data with each cell containing a location and a value. Both vector and raster geospatial data models are used in GIS analysis and visualization of marine habitat data.

Vector and raster data are useful for spatial analysis; however, they have different methods for displaying and using maps and their associated data. A vector model tells where features occur and gives a location to every object (defined by x,y coordinates in space). A raster model tells what occurs everywhere and assigns a value to each place (cell) in the area. Raster models could represent a point as a single cell, a line as a continuous series of cells, and a polygon as an area of continuously touching cells.

Data collected using the vector model can be converted to raster, either in "real-time" during data collection or in post-survey processing phases. For example, single-beam and multibeam echosounders continuously record water depth as bathymetry points (vector format). The data can be converted to raster using two different geospatial processing techniques.

Point data (for example, bathymetry) are converted to raster through either interpolation (that is, single-beam) or point-to-raster conversion (that is, multibeam) algorithms. Courtesy: Science Applications International Corporation

• Single-beam sonar data are collected along the survey track lines; however, the area between adjacent survey lines is not surveyed. The depth data must be interpolated between track lines when constructing gridded raster maps. This involves calculating values for unsampled points based on the values of nearby sampled points.
• Multibeam sonars provide full-bottom data coverage consisting of dense sounding data both along and between track lines. These data are converted to a raster data model using GIS processing routines that essentially convert each point to a square pixel (cell) without interpolation.

The data formats have several distinct differences. Vector maps are more specific and more precise. They require less computer storage space and are displayed faster. Raster data requires large storage capacity and are generally slower to display. Raster layers are easy to manipulate and compare layers of information.

The Role of GIS in Benthic Habitat Mapping

Bathymetry data displayed in 3D perspective. A GIS can create very accurate and realistic-looking visualizations. Courtesy: UNH/NOAA Joint Hydrographic Center

GIS combines data from a wide variety of marine sensing instruments, including satellites, aerial photographs, sonars, underwater cameras, sediment profilers, and grab samplers.

GIS can create very accurate and realistic-looking images of marine environments. Recent advances in mapping, charting, and visualizing the data in three dimensions have improved the presentation of spatial information.

• These maps and images help effectively communicate results to coastal managers, researchers, and the general public.
• They can help identify what benthic habitats exist at a particular location.
• They can be used to identify areas where certain conditions exist (for example, oyster beds within 1 km of a sewage outfall).
• They are useful for detecting and monitoring changes within an area over time, such as calculating the volume of sediment deposited at a dredged material disposal site.

The section on Spatial Analysis demonstrates how GIS is used to integrate, analyze, and visualize geospatial benthic habitat data.

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