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Benthic Habitat Mapping

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Project Planning: Data Accuracy and Verification

Minimizing positional and thematic errors is critical for benthic habitat mapping. Positional errors are those that result from miscalculation of sampling location. Thematic errors are the result of misinterpreting benthic data such that a given feature is wrongly identified. While the precise techniques used to evaluate and quantify such errors are usually specific to the data collection method used, a few general issues are discussed below.

Positional Accuracy

Positional or spatial accuracy is a measure of the possible difference between the locations of features in the data versus their actual location on the ground. It is usually expressed in units plus or minus some distance from actual ground position in either the horizontal or vertical plane. These values typically range from centimeters for engineering-grade work to meters for environmental work. The determination of the required positional accuracy for a project should be driven by the resolution of the data being produced and by the anticipated uses of the final data.

The positional accuracy of data collection has improved with increased use of Global Positioning System (GPS) technology. However, it is still necessary to further check the accuracy of aerial remotely sensed imagery and image-derived data. One strategy for determining positional accuracy in estuarine environments is to use GPS to measure shoreline features on peninsulas, spits, and long piers that extend into the estuary and compare these coordinates with those drawn from the rectified imagery. This method takes advantage of the stability of shore features, but still requires that measurements be inferred to the adjacent submerged landscape. As distances from shore increase, the probability that the positional accuracy has decreased becomes greater. If the water conditions are calm and allow the bottom to be seen from a boat, it is sometimes possible to anchor over a submerged feature and record a GPS coordinate by direct observation. Unfortunately, this type of situation is relatively rare.

The NOAA Coastal Services Center uses the Federal Geographic Data Committee (FGDC) C.E. 90 method for determining and reporting spatial accuracy of imagery-derived data products. This testing methodology compares the data set coordinate values with coordinate values from a higher accuracy source for easily visible features on the ground. Although this standard evaluates positional accuracy at points, it is applicable to geospatial data sets that contain point, vector, or raster objects. To learn more about this methodology or to download a copy of the standard, please visit the National Standard for Spatial Data Accuracy.

Thematic Accuracy

Thematic accuracy is a measure of the probability that a habitat on the ground is correctly mapped and how likely a mapped habitat type is likely to be found on the ground. This measure documents the "false positives." Features must be identified and classified correctly in order to create accurate benthic habitat maps. Users must have an understanding of the reliability of their benthic data in order to use it appropriately. Assessing thematic accuracy is especially important for sensors with a wide survey area, such as aerial photography or side-scan sonar, or in cases where point data from a single-beam sensor are being interpolated for locations between survey transect lines. These methods produce data across relatively large portions of the seafloor where no direct observation has been made and that often encompass several different habitats.

Acoustic techniques vary in cost and resolution capabilities, but all require ground-truthing to calibrate or validate derived data. So, any mapping effort that uses acoustic data must include an additional field sampling component that includes fine-scale sampling techniques for accuracy assessment and verification.

Conducting a thematic accuracy assessment involves collecting reference (ground truth) information and then comparing these data to habitat data derived from a mapping process. A thematic accuracy assessment has three major components:

  • Determining the sample unit. What will the field observation consist of? Often this is a physical area of a certain size or a certain number of pixels.
  • Selecting a sampling approach. Often this is some variation of a random sample selection process and is stratified by habitat class.
  • Reporting the results. This involves creating an error matrix and reporting overall accuracy, as well as individual habitat category accuracies.

Conducting a thematic accuracy assessment of benthic habitat data is a challenging process that can include diving, underwater videography, a grab sampling effort, or other means of recording actual bottom conditions over large areas.

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