Each polygon represents what is believed, with varying degrees of confidence, to be fringing marsh lining the mainland coast of Casco Bay as indicated in the 2003 Topobird imagery. Marsh is sometimes differentiated as high or low marsh, but is more often undifferentiated and labeled (see attribute definitions later in metadata) as mixed.
Processing the GPS data points created polylines that were then buffered in a GIS system with buffers of varying width. The buffered lines were then geographically joined with the image-delineated marshes to determine the fraction of existing 2007 marshes (from the boat survey) were approximately identified by the image-based process using 2003 imagery.
Results indicated that 60% of the boat-identified marshes were identified from the imagery with a 20 meter buffer. That fraction rose to nearly 70% of the boat-identified marshes identified from the imagery with a 200 meter buffer. Given that marshes are created, destroyed, and changed in extent over time, this is considered a lower bound of the accuracy of identification of marshes in the area.
Marshes were identified as accurately as possible, and the coverage of the marsh outlined as a polygon.