Vegetation Classification and Mapping Project Report - USGS

USGS-NPS Vegetation Mapping Program
Colonial National Historical Park
boundary data obtained Colonial National Historical Park in May 2003. The mapping boundary
includes lands under a scenic easement at Swanns Point and it excludes the Cheatham Annex, an
area that returned to US Navy ownership in February 2004. The vegetation map was clipped at
the park boundary because areas outside the park were not surveyed or included in the accuracy
assessment.
Accuracy Assessment
Two sources of potential error in the vegetation map include: 1) horizontal positional accuracy,
in which a location on the photomosaic does not accurately align with the same location on the
ground due to errors in orthorectification or triangulation; and 2) thematic accuracy, in which the
vegetation type assigned to a particular location on the map does not correctly represent the
vegetation at the same location in the park due to mapping error. The USGS-NPS VMP protocols
(The Nature Conservancy and Environmental Systems Research Institute 1994c) were followed
to assess the horizontal positional accuracy of the digital photo mosaics (leaf-on and leaf-off) and
the thematic accuracy of the vegetation map.
Positional Accuracy Assessment
The horizontal positional accuracy of the leaf-on and leaf-off mosaics was assessed using
guidelines of the USGS-NPS VMP (The Nature Conservancy and Environmental Systems
Research Institute 1994c). For purposes of accuracy assessment, each set of mosaics (leaf-on
and leaf-off) was treated as a single entity because the photography for each was acquired in a
single flight, with the same camera, and with one set of airborne GPS and IMU data. Welldefined
positional accuracy ground control points were placed throughout all quadrants of each
set of mosaics in ArcMap. Ground control points and zoomed-in screenshots of each point were
plotted on hard copy maps with the mosaic as a background. These maps and plots were used to
locate the ground control points in the field. For each plotted ground control point, field staff
noted any alterations to the locations in the field, and then recorded the coordinates with a
Trimble Pro XR/XRS or GeoXT. Mapped ground control points that were physically
inaccessible were also noted. The field crew correctly located and collected accuracy assessment
data at 62 ground control points for the leaf-on mosaics and at 69 ground control points for the
leaf-off mosaics. The coordinate data were collected with real time GPS and post processed with
differential correction using Pathfinder Office software. Prior to calculating accuracy, five
ground control points for the leaf-on mosaics and four ground control points for the leaf-off
mosaics were identified as outliers with SAS’s JMP program and removed. The field-collected
“true” or “reference” GPS coordinates for the remaining 57 points for the leaf-on mosaics and 65
points for the leaf-off mosaics were compared to the coordinates obtained from each mosaic
viewed in ArcMap. Both pairs of coordinates for each point were entered into a spreadsheet in
order to calculate horizontal accuracy (in meters). The accuracy calculation formula is based on
root mean square error (FGDC 1998b; Minnesota Governor’s Council on Geographic
Information and Minnesota Land Management Information Center 1999). Figures 4 and 5 show
the distribution of the ground control points for the leaf-on and leaf-off mosaics, respectively.
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