Coastal Erosion Responses for Alaska - the National Sea Grant ...

44 Richmond—Understanding Shoreline Change
tific preference, different proxies for shoreline position are used to document
coastal change, including the high water line, wet-dry line, vegetation line,
dune toe or crest, toe of the beach, cliff base or top, and the line of mean high
water (MHW).
Aerial photography has several types of inherent distortions and displacements
associated with the geometry of the camera system, the change in the
position of the aircraft from one photo to another, and the relief of the terrain
being mapped. Digital photogrammetry is the current preferred method used
to remove these displacements using digital imagery (usually scanned diapositives
of vertial stereopair photographs). GPS ground control points are used
to relate the imagery to true ground space so that the images can be georeferenced.
The processing provides a fully orthorectified, or corrected, image
that can be used as a base map. The orthorectification process requires the
generation of a digital terrain model (DTM), which involves taking user-input
measurements (ground control points and tie-points) and deriving the DTM
from a stereo model through interpolation (Ackermann 1996). The DTM itself
is a network of grid points containing XYZ information. Additionally, breaklines
can be added to areas between grid points where the topography changes
abruptly, such as the top edge or base of a sea cliff. Breaklines, which allow
for more accurate definition of topographic changes, can only be added to the
DTM while viewing in stereo, in order to see the elevation changes in 3 dimensions.
Once the imagery is orthorectified, orthophotographs and mosaics can
be created that provide distortion-free, georeferenced base maps. The rectified
stereo model can also be displayed on a stereo-viewing monitor, and accurate,
georeferenced measurements can be acquired directly from the model.
Airborne lidar
Airborne lidar surveys ground elevation using an elliptically rotating bluegreen
laser. GPS (global positioning system) positions and inertial navigation
systems are used to correct for aircraft pitch, roll, and heading, providing
ground elevations with accuracies of about ±15 cm (Sallenger et al. 2003). To
compare lidar-derived datum-based shorelines with historical shorelines, a
comparable shoreline reference feature must be used. In a study by Morton
et al. (2004) comparing historical NOAA T-sheet shorelines with a modern
lidar-derived shoreline the MHW shoreline was the reference feature. The lidar
shoreline was determined from densely spaced airborne lidar data, using a
method developed in 2002 by Stockdon et al., where shorelines were extracted
from cross-shore profiles which consist of bands of lidar data 10 m wide in the
alongshore direction and spaced every 20 m along the coast. Repeating this
procedure at successive profiles 20 m apart generates points that can be connected
to create a continuous shoreline. An example of a historical analysis
utilizing historical T-sheet derived shorelines compared with a modern lidarderived
shoreline is shown in Fig. 1.