Quantcast Historical Analyses (cont.)

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Solana Beach Coastal Preservation Association
August 20, 1998
Project No. 1831
Page 30
through the communities of Encinitas and Leucadia, are undercut at an elevation
of about +15 feet, but this appears to be the result of the cutting action of
windblown sand rather than wave erosion.
The San Diego County coastline has been portrayed in various maps and charts dating
back to the 1800s.  A prime source of coastal maps is the National Oceanic and
Atmospheric Administration (NOAA), which in cooperation with the Los Angeles District
(COE) has produced shoreline-movement maps at a scale of 1:24,000 extending from
Portuguese Point (Long Beach) on the north to the Mexican Border on the south (USCOE,
1985, 1987). The maps compile coastline data, in part from the U.S. Coast and Geodetic
Survey, extending back to 1887 along the coast, and back to the 1850s in the bays, and
show gross changes in the shoreline. However, at this scale, erosion amounts of less than
50" feet are not distinguishable.
Considerable work has also been done to evaluate the accuracy of comparing historic and
contemporary small-scale mapping (Crowell and others, 1991). In Crowell=s study, maps
with scales as large as 1:10,000 were considered. A computed erosion rate is based on an
apparent map difference (subject to mapping resolution inaccuracies) divided by the time
span between maps. A very old map of lesser accuracy may yield a more accurate erosion-
rate estimate than a recent map, because more time allows coastal change to accumulate
to detectable amounts.  The results of these studies generally indicate that typical
resolution of principal identifiable features in mapping performed prior to 1930 may have a
horizontal error of 13 feet, indicating that erosion rates estimated by comparison to these
maps have a resolution of at best two inches per year. Maps produced from 1934 to 1938,
using early photogrammetric methods, are highly variable in quality, with horizontal error
exceeding 36 feet in some cases, indicating erosion-rate resolutions of at best nine inches
per year. Topographic maps produced through the late 1950s, using more contemporary
photogrammetric methods, have horizontal error of about eight feet, yielding potential
erosion-rate resolutions of at best three inches per year. Since the early 1960s, map quality
based on photogrammetric methods has improved to the point where a typical horizontal
error would be less than five feet.
Larger-scale topographic maps dating back to the early 1950s are available for most of San
Diego County at a scale of 1:2400. These maps were prepared using photogrammetric



 


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