The Alaska Geospatial Data Committee

Shoreline Subcommittee

Issues briefing on shoreline mapping and delineation in Alaska

Shorelines are recognized by the International Geographic Data Committee
(IGDC) as one of the most important terrain features on earth's surface.
In Alaska the shoreline is poorly defined at high resolutions by
existing digital delineations derived from existing charts or
topographic basemaps. This is partly due to the extraordinary length of
the state shoreline and the costs associated with producing a high
resolution product that includes small scale features but also because
of the expansive tide lands particularly in the Gulf of Alaska region.
Existing linear delineations based on tidal datums at either mean high
water (MHW) or mean lower low water (MLLW) cannot account for the area
of tidal inundation, and thus, significant areas of lands are
unaccounted for on existing digital shoreline representations and
subsequent derived products such as shoreline habitat maps. The Alaska
Geospatial Data Committee (AGDC) Shoreline Subcommittee is currently
addressing this issue with the goal of providing guidance for land and
ocean management agencies involved in mapping shorelines to avoid
duplication of efforts particularly in data acquisition which is often
the most costly phase of any mapping project. This briefing paper
attempts to summarize the principle issues currently associated with
shoreline mapping with a focus on Alaska.

The U.S. coastal zone encompasses a bewildering array of shoreline
designations and definitions of shoreline position leading to a
confusing management and regulatory environment. A single nationally
accepted and consistent shoreline does not exist and the use of
inconsistent shoreline definitions between maps, charts, and geographic
information system (GIS) products leads to user confusion and
misinformed decision making. Shorelines are currently mapped by
different federal and state agencies using different delineation
criteria often aligned with the specifics of a particular agency
mission. The current Mean high water digital shoreline delineation for
the State of Alaska is a product derived from U.S. Geological Survey
(USGS) topographic maps and National Oceanic and Atmospheric
Administration (NOAA) charts leading to a situation where the legal MHW
boundary between private and state lands is actually defined by MHHW in
some cases, MHW in others, and MLLW is still others. The amalgamation of
inconsistent shoreline definitions and boundaries between maps, charts,
GISs and other sources leads to jurisdictional confusion with
significant consequences as highlighted most recently in Alaska by the
Dinkum Sands Case (Reed, 2000).

By definition, a shoreline is a linear intersection of coastal land and
the surface of a water body. Because of the dynamic nature of the water
body and the coastal land, shoreline position is constantly changing and
has never been stable. Changes are caused by both natural processes and
human activities and as humans continue to encroach on coastal lands the
fine scale location and attributes of a shoreline tend to become more
highly valued by an increasingly diverse user community. For example,
shoreline mapping and change detection at relatively high resolutions
becomes critical to safe navigation, coastal resource management,
coastal environmental protection, sustainable coastal development, and
planning.

Differences in shoreline definition and subsequent delineation can lead
to unnecessary duplication of data acquisition efforts with individual
agencies collecting independent datasets in accordance with their own
definitions of the shoreline. There appears to be significant overlap
among federal agencies and between federal and state efforts to acquire
shoreline data and in the subsequent generation of derived products such
as habitat maps, shoreline erosion and accretion maps, etc.. The Bureau
of Land Management for example collects data to define the vegetation
line as the basis for its shoreline, the USGS delineates the shoreline
at MHW based on aerial photographs taken at high water, and the Minerals
Management Service uses the NOAA MLLW shoreline position based on aerial
photos taken at low water. Other agencies involved with coastal mapping
or assessments include the Federal Emergency Management Agency, the U.S.
Army Corp of Engineers, and the U.S. Forest Service. How the different
data collection efforts and resulting shorelines produced by these
agencies will be reconciled is not clear and are likely to generated
considerably confusion to decision makers in the future unless steps are
taken to develop a standard shoreline based on a nationally recognized
tidal datum or shoreline definition.

In a GIS it is currently impossible to depict the dynamic
characteristics of a shoreline. In practice, an instantaneous shoreline
is used but these cannot be directly used for shoreline mapping and
navigation, nor can it be employed for quantifying shoreline changes. A
shoreline that is delineated based on a stable vertical datum can be
treated as a reference shoreline and used to differentiate shoreline
changes. Such a shoreline is called a tide-coordinated shoreline, that
is, the linear intersection between the coastal land and a desired water
level. In the United States, internal shoreline mapping is the
responsibility of the NOAA National Geodetic Survey (NGS) which uses MHW
and MLLW to delineate the tide-coordinated shorelines. MHW and MLLW are
the averages of high and lower low water levels, respectively, over a
period of 19.2 lunar years. All high water heights are included in the
average of MHW, where the type of tide is either semidiurnal or mixed.
Where the type of tide is predominantly diurnal, only the higher high
water heights are included in the average on those days when the tide is
semidiurnal. The lower low water is the lower of the two low water
levels of any tidal day where the tide is of the semidiurnal or mixed
type. The single low water occurring daily during periods when the tide
is diurnal is considered to be lower low water. On NOAA nautical charts
both MHW and MLLW coordinated shorelines are shown on tidal areas. In
current practice, aerial photographs for shoreline mapping are taken
when the water level reaches the desired value (MLLW). This requires
coordination between the water gauge reading and aerial photographing to
make sure that the shoreline that appears in the images is the
tide-coordinated shoreline. In the case of satellite imaging, the
imaging technology has improved so much that the image resolution is
comparable to that of aerial photographs, and it also has stereo mapping
capability. In principle, the images can be taken repeatedly within a
short period. However, it not realistic to arrange satellite imaging at
the desired water levels. The shorelines delineated from the satellite
images are therefore instantaneous shorelines.

One of the key findings of the National Research Council, Ocean Studies
Board, Committee on National Needs for Coastal Mapping and Charting
(2004) was that numerous federal and state agencies had identified the
lack of a consistently define shoreline as a major obstacle to informed
decision making in the coastal zone. But while a consistent shoreline is
most desirable the reality is that many different definitions of the
shoreline are now embedded in local, state, and federal laws making a
single shoreline impractical. With regard to a nationally consistent
shoreline delineation, the NRC committee recommendation was "all parties
should define their shorelines in terms of a tidal datum, allowing
vertical shifts to be calculated between and among the various shoreline
definitions, while at the same time permitting different agencies and
users to maintain their existing legal shoreline definitions. In
situations where legislation or usage does not preclude it, the
committee recommends that the internationally recognized shoreline
established by the NOAA National Geodetic Survey be adopted". The
implementation of this recommendation will rely to a large extent on the
cooperation of and among several agencies. In addition, the tidal models
will depend on real-time tidal measuring stations, the development of
hydrodynamic models for coastal areas, and the development of protocols
and tools for merging bathymetric and topographic datasets. Costs can be
considerably reduced through active coordination among agencies in the
data acquisition phase. Agencies must be cognizant of the needs and data
standards of other agencies to avoid duplication of effort. The Alaska
Geospatial Data Committee Shoreline Subcommittee encourages the
implementation of these recommendations.

References

A Geospatial Framework for the Coastal Zone: National Needs for Coastal
Mapping and Charting, 2004. ISBN-10: 0-309-09176-4.

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