The Alaska Geospatial Data Committee

Shoreline Subcommittee

The mapping, delineation, and graphical depiction of shorelines in
Alaska

Shorelines are recognized by the International Geographic Data Committee
(IGDC) as one of the most important geographical features on earth's
surface. In Alaska the shoreline is poorly depicted at high resolutions
by digital delineations derived from existing charts or topographic base
maps. This is due to the extraordinary length of the state shoreline and
the costs associated with producing a high resolution product that
includes fine scale features. The existing linear shoreline delineations
do not account for some areas of tidal inundation particularly in the
Gulf of Alaska, and thus, significant areas of lands are unaccounted for
on digital delineations and subsequent derived products such as
shoreline habitat maps. There are considerable differences among agency
shoreline depictions due to how the agencies collect the data used to
approximate the location in lieu of more precise and thus more expensive
methods. The Alaska Geospatial Data Committee (AGDC) Shoreline
Subcommittee is currently addressing these issues 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 delineation does not exist and the use
of inconsistent shoreline datums between maps, charts, and geographic
information system (GIS) datasets 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 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 graphically depicted by the mean
higher high water (MHHW) in some cases, mean high water (MHW) in others,
and mean lower low water (MLLW) in still others. The amalgamation of
inconsistent shoreline delineations 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). While there was little confusion over the definition of the
boundary in the Dinkum Sands case, the depiction of the feature as an
island on some maps and not on others was used as part of the evidence
presented in the case. The differences in map depictions and the dispute
were both caused by the constantly changing nature of the feature
depending on the time the mapping data was collected.

The geographical definition of 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,
resource management, development, and planning.

Differences in the definition of shorelines can lead to unnecessary
duplication of data acquisition efforts with individual agencies
collecting independent datasets in accordance with their own needs for
shoreline delineation. There appears to be some 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 to
aid in the determination of MHW, 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, the U.S. Forest Service, and the National Park
Service. Legislative mandates will continue to require different datums
for specific agency missions. Coordination of data collection can result
in considerable cost savings. Coordinated data collection will also
allow participating entities to define their respective boundaries
simultaneously, thereby reducing the occurrence of apparent overlaps or
gaps in jurisdictional boundaries.

In the United States shoreline mapping is the responsibility of the NOAA
National Geodetic Survey (NGS) which uses MHW and MLLW to delineate the
tide-coordinated shorelines. However, only those shorelines important to
commerce have been mapped at the fine-scale in Alaska by the NGS to date
and it will likely be decades until the entire state is mapped. MHW and
MLLW are the averages of high and lower low water levels, respectively,
observed over the National Tidal Datum Epoch (NTDE), the specific
19-year period adopted by the NGS as the official time segment over
which tide observations are taken and reduced to obtain mean values for
tidal datums.

![](media/image1.jpeg){width="4.190277777777778in"
height="3.0444444444444443in"}

> *Fig 1. NOAA representation of tidal datums where abbreviations are
> defined in the text except for: DHQ is the Mean Diurnal High Water
> Inequality, MN is the Mean Range of Tide, DLQ is the Mean Diurnal Low
> Water Inequality, GT is the Great Diurnal Range, MTL is the Mean Tide
> Level, and DTL is the Diurnal Tide Level
> (http://tidesandcurrents.noaa.gov).*

It is necessary for standardization because of periodic and apparent
secular trends in sea level. The present NTDE is 1983 through 2001 and
is actively considered for revision every 20-25 years. Tidal datums in
certain regions with anomolous sea level changes (Alaska, Gulf of
Mexico) are calculated on a Modified 5-Year Epoch
(http://tidesandcurrents.noaa.gov). 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 tide 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 (MHW or MLLW). This
requires coordination between the tide 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 to
be tide-coordinated.

One of the key findings of the National Research Council (NRC), 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 defined 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". With a seamless bathymetric/topographic dataset across the
land-water interface, appropriate difference or tidal models, and
consistent horizontal and vertical reference frames, any shoreline
definition can be transformed and integrated within the common
framework. 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 the recommendations of the NRC committee with
particular emphasis on interagency coordination of mapping efforts, data
standards, and development of tools that allow for the transformation of
mapping data to create as much seamless data across the land-water
interface as possible.