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Coastlines and Land Masks

Contents


Background

This article is about the two basic methods scientists use to distinguish objectively between the sea and the land: vector coastlines or raster land masks. In both cases, a digital file must be found or created with sufficient detail to portray the necessary features of interest; a global ocean map may require far less resolution than the map of a bay where seabird nests are to be located. The scope of this article is purely global, public datasets, because that is the only level where generalizations may be drawn. At regional, national and local scales, countless individual datasets have been developed of all types and resolutions. The reader is urged to investigate local availabilities even as the materials discussed here are considered

Scales

The topic of Map Scale is covered in another article. Please read that material before continuing here.

Vector Coastlines

A coastline datafile is a vector file that draws "the coastline." The definition of coastline is very open to debate, and there are many different approaches, both scientific and legal. The morphological features of the coast are discussed in the Wikipedia reference below.

  • While global-scale databases have probably been developed using a mixture of high-water to low-water datasets, smaller area databases are often obtained under controlled conditions that allow them to be specified as to water levels. Terms like MHW (mean high water) or MLW (mean low water) may be encountered, in these cases.
  • The physical line of the land/water interface, however and whenever obtained, is the usual feature captured in coastline databases. However, national and international legal principles involve another type of coastline often measured as a set of straight lines between carefully selected "salient points" along the physical shoreline. These legal coastlines tend to be quite low-resolution. Users of coastline data must be aware of these differences, and remain sensitive to political and legal issues that can arise from the use of the wrong "coastline" in some cases.

Surfer Blanking/Boundary Lines

The BLN line file type is both the lowest-resolution coastline set in common use today, providing very rough maps of large landmasses, and it provides a vector method, in Surfer, to "blank" land or sea areas. This means that data in the blanked region are neither gridded nor contoured. BLN files can be configured to blank out either the area within a closed loop, or the area outside it. If large land/sea areas are involved, then the rough resolution of the available BLN files is good enough. If smaller, more detailed mapping is required, then special BLN files must be digitized or converted from other vector sources. OceanTeacher has these BLN files from an old public provided by the Surfer publisher:

  • Africa
  • Antarctica 1
  • Antarctica 2
  • Asia
  • Central America
  • Canada
  • China
  • Europe
  • Mexico
  • North America
  • Oceania
  • Russia
  • South America
  • World

Digital Chart of the World (DCW)

A US government product, dated nominally 1992, originally published in VPF format at a scale of 1:1,000,000. It has been succeeded by the VMap series of products. Sometimes a derivative product, named the Micro World Database is encountered, which probably has a scale of 1:10,000,000 or smaller. See reference below.

Vector Map (VMap)

The US National Geospatial Intelligence Agency is developing a new, updated version of the DCW, also in VPF format (see Wikipedia reference below). The dataset is proposed to be in three versions: VMap0 will be at the same scale as the DCW, but with updated themes and somewhat more detail. It is already in the public domain (see download sites below). Version VMap1, with a higher resolution, is only partially released. VMap2, the highest resolution, is not expected to be public.

World Vector Shoreline (WVS)

Also a US government product, this 1:250,000 product was originally derived from a global set of 3-second rasterized image maps. An enhanced version, WVS Plus, also contains bathymetry and geopolitical boundaries.

Prototype Global Shoreline Data

A new US governmental product based on satellite imagery, still being published in "tiles." The nominal scale is given variously as being 1:50,000 or 1:75,000.

A Global Self-consistent, Hierarchical, High-resolution Shoreline Database (GSHHS)

"GSHHS is a high-resolution shoreline data set amalgamated from two databases in the public domain. The data have undergone extensive processing and are free of internal inconsistencies such as erratic points and crossing segments. The shorelines are constructed entirely from hierarchically arranged closed polygons. The data can be used to simplify data searches and data selections, or to study the statistical characteristics of shorelines and land-masses. It comes with access software and routines to facilitate decimation based on a standard line-reduction algorithm." [From the HSHHS download site, referenced below]

Land Masks

A landmask (or, conversely, a watermask) is a gridded file, georeferenced to the earth surface, that has a data value (typically 1 or 0) indicating land or water for each physical area that it represents. There are many landmasks of the earth, and of particular regions, cited on the Web. Curiously, many of these are described in excruciating detail, but are not actually available for download and/or their format specifications are vague. Typically, they are simply not included in lists of published products. We could include here a short list of the masks that are easily available, but that would probably not provide must users with solutions to their specific situations, for two reasons: (a) the region covered would be too large (i.e. global) or too small, and (b) the grid resolution would not match user requirements. OceanTeacher provides instead the Data Roadmaps Universal Method. This method, fully described in the Roadmaps section, uses a global landcover dataset that can be subsetted (with a routine provided in OceanTeacher) and manipulated in the Saga GIS software to yield either landmasks or watermasks. The source resolution is 1-km, which is extremely good for most purposes, but masks of any desired resolution can be created in a second step

Additional Resources


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Information about this article

Short title: Coastlines and Land Masks

Description: none

Expertise level: beginner

Author: Murray.Brown

Approval status: approved

Approved by: Murray.Brown

Last change: 2010-9-22

Subsection of: Geospatial Data Concepts

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