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Charting the water's edge

Coast Lines: How Mapmakers Frame the World and Chart Environmental Change

University of Chicago Press: 2008. 224 pp. $25 0226534030 9780226534039 | ISBN: 0-226-53403-0

Coastal margins are drawn as precise lines on maps, yet anyone who has walked on a beach knows that coastlines are unstable. Cliffs, rocks and sands change under the influence of tides, storms, tectonic movements, global climate change and other natural and artificial phenomena.

Mark Monmonier, professor of geography at the Maxwell School of Syracuse University in New York, seeks to inform the public about how cartography and society intersect. He wishes us to look closely at maps, to recognize which features are shown or missing, and understand why. In Coast Lines, he offers an assortment of eclectic and fascinating information about how coastlines have been defined, determined and depicted, focusing on the United States in the twentieth century.

Different maps and charts of the same coastal area show different cartographic coastlines. Monmonier calls our attention to four types, explaining that each is a human construct designed to serve a specific purpose, and the result of many observations and assumptions (the latter sometimes gaining the upper hand). One cartographic coastline is the high-water line visible from offshore. Another, introduced in the nineteenth century to aid safe navigation, is the low-water line. Two are more recent: storm surge lines are designed mainly for evacuation planning and flood insurance, and inundation lines describe the plausible effects of changing geological and meteorological conditions.

Monmonier discusses the international boundaries of territorial waters. These comprise the region beyond a nation's coastline, however defined, in which that nation has sovereignty, where it can “enforce laws, levy taxes, and exclude foreign vessels not pursuing expeditious 'innocent passage' along the coast or into a port”. Under pressure from military and economic quarters, territorial waters have, in recent decades, been extended from the traditional 5.5 kilometres to 22 kilometres from shore.

Satellite images help map-makers to chart changes in coastal margins, such as shifting sands from Louisiana to Alabama. Credit: NASA/JPL-CALTECH

Coast Lines introduces the exclusive economic zone (EEZ) in which maritime nations can “manage fisheries, mine the sea bed, and extract oil and natural gas” but cannot exclude foreign vessels. The EEZ extends 370 kilometres from shore, except where it encounters the EEZ of another nation, in which case the boundary is drawn equidistant between the two coastlines. Islands can affect where the line is drawn. For instance, because of the position of Key West and the Dry Tortuga islands off the southwestern coast of Florida, the compromise boundary between the United States and Cuba is much farther south than would be expected.

To show that “good cartography is seldom cheap and rarely happens overnight”, Monmonier tells the story of the ambitious International Map of the World project. Following a German geographer's proposal at an international conference in 1891, cartographers from the industrialized nations agreed to produce a series of maps with uniform projection, symbols, style, scale and technique, which would adequately describe all of the mountains, rivers and key coastal features of the world. When complete, the mosaic of assembled 'millionth maps' — drawn to a scale of 1:1,000,000 — would have covered more than 185 square metres (half the size of a basketball court) at a cost of £100,000 (US$200,000), a significant sum at the time. Some maps were eventually produced but, with most nations reluctant to pay their share and with two intervening world wars, this grand project ended in a whimper in the second half of the twentieth century.

The failure of the International Map of the World project can, to some extent, be explained by emerging technologies that made the old standards obsolete. Some advances have revolutionized data collection and display, and some (such as intercontinental ballistic missiles) expanded the military's need for maps that were increasingly accurate and sophisticated. Monmonier has good knowledge of cartographic technologies. In a section on overhead imaging, he introduces the problem of consolidating images taken from different angles and discusses techniques for remote sensing. In another chapter he surveys the types of electronic chart that are now available.

Today, the mapping of coastlines has environmental and political implications. Proposing that cartographers can document global climate change, Monmonier juxtaposes two charts of the Outer Banks of North Carolina that show how the coastline in this area has receded over the past 125 years. Turning to predictive cartography — that is, maps and charts that indicate the probable future effect of rising seas and coastal subsidence — he admits that their foundations incorporate “multiple sources of uncertainty”. Monmonier acknowledges the sensitive political nature of materials of this type. For example, although he has seen the latest maps of tidal marshlands prepared by the US Environmental Protection Agency, he is prevented from discussing them before their authorized release.

By highlighting a selection of topics, Coast Lines may succeed in its goal of getting the public to think about what maps show and why. However, Monmonier cautions prospective navigators to be aware that modern global-positioning technology enables them to know their latitudes and longitudes with greater precision than the scientists who made their maps in the first place.

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Warner, D. Charting the water's edge. Nature 453, 285–286 (2008).

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