Warning to Navigators: Geodesy & Datums Are Here

By Lieutenant Commander Thomas Cutler, U.S. Navy (Retired)

The cartography that creates charts is based on geodesy. With the advent of satellites and other sophisticated navigational methods, nautical navigators do not have to be geodesists—but they must realize the importance of such things as verifying the datum used to create the chart in use.

Principles

To deal mathematically with our irregularly shaped planet, certain assumptions simplify calculations and have the least deleterious effect on the results. When a cartographer is dealing with a relatively small part of Earth's surface—for example, a city—it is acceptable to assume that the Earth is flat. However, as the area to be covered grows larger, the curvature of its surface must be taken into account.

Although Earth is not a perfect sphere, it is a close cousin to one. In the science of geodesy, several terms are used to describe it: spheroid, ellipsoid, and geoid, depending on what the geodesist is trying to accomplish. These terms are theoretical shapes that are listed in order of their degree of accuracy—the geoid is a much closer approximation of Earth's shape than the spheroid, and the ellipsoid is a compromise between the two.

The ellipsoid can be thought of as a basketball with a small boy sitting on it. His weight flattens the basketball somewhat, causing it to bulge at the middle, so that it is no longer a sphere. This flattening will cause the basketball's diameter to be slightly larger at the middle than at the top. In mathematical terms, the ellipsoid can be defined by the terms "major axis" (equatorial diameter), "minor axis" (polar axis), and "eccentricity" (flattening). Actual measurements reveal the flattening results in a difference between the equatorial and polar diameters of Earth of about 23 nautical miles. Against an equatorial diameter of nearly 7,000 nautical miles, the degree of flattening is very small.

Taking this eccentricity into account, the geodesist has a working model—an ellipsoid—that can be used in calculations yielding small enough errors to be acceptable for most nautical cartographic purposes. The problem is that it changes somewhat, depending on which part of Earth is being measured or charted. An ellipsoid that works well in the Atlantic Ocean does not necessarily fit well in the Indian Ocean. Thus, different ellipsoids based on different measurements must be used for different parts of the earth.

In earlier times, navigators used charts for a local area based on measurements taken in that area; because they navigated by taking bearings on local objects, different mathematical models did not matter. When transferring visual bearings to a chart, the exact longitude and latitude of a fix are not significant—you care where you are relative only to the features on the chart you are using , not where you are in the absolute sense. But modem navigators using satellite fixes do not always construct them geometrically using intersecting bearings; instead, they often accept satellite fixes and plot them on the chart. A potential problem not faced in earlier times is that today's navigator may be using a chart based on a local ellipsoid while simultaneously receiving information from satellites that are using a different set of assumptions.

This is difficult to grasp, since for most of our lives we were perfectly safe in assuming "a map is a map," without thinking of the geodesy used by cartographers to create it. The great advantages realized by the advent of modern technology also bring dangers—and ships have gone aground recently because of failure to realize them. Like their forebears, modern navigators must know the pitfalls of the navigational techniques they are using.

Datums

In the modern world of geodesy and navigation, the word datum takes on new meaning and defies conventional English syntax. Discussing datums in the geodetic or cartographic sense can be confusing because navigators traditionally referred to the reference level of tides on charts as datums. The geodetic datum that has taken on new significance for modern navigators is quite different and must be taken into account to avoid disaster. More than one vessel has been lost because this new definition was ignored or misunderstood.

Before satellite technology, all measurements taken by surveyors to establish known points, lines, and areas were taken relative to some other point. Obviously, there has to be an original (or first) point. In the United States, that point was established in 1879 in Maryland. Once a point has been defined using sophisticated measurements and calculations, other points may be defined relative to it by using techniques such as triangulation. Combining these known points into a network forms a horizontal geodetic reference system. For geodetic purposes, the datum is the starting (or reference) point for the network, but the whole network often is referred to as a datum. For navigational purposes, think of a datum as a system consisting of a base (or reference) point combined with an appropriate ellipsoid so that consistent measurements can be made.

In geodesy, datums can be horizontal or vertical, the latter pertaining to altitude and depth. Horizontal datums are of primary concern in nautical navigation. Many datums are used by cartographers and many nations have developed datums that differ from those of neighboring nations. The Tokyo Datum has its origin in Tokyo, Japan, and the European Datum originates in Potsdam, Germany. In the United States, North American Datum, 1927 (NAD 27) was replaced in 1989 by a new datum designated as NAD 83, which is the current standard used by the National Ocean Service (NOS) in producing its charts.

Modern measuring techniques have greatly improved the geodesist's ability to measure the Earth, resulting in a more accurate entity called the World Geodetic System (WGS). Based on numerous points that have been fixed with precision by satellite technology and modern statistical methods, the WGS is based on a geoid model that can be used worldwide. The first WGS was completed in 1960 and known as WGS 60. Subsequent improvements led to WGS 84, the standard used by the Navigation Satellite Timing and Ranging GPS and by the National Imagery and Mapping Agency (NIMA) in producing its charts. For all intents and purposes, WGS 84 and NAD 83 may be considered as equivalent—meaning that NOS and NIMA charts are compatible.

Datum Shift and Identification

Nautical navigators must remember that datums can be different and these differences result in datum shift, something that is not to be ignored. A problem develops when a navigation system (such as GPS) provides a fix based on a datum different from that used to create the nautical chart in use. The resulting plotted position may be different from actual location on the chart. The difference varies greatly depending on the datums in use. For example, if a GPS position (referenced to WGS 84) in New York Harbor were plotted on a chart that was made using the old NAD 27 datum, the datum shift would be about 11 meters. A GPS position in the Sea of Japan plotted on a chart using the Tokyo Datum would have a datum shift of more than 700 meters.

A similar problem can result if the navigator shifts from one chart to another and the two charts were made using different datums. Simply transferring a position using longitude and latitude will result in the new position having a different location relative to the charted features. If one of the features happens to be a shoal, the datum shift could be dramatic.

Most charts produced by NIMA and NOS include a datum note. Usually located in the title block or in the upper left margin of the chart, it will read "North American Datum of 1983 (World Geodetic System 1984)," "World Geodetic System 1972 (WGS-72)," "World Geodetic System 1984 (WGS 1984)," or sometimes just "World Geodetic System (WGS)." The latter note is sufficient because differences between the four WGS datums (60, 66, 72, and 84) are not significant enough to concern the nautical navigator.

Accompanying the note may be an additional notation indicating that positions obtained from satellite navigation systems can be plotted directly on the chart without corrections. If the chart is a NIMA reproduction of a foreign chart based on a different (non-WGS) datum, it willhave an additional note, such as "Positions obtained from satellite navigation systems referred to the WGS 84 datum must be moved 7.36 minutes northward and 3.65 minutes eastward to agree with this chart." And there may be other notations, giving corrections to other datums. On charts based on such early surveys that a conversion factor cannot be determined accurately, notes will say "Adjustments to WGS cannot be determined for this chart" or "From various sources to (year)," without reference to the WGS.

Minimizing Datum Shift Errors

  • If the chart you are using is based on a WGS datum or the NAD 83 datum, no corrections will be necessary. If it is based on any other datum, you must be aware of—and deal with—datum shift. Because datum shift affects the latitude and longitude grid (graticule) of a chart, but not the relative position of features shown, the most effective way to correct for datum shift and determine the correction factor for satellite information is to compare an accurate fix taken by local means (visual or radar) to a satellite fix taken at the same time. The difference between these two fixes then can be applied to all satellite fixes subsequently taken and plotted on that chart. The prudent navigator will repeat this process periodically to confirm or adjust the correction factor, especially as the vessel moves to other parts of the chart.
  • Exercise caution in applying chart corrections. Make sure that chart corrections are plotted only on the specific charts and editions for which they are intended. Mixing datums when applying chart corrections can introduce errors. Remember that some chart features—drill rigs, for example—are temporary. Since normally they are positioned using satellites, WGS is the appropriate datum. If position information on such a feature is plotted on a chart of a different datum, a correction for datum shift probably will be necessary. If you are doubtful as to the reference datum for plotted in formation, be wary and allow a margin of error that will compensate for any possible datum shifts.

You do not have to be a geodesist or cartographer to be a good navigator. You do not even have to understand geoids, ellipsoids, and datums fully. However, you must be aware of the importance of datums in terms of compatibility. In the Navy, "check chart datum" has been added to every navigational checklist. This awareness, coupled with corrections that may be required as a result, is absolutely imperative until all nautical charts and navigational systems in the world are referenced to the same datum.

Commander Cutler is the senior acquisitions editor of the Naval Institute Press. He is now revising Dutton's Navigation and Piloting (Annapolis: Naval Institute Press, 1985).

 

 
 

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