A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Sextant Positions versus Map Datums?
Date: 2002 Jan 17, 11:33 AM
Date: 2002 Jan 17, 11:33 AM
On Wed 16 Jan 2002 10:07 PM in Navigation-L, Trevor Kenchington [mailto:Gadus@ISTAR.CA] said: > As I understand it, the various datums are different ellipsoids, > each of which is a mathematically-perfect surface which closely > (but not perfectly) matches the shape of the real globe, the > early ellipsoids differing in which part of the real globe they > most closely matched. Surely then it is not the landmarks which > move (they, after all, are rooted on bedrock) but the positions > of latitude and longitude lines which are defined relative to > the particular ellipsoid? Actually, it is the landmarks that appear to "move" between charts with differing datums. Underlying each datum is an ellipsoid. The ellipsoid is an idealized smooth surface chosen to minimize differences between itself and the geoid (a theoretical earth with all points at "sea level" and with the surface everywhere perpendicular to the local gravity vector) for the area covered by the datum (North America for NAD-27) or worldwide (WGS-84). The equator is a plane through the center of the earth perpendicular to the axis of the poles. The prime meridian is a plane through the poles that passes through a selected point (Greenwich). The longitude of any other point is the angle between its meridian (plane through the poles) and the prime meridian. The latitude is similarly measured as the angle between the equatorial plane and its prime vertical, a plane through the point (and the center of the earth) and perpendicular to the point's meridian. The point to bear in mind is that lat and lon are angular measures between intersecting planes. Project these planes to the stars and you have celestial lat/lon, which, translated into declination and hour angle, is the basis for celestial navigation. A position calculated from celestial sights references these planes. Where these planes intersect the surface of the earth, or of an earth-size sphere or ellipsoid, you have the geographic lat/lon of a point. However, the difference between the lat/lon on the celestial sphere and the not-quite spherical earth is vertical. The planes and the angles between them remain the same. Only the distance from the center of the earth changes with the surface we've chosen. [Diagram of lat/lon planes intersecting concentric sphere and ellipsoid desperately needed here!] So what's the datum shift between charts? A plotting sheet for a given latitude gives you a place to plot a position by lat/lon, or by reference to some other point previously determined. A chart is a plotting sheet with landmarks (land, shoals, channels, etc.) already plotted for you. Plotted how? Each horizontal datum begins with a set of reference points whose lat/lon is determined as accurately as possible. From these reference points, ancient or modern surveying methods are used to assign latitude and longitude to other landmarks, and you have a chart. Newer datums have more accurately determined base references and more accurate surveys from them. But it's not the underlying latitude and longitude that shifts between datums, its the assignment of lat/lon to landmarks. Thus, if you take a lat/lon and plot it on charts with two different datums, it's still the same lat/lon on both, but the distance and bearing from that lat/lon to a given landmark may change. Likewise, a landmark, or a point determined by range and bearing from it may plot at different lat/lons on the two charts. -- Peter (where did my lunch time go?!)