At 11:12 AM 9/18/02 -0800, Declan Troy wrote: >There were four suggestions for the proper terminology for a line >projected from a starting point and a bearing; ... >rhumb: a line or course on a single bearing >strike: the direction of the line of intersections of a horizontal plane >with an uptilted geological stratum; a distance >transect: a line along which a survey is made >vector: a course or compass direction especially of an airplane
"Strike," "vector," and "bearing" all refer to a direction, not a line or curve. A "rhumb line," or "loxodrome," is a curve on the earth's surface whose bearing is constant from point to point. A "geodesic" is a curve on the earth's surface that locally realizes the shortest distance between its points. Geodesics are the (straight) "lines" of Riemannian geometry. (The bearings of most geodesics vary from point to point (geodesics lying along the equator and along meridians are the exceptions). For spherical earth models, a loxodrome will appear straight in a Mercator projection and all geodesics through a given point will appear straight in a polar projection centered at the point; otherwise, projections of loxodromes and geodesics tend to be noticeably curved. Conversely, portions of (euclidean) lines on a map usually correspond to curved routes on the earth's surface. Thus, the question has multiple answers depending on whether you mean "line" to be straight and whether by "projected from ... a bearing" you mean straight on the earth's surface, straight on a map, or of constant bearing.) Trivial matters like this are fascinating because they expose both the richness and the frailties of our shared interest: multiple terminology reflects not only multiple points of view but also the convergence and collaboration of multiple disciplines, a decided strength of GIS; however, lack of common terminology, and--far worse--the pandemic tendency to use terms in a loose and ill-defined way, expose GIS as a young area without (as yet) a solid professional or scientific grounding. Cheers, Bill Huber Quantitative Decisions --------------------------------------------------------------------- List hosting provided by Directions Magazine | www.directionsmag.com | To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED] Message number: 3116
