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# Measurement on Maps

## distance, map and distances

MEASUREMENT ON MAPS Measurement of Distance.—The shortest distance between two places on the surface of a globe is represented by the arc of a great circle. If the two places are upon the same meridian or upon the equator the exact distance separating them is to be found by reference to a table giving the lengths of arcs of a meridian and of the equator. In all other cases recourse must be had to a map, a globe or mathematical formula. Measurements made on a topographical map yield the most satisfactory results. Even a general map may be trusted, as long as we keep within ten de grees of its centre. In the case of more considerable distances, however, a globe of suitable size should be consulted, or—and this seems preferable—they should be calculated by the rules of spherical trigonometry for the solution of a spherical triangle.

Orthodromic, i.e., great-circle, distances are of course shorter than those measured along a loxodromic line, which intersects all parallels at the same angle. Thus the distance between New York

and Oporto, following the former (great-circle sailing), amounts to 3,000m., while following the rhumb, as in Mercator sailing, it would amount to 3,120 miles.

Direct distances may of course differ widely from the distance which it is necessary to travel between two places along a road, down a winding river or a sinuous coast-line. Thus, the direct distance, as the crow flies, between Brig and the hospice of the Simplon amounts to 4.42 geogr. m. (slope nearly 9°), while the distance by road measures 13.85 geogr. m. (slope nearly 3°). Dis tances such as these can be measured only on a topographical map of a fairly large scale, for on general maps many of the de tails needed for that purpose can no longer be represented. Space runners for facilitating these measurements have been devised in great variety.