The representation of relief by hachures is a graphic system, and in the best examples we have is an attempt to show, upon a plane surface, the actual appearance of a given area under given conditions of lighting,--as in the Dufour map of the Alps. Of course certain details that would really disappear if the assumed conditions were actual ones, must be shown upon the map,--so that it is, after all, but a conventional representation. The very best examples are, for this and other reasons, unsatisfactory, and far more so is this the case in the vastly larger class of medium grade and poor work. The contour system represents relief by a series of lines, each of which is, at every point throughout its length, at a certain stated elevation above sea-level, or some other datum-plane; in other words, each contour line represents what would be the water's edge, if the sea were to rise to that elevation. It possesses the advantage of great clearness, but fails to a large degree in the representation of surface detail; moreover, one must have considerable knowledge of topography, in order to read the map correctly.[1] [Footnote 1: For specimens of representation of the same subject on different scales, in both the hachure and contour systems, see plate from "Enthoffer's Topographical Atlas."] To those who must give first place to the quantity of relief rather than the quality, as, for example, the geologist or the engineer, a contoured map is now considered essential. On the other hand, where quality of relief is the prime consideration and the quantity a secondary one, as, for example, for the use of the army, a hachured map is considered the best. The method of hachures may be roughly characterized as a graphic system with a conventional element, and the contour method as a conventional system with a graphic element,--for if the contour interval is small enough a sort of shading is produced which helps considerably the idea of relief.