[1883]. In all maps that are to be contoured some plane, called the datum plane, must be used to which all contours are referred. This plane is usually mean sea level and the contours are numbered from this plane upward, all heights being elevations above mean sea level.
In a particular locality that is to be sketched there is generally some point the elevation of which is known. These points may be bench marks of a survey, elevation of a railroad station above sea level, etc. By using such points as the reference point for contours the proper elevations above sea level will be shown.
In case no point of known elevation is at hand the elevation of some point will have to be assumed and the contours referred to it.
Skill in contouring comes only with practice but by the use of expedients a fairly accurate contoured map can be made. In contouring an area the stream lines and ravines form a framework or skeleton on which the contours are hung more or less like a cobweb. These lines are accurately mapped and their slopes determined and the contours are then sketched in.
If the sketcher desires he may omit determining the slopes of the stream lines and instead determine the elevations of a number of critical points (points where the slope changes) in the area and then draw in the contours remembering that contours bulge downward on slopes and upward on streams lines and ravines.
If time permits both the slopes of the stream lines and the elevation of the critical points may be determined and the resulting sketch will gain in accuracy.
[Figs. 5], [6], [7], [8], and [9] show these methods of determining and sketching in contours.
