he 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.

[Illustration: Fig. 5]

[Illustration: Fig. 6]

[Illustration: Fig. 7]

[Illustration: Fig. 8]

[Illustration: Fig. 9]

=1884. Form lines.= It frequently happens that a sketch must be made
very hastily and time will not permit of contouring. In this case form
lines are used. These lines are exactly like contours except that the
elevations and forms of the hills and depressions which they represent
are estimated and the sketcher draws the form lines in to indicate the
varying forms of the ground as he sees it.

=1885. Scales.= The Army Regulations prescribe a uniform system of
scales and contour intervals for military maps, as follows:

Road sketches and extended positions; scale 3 inches to a mile,
vertical (or contour) interval, 20 feet.

Position or outpost sketches; scale 6 inches to a mile, vertical (or
contour) interval, 10 feet.

This uniform system is a great help in sketching as a given map
distance, Par. 1867a, represents the same degree of slope for both the
3 inch to the mile or the 6 inch to the mile scale. The map distances
once learned can be applied to a map of either scale and this is of
great value in sketching.

Construction of Working Scales

=1886. Working scale.= A _working scale_ is a scale used in making a
map. It may be a scale for paces or strides or r