(Figure 20). By examining the
profile it is seen that an observer, with his eye at D, looking along
the line D--XX, can see the ground as far as (a) from (a) to (b), is
hidden from view by the ridge at (a); (b) to (c) is visible; (c) to
(d) is hidden by the ridge at (c). By thus drawing the profiles, the
visibility of any point from a given point may be determined. The work
may be much shortened by drawing the profile of only the observer's
position (D) of the point in question, and of the probable obstructing
points (a) and (c). It is evidently unnecessary to construct the
profile from D to x, because the slope being concave shows that it
does not form an obstruction.

The above method of determining visibility by means of a profile is
valuable practice for learning slopes of ground, and the forms of the
ground corresponding to different contour spacings.

Visibility of Areas

=1877.= To determine the area visible from a given point the same
method is used. First mark off as invisible all areas hidden by woods,
buildings, high hills, and then test the doubtful points along lines
such as D--XX, Figure 20. With practice the noncommissioned officer
can soon decide by inspection all except the very close cases.

This method is a rapid approximation of the solution shown in the
profile. In general it will not be practicable to determine the
visibility of a point by this method closer than to say the line of
sight pierces the ground between two adjoining contours.

CHAPTER II

MILITARY SKETCHING

(While this chapter presents the principal features of military
sketching in a simple, clear manner, attention is invited to the
fact that the only way that any one who has never done any
sketching can follow properly the statements made, is to do so
with the instruments and the sketching material mentioned at hand.
In fact, the only way to learn how to sketch is to _sketch_.)

=1878.= A military sketch is a rough map showing the features of the
ground that are of military value.

Military sketching is the art of making such a military sketch.

Military sketches are of three kinds:

Position sketches, Fig. 1;

Outpost sketches;

Road sketches.

All kinds of military sketches are intended to give a military
commander detailed information of the ground to be operated over, when
this is not given by the existing maps, or when there are no maps of
the area.

The general methods of s