the point of aim in which case the position of the contours must be
estimated by eye.

If your elevation above the datum or sea level is unknown at the start
assume any elevation which is great enough to put the datum lower than
the lowest spot of the area to be sketched.

The sketching board is easily made to serve as a slope board in this
manner. Hang a plumb bob about an inch below the center of a straight
edge of the board while pointing at the horizon, using the back of the
board. Mark a point 5.7" directly below and draw a semicircle through
it with the same radius. Now mark the point below the center zero and
from it divide the arc using chords one tenth of an inch long. This
will give a scale reading in degrees. By sighting along the top of the
board at some object at the height of the eye from the ground the
degree of slope is shown by the plumb bob on the scale below. Care
must be exercised to prevent the wind from disturbing the reading. A
protractor may be used in the same manner by sighting along the top
and using a plumb bob to record the angle.

In reading maps it is important to know whether points are visible
from each other due to intervening ridges or other topographical
features. This can be told by laying off accurately the distance on
the map between the points in question and using as datum the lowest
of the 3 points, then draw vertical lines, from the 2 higher points,
making them in proportion to their elevation with any convenient
scale. Draw a line between the first and last points and, if the
intervening vertical cuts this line the second point is not visible
from the first. Take for example, two points A and B, 1,760 yards
apart, by the map, A 500 feet and B 450 feet above sea level, the