rp and make clear, distinct signs.

LESSON 5. (CLASS ROOM--FOUR HOURS.)

PROBLEM.--Contours, the Vertical Interval, Use of the Slope Board, Map
Distance, Visibility and Profiles.

A contour is an imaginary line on the surface of the earth all points
of which have the same elevation from a base or datum level, sea level
usually being this base. Slice an apple into pieces 1/2-inch thick;
where the cuts come may represent the contour lines. Take these
individual slices, beginning at the bottom and outline them on a sheet
of paper with a pencil (having run a nail through the apple first to
keep each piece in place). The resulting circles will represent the
apple's outline at 1/2-inch intervals.

Contours are always at equal elevations from each other, and the
Vertical Interval (known by the abbreviation V.I.) is the measure
between successive contour lines. In military maps the V.I. is always
the same for each map scale:

1 inch to the mile, the V.I. is 60 feet.
3 inch to the mile, the V.I. is 20 feet.
6 inch to the mile, the V.I. is 10 feet.
12 inch to the mile, the V.I. is 5 feet.

Note that the V.I. changes in proportion to the scale, a map on a 3
inch to the mile scale is 3 times as large as one on a scale of 1 inch
to the mile, while the V.I. is 1/3 as great, hence the former shows 3
times as many contours as the latter.

Map Distance means the horizontal distance between two contour lines
on a map and indicates a certain degree of slope. As the scale
increases the V.I. decreases in proportion and the M.D. therefore
remains the same for the same degree of slope whatever the scale of
the map. By computation we find that a one degree slope rises one foot
for every 57.3 feet horizontal distance, so a one degree slope would
have a 20 foot rise in 1,146 feet horizontal distance, this distance
equals .65 of an inch on the map if the scale is 3" to 1 mile.

The term "Map Distance" is also loosely used to denote distance
between points as measured on the map. Care should be taken to
distinguish between these two meanings.

Distances between contours, scale 3" to 1 mile: 1/2 deg. slope = 1.3", 1 deg.
slope =.65", 2 deg. slope =.32", 3 deg. slope =.22". These distances are
already on the alidade and if you get a slope of 2 deg. with the slope
board and have the distance from your station on the map to the point
of aim either by pacing, intersection or resection, apply the M.D.
scale as many times as