at your present station and resting the side of the alidade
against it, swinging the other end for sighting.

After sighting and lining the sight on your sketch, step off evenly to
pace the distance. Time is always a factor in military mapping and
where possible make mental notes as you go along as to where roads or
other important features are located, so that you can place them in
their proper place on the map when you have reached the next station.
It is well always to set a good pace for here time can be readily
saved.

Making an _intersection_ is very simple. For as the sketcher moves
along he ties his map together by sighting at any prominent object
near his area, running these lines very lightly and only where he
assumes the points to lie on his map. An abbreviation on the line or a
number referring to a list off to one side will answer to recall the
object. At any other station where the same point can be seen a
similar line is drawn and where the two lines cross will be the
location of the object. In the case of three lines not crossing at the
same point take the middle of the triangle so formed.

_Resection_ is just the reverse of this process. The mapper wants to
know where he is located on the map. If he is properly oriented and
can aim at two points on the ground which he has located on the map,
he places a pin at one of these locations on the map and aims with the
alidade at the object on the ground drawing a line towards himself;
this is repeated with the other known point and where the two lines
cross on the map will be the point he is standing at.

In intersection the greatest accuracy is obtained by running the rays
so as to meet as nearly as possible at right angles.

In running a traverse the sketcher must expect to find some error at
his closing point. This error must be distributed over the whole
traverse so as not to have all the error concentrated at one point.

LESSON 4. (FIELD WORK--FOUR HOURS.)

PROBLEM.--Make a simple sketch, containing topographical details using
the traverse made during the preceding lesson. Use of conventional
signs should be emphasized and the appreciation of features of
military importance impressed. A tendency is to put in details to a
point of confusion. Judgment must be developed to choose telling
points.

A sharp pencil is always needed in sketching; in putting in the
topographical details special attention must be given to the pencil.
Keep the point sha