ketching are:

(1) The location of points by intersection.

(2) The location of points by resection.

=1879. Location of points by intersection.= To locate a point by
intersection proceed as follows: Set up, level and orient the
sketching board (Par. 1872), at A, Fig. 1. The board is said to be
oriented when the needle is parallel to the sides of the compass
trough of the drawing board, Fig 2. (At every station the needle must
have this position, so that every line on the sketch will be parallel
to the corresponding line or direction on the ground.) Assume a point
(A) on the paper, Fig. 1 Y, in such a position that the ground to be
sketched will fall on the sheet. Lay the ruler on the board and point
it to the desired point (C), all the while keeping the edge of the
ruler on the point (A), Fig. 1 Y. Draw an indefinite line along the
edge. Now move to (B), Fig. 1 X, plotted on the map in (b), Fig. 1 X,
and having set up, leveled and oriented as at (A), Fig. 1 Y, sight
toward (C) as before. The intersection (crossing) of the two lines
locates (C) on the sketch at (c), Fig. 1 X.

[Illustration: Fig. 1]

=1880. Locating points by resection.= A sketcher at an unknown point
may locate himself from two visible known points by setting up and
orienting his sketching board. He then places his alidade (ruler) so
that it points at one of the known points, keeping the edge of the
alidade touching the corresponding point on the sketch. He then draws
a ray (line) from the point toward his eye. He repeats the performance
with the other visible known point and its location on the map. The
point where the rays intersect is his location. This method is called
_resection_. However, local attractions for the compass greatly affect
this method.

=1881. The location of points by traversing.= To locate a point by
traversing is done as follows: With the board set up, leveled and
oriented at A, Fig. 1 Y, as above, draw a line in the direction of the
desired point B, Fig. 1 X, and then move to B, counting strides,
keeping record of them with a tally register, Fig. 3, if one is
available. Set up the board at B, Fig. 1 X, and orient it by laying
the ruler along the line (a)-(b), Fig. 1 X, and moving the board until
the ruler is directed toward A, Fig. 1 Y, on the ground; or else
orient by the needle as at A. With the scale of the sketcher's strides
on the ruler, lay off the number of strides found from A, Fig. 1 Y, to
B, Fig. 1 X, and mark