Elements of descriptive geometry, with applications to isometric projection and othe forms of one-plane projection; a text-book for colleges and ingineering schools by O. E. Randall.

CHAPTER V

METHOD OF LOCATING GIVEN PARTS
81. Coordinate Planes of Reference. Conceive a profile plane of
projection P perpendicular to G-L, passing through the center
of the drawing space and cutting the plane of the drawing in a
straight line to be known as the axis of Y.
All distances in space will be referred to the planes P, V, and
H, and measured on straight lines perpendicular to them.
Distances measured to the right of P, in front of V and above
H, will be considered plus distances.
Distances measured to the left of P, back of V and below H,
will be considered minus distances.
82. The Point. A point will be located by giving its distances
from P, V, and H. The distance of a point from P is the same as
the distance of its horizontal projection from the horizontal trace
of the plane P, or as the distance of its vertical projection from
the vertical trace of the plane P. The distance of a point from V
is the same as the distance of its horizontal projection from G-L.
The distance of a point from H is the same as the distance of its
vertical projection from G-L.
In giving the position of a point in space its distance from P
will be mentioned first, its distance from V second, and its distance
from H last. M3 = 2, 5, 4 means that the point M is 2 units to the
right of P, 5 units in front of V, and 4 units above H; that the
horizontal projection of M is 2 units to the right of Y and 5 units
in front of G-L; that the vertical projection of Ml is 2 units to the
right of Y and 4 units above G-L. The letters M, N, 0, P, Q,
and R will be used in connection with points.
83. The Straight Line. A straight line will be located by giving
the position of two of its points. [M= - 2, 2, - 5; N = 2, - 4, 6]
indicates a straight line passing through the points M and N but
not necessarily limited by them.
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