Textbook of Engineering Drawing, Second Edition

• ____ Orthographic Projections 5.19

To determine the true length of a line, given its projections -Rotating line method

In this, each view is made parallel to the reference line and the other view is projected from it. This
is exactly reversal of the procedure adopted in the preceding construction.

Construction: (Fig.5.22)

f

x--+----+--r--~---- y

--~~~-------t

Fig. 5.22 Obtaining true length

1. Draw the given projections allJl and abo


2. Draw f-f and t-t, the loci passing through bl and b and parallel to xy.


3. Rotate al bl to al b 21 , parallel to xy.


4. Draw a projector through b 21 to meet the line t-t at bz.


5. Rotate abl parallel to xy.


6. Draw a projector through bl, to meet the line f-f at bll.

7. Join al, b/ and a, bz•

8. Measure and mark the angles e and ~

The length alb/ (=abz) is the true length of the given line and the angles e and ~, the true

inclinations of the line with H.P and V.P. respectively.

5.7 ·Projection of Planes

A plane figure has two dimensions viz. the length and breadth. It may be of any shape such as

triangular, square, pentagonal, hexagonal, circular etc. The possible orientations of the planes with

respect to the principa~ planes H.P and v.p ofprojection are: