4.Pentagon

Enclose the given pentagon in a rectangle and obtain the parallelogram as in Fig. 13 (a) or (b) or (c). Locate points 1, 2, 3, 4 and 5 on the rectangle and mark them on the parallelogram. The distances A–1, B–2, C–3, C–4 and D–5 in isometric drawing are same as the corresponding distances on the pentagon enclosed in the rectangle since the edges of the rectangle are isometric axes.

Figure 13. Method of obtaining the isometric views of a pentagon.

5.Circle

The isometric view or isometric projection of a circle is an ellipse. It is obtained by using four-centre method explained below and illustrated in Figure 14.
Four-Centre Method: First, enclose the given circle into a square ABCD. Draw rhombus ABCD as an isometric view of the square. Join the farthest corners of the rhombus, i.e., A and C. Obtain midpoints 3 and 4 of sides CD and AD respectively. Locate points 1 and 2 at the intersection of AC with B–3 and B–4 respectively. Now with 1 as a centre and radius 1–3, draw a small arc 3–5. Draw another arc 4–6 with same radius but 2 as a centre. With B as a centre and radius B–3, draw an arc 3–4. Draw another arc 5–6 with same radius but with D as a centre.

Figure 14. Method of obtaining the isometric views of a circle by four-centre method.

6.Isometric view of  irregular Shape

The method of drawing the isometric view of an irregular shape 1–2–3–4–5–6–7 is illustrated in Figure 15. First the  figure is enclosed in a rectangle. The parallelogram is obtained in isometric for the rectangle as shown. The distances B–2, D–2, C–3, E–3, G–4, F–4, H–5, H–6 and A–7  has the same length as in original shape since they are along the isometric axis. The points 1 to 7 are located  by travelling along the isometric lines. After locating the points, the points are joined for lines which lie along non-isometric lines viz.  1-2, 2-3, 3-G, 6-5, 7-6.

Figure 15. Method of obtaining the isometric views of an irregular shape.