Page 339 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

318 Maritime Archaeology: A Technical Handbook, Second Edition VIII. PROJECTIONS
A. OBJECTS WITH AXIAL SYMMETRY
An object with axial symmetry is usually illustrated by a side elevation. Pots, for example, are often made on a wheel and generally have a central axis of symmetry, i.e., rotating the pot gives the same profile. This is also the case for a number of metal and wooden objects that are turned on a lathe. Thus, on the simplest level, the only view required to illustrate a pot is a front elevation. The drawing of the object should then be divided down the middle, one side used to illustrate the exterior of the object, and on the other side of the illustration, the interior view including the profile of the wall of the object. It does not matter on which side the profile is shown, provided one maintains that convention. A cursory perusal of Old World archaeological reports seems to favor a left-sided profile.
B. ISOMETRIC PROJECTIONS
As well as the standard orthographic projection, another means of illus- trating a three-dimensional object in two dimensions is by using an iso- metric projection. In this projection, an object is drawn in perspective, but without perspective scale change. This is particularly effective for rectilin- ear objects and architectural views. Two of the rectangular coordinate axes are set at right angles, and the third is angled at 30° to the X-axis, between the two, as if viewed in perspective. The scale along the third axis is linear and usually set slightly smaller than the (equal) scales on the other two axes. The resultant drawing shows the objective in perspective view, but in a way which measurements can usually be made in three dimensions (Figure 11.15). All measurements in the isometric projection must be made on or parallel to the isometric axes. A line which is inclined to the isometric axes is called a nonisometric line and does not appear as a true length. There- fore it cannot be used to take measurements from the drawing. As a general guide to the construction of an isometric projection, all vertical and horizontal lines, i.e., all lines parallel to the X- and Y-axis, are drawn hori- zontally (for the X-axis) and vertically (for the Y-axis) and lines that are parallel to the Z-axis make an angle of 30° to the X-axis (see Morris, 1988).