Morphometric Data - An Invariant Approach to Statistical Analysis of Shapes - page 28

on a stage secured to the stand over a piece of ordinary paper. The
paper is taped to the writing surface so that it does not move during
data collection. Two pointers are also mounted on the stand. These
pointers are plumb with each other and move in concert in X and Y
space. One of the pointers is used to locate the landmark on the object
being digitized, while the other pointer traces this location in X,Y
space near the bottom of the stand. When a landmark is located by
touching a location using the upper pointer, the lower pointer is
depressed to mark the landmark location in X,Y space on the paper.
That location is then labeled. The Z coordinate is read from a scale
mounted on the stand that indicates the height of the upper pointer
that is in contact with the landmark and recorded. This is done for
each landmark. After the X and Y coordinates of all landmarks are
marked on the paper, they are most easily entered into a computer
using a two-dimensional digitizer pad. The Z coordinate associated
with each X,Y pair is entered by hand. If a digitizing pad is not avail-
able, a coordinate system superimposed onto the paper could allow the
researcher to determine the X and Y coordinates. These could then be
entered into a computer along with the Z coordinate. Diagraphs can be
custom built or bought from anthropometric instrumentation supply
houses for less than $1500. The diagraph is highly portable and does
not require a computer during the data collection phase. It is techni-
cally simple and easy to use.
Digitizers
A digitizer is a machine that enables either two- or three-dimensional
coordinate data to be collected directly from a specimen by direct con-
tact of a mouse (2D) or stylus (3D) to the point of interest.
Two-dimensional tablets are inexpensive, manufactured by many com-
panies, and come in a large selection of designs and sizes. All
two-dimensional digitizers are intended to record the location of spec-
ified points of interest within a 2D grid. Three-dimensional digitizers
are more expensive, more limited in availability and design, and oper-
ate using either sonic or electrostatic signals. The 3D digitizers are
able to sense the location of a pointer or indicator in a defined 3D vol-
ume and to assign a specific coordinate location to that point within
the defined volume.
The Polhemus 3Space tabletop digitizer is the machine with which
we have had the most experience (see Corner, Lele et al., 1992), but it
is no longer available commercially. The next generation of 3D digitiz-