Morphometric Data - An Invariant Approach to Statistical Analysis of Shapes - page 29

ers available from Polhemus includes the 3DRAW digitizing tablet.
The 3DRAW enables direct digitization of specimens in either two- or
three-dimensions. The 3DRAW includes a stylus that is shaped like a
pencil connected to a tablet. The 3DRAW uses electromagnetic tech-
nology that precisely measures the location of the stylus tip and, if
desired, its orientation in reference to the source located within the
tablet. 3DRAW requires connection to a host computer via the RS-
232C serial port. Data can be continuously input as the stylus moves
through space around the object, or single data points can be collected.
A data capture switch is located on the stylus as well as on a footplate.
We have found the capture switch on the stylus to be less useful for
landmark data collection, as depressing the switch invariably causes
movement of the stylus, which should remain motionless and in con-
tact with the landmark. The footplate is relatively easy to use,
especially if placed on a tabletop and depressed by hand. Once the data
capture switch is depressed, the X, Y, and Z coordinates of that point
are written to an ASCII file on the host computer. The 3DRAW tablet
is portable (approximate dimensions: 21.5'' x 16.4'' x 3.1''), weighing 5.5
pounds. Objects are measured within a volume measuring 17.75'' x
12.75'' x 12'' which limits the user to smaller specimens. The resolution
of the 3DRAW is quoted by Polhemus to be within 0.005, with an accu-
racy of 0.01 inch root mean square. The cost of the 3DRAW exceeds
$5000. It is portable, but requires a computer. Elementary software for
digitizing landmarks is included.
Immersion Corporation produces the MicroScribe-3D, a highly
portable digitizer with a mechanical arm and sensors that track the
position and orientation of the stylus tip. Digitizing software available
with the MicroScribe-3D allows graphical modeling using points, lines,
polygons, or splines. Data can be exported in several standard formats,
and drivers are available that allow the digitizer to function with
graphic modeling packages as well as spreadsheet software. The digi-
tizer requires a computer connection but is compatible with PC,
Macintosh, and Silicon Graphics workstations. Three models are avail-
able which differ in cost and resolution. Steve Leigh (personal
communication) has reported an average error estimate of .5 mm (cal-
culated as the average distance between multiple digitizations of a
single landmark). He reports a lack of statistically significant differ-
ences between linear distances calculated between points digitized
using the MicroScribe-3D and those same linear distances measured
using calipers. We have not conducted measurement error studies
using this digitizer.