Vol. VII, No. 3

November, 1994

Vessel Capacity from Pottery Profiles: Notice of an Upcoming Publication

by Louise M. Senior
and Dunbar P. Birnie, III

Prehistoric vessels were made to put things in, yet most archaeological analyses study only what is on the vessels. Whole or reconstructible vessels from which capacity can easily be measured are valuable rare finds given the highly fragmented nature of most archaeological ceramic assemblages. Vessel volume has rarely been systematically quantified because convenient methods dealing with sherds and partial vessels have been lacking. A new technique for measuring accurate vessel capacities from profile illustrations has been developed. It is based on the use of a computer and digitizing tablet to quantify the shape of a vessel cross-section illustration and calculate the volume of that vessel. Whole or reconstructed vessels are not needed, although this method will also work on complete artifacts. Additionally, capacity of symmetrical archaeological features can be calculated using this method if accurate feature sections are digitized.

This method is presented in an upcoming American Antiquity 1995 article ("Accurate Estimation of Vessel Volume from Profile Illustrations," by Louise M. Senior and Dunbar P. Birnie, III). Highly accurate volumetric capacities of fragmented vessels can be quickly calculated from carefully prepared vessel profile illustrations. The profile is digitized by tracing the profile on the digitizing tablet using a small number of points per vessel (20 to 30 points are usually sufficient). These data are then converted to a volumetric measure using a computerized algorithm based on the geometry of stacked beveled-walled cylinders. This method of determining vessel volumes was tested and shown to be highly repeatable and accurate. Quantifiable sources of error are generally limited to less than 1% per vessel, with the final accuracy limited chiefly by the quality of illustration. With this computerized technique, fragmented vessels no longer need to be fully reconstructed in order to obtain volumetric information.

Future investigations might also utilize photographs, rather than vessel renderings for production of profile information. New illustration techniques developed by Harrison Eiteljorg and Nancy Wilkie may also greatly facilitate use of this method of capacity estimation: pottery profiles can now be accurately drawn using AutoCAD 12 and a digitizing tablet (Eiteljorg 1994).

One advantages of this technique is that this program runs very quickly on a standard personal computer (PC). Most vessels can be accurately measured from scale drawings in only a couple of minutes. The brief time required for this method allows investigators to average several readings for the best possible assessment of the vessel capacity. Most archaeological projects are now equipped with a computer adequate for this computation; additional costs of obtaining a digitizing tablet currently run approximately $250-300. The method described herein provides a low budget alternative that supplies very fast, accurate, and repeatable measurement of vessel or feature capacities using scaled illustrations.

Reasons why vessel volume is important are discussed in the introduction, as well as all other methods of calculating capacity cited in the archaeological literature. The limitations of each method of capacity calculation [fluid volume (water), dry volume (rice/beans/etc.), density estimation, geometric solid models, and calculus (stacked-cylinders)] are all discussed, and error calculations are compared to the digitized method introduced in the article. The new method utilizes an improved stacked-cylinder calculus method. The algorithm, rather than being restricted to vertical-walled cylinders, incorporates the calculus for cylinders with slanted edges (frustra); their edges match as they are stacked, and thus provide a smoother, more continuous, rendition of the vessel profile. The algorithm, special programming considerations, and potential sources of error are discussed fully in the article as well.

This technique has been implemented using an IBM AT compatible personal computer and a Summagraphics "Summasketch" digitizing tablet. A powerful computer is not required to run this program efficiently and quickly. In fact, a hard drive is not even needed to run this program successfully; it has also been used with a dual-floppy portable PC computer connected to the digitizer. The computer and digitizer are attached through the parallel communications port (using RS232 protocol). The present program for communicating with the tablet and for later integrating the volume is written in PASCAL, although any language would suffice.

As with all techniques, reasonable care must be taken when measuring the volume. The illustration must be carefully sized and oriented to guarantee accurate data collection. As an example, we describe the measurement of a half-pot cross- section illustration since this is slightly more complex and, as noted above, it is our preferred method.

The scale drawing is first taped to the digitizing tablet. The entire drawing must be digitized without removing it; thus, the illustration must be proportioned to fit within the active area of the digitizer (usually about 1 foot square). This may sometimes require photocopy reductions to make the drawing fit. As long as the illustration's scale is reduced uniformly the potential distortion of the photocopying process will have little impact on the final measured volume. Photocopy distortion is often greatest around the edges of the image and drawings should therefore be scrutinized in these regions for error.

Care must be taken when affixing the drawing to the tablet; it must be directly vertical. To assure this, the illustration's center line should be aligned such that it is perpendicular to the edge of the digitizer's active area. This can easily be done with a drafting triangle or T-square. After attachment, the illustration scale is digitized by pointing to each end of the scale marker and then entering the measured length value through the computer keyboard. The pointing device is called a puck; it has cross-hairs in a target area to aid in aiming the puck precisely and buttons comparable to the buttons on a mouse. Next, point after point along the vessel profile is entered into the program. Each point is entered by carefully moving the puck so that, at each new point, the cross-hairs are directly on the contour. Once the puck is in position, one of the buttons is pressed to register its location. Then the puck is moved to the next position along the contour, thus tracing the outline. Measured points can be taken with arbitrary spacing along the contour; the program does not require that they be evenly spaced to produce a capacity measurement.

There is no specific guideline on how closely the data points should be spaced along the contour. It is usually better to gather as many data points as patience will allow, but the points can usually be spaced somewhere between a millimeter and a centimeter apart and still yield excellent volumes, as discussed below. The spacing of the data points can also be adjusted by the user when entering the data; portions of the cross-section that have sharp curvature, carinations, or other odd shapes will require more closely spaced input points. With practice, most vessel profiles can be digitized in less than two minutes using this program.

After the half-contour is completely specified, then the center line is specified. When these data have been collected, the program quickly goes about integrating the sequential volume elements and presenting the final vessel characteristics. These calculations are completed within only a few seconds.

The authors can provide either a printed copy of the PASCAL program or the compiled program on diskette as "shareware." No Macintosh version is available. Send a stamped, self-addressed envelope for return of materials, and a blank diskette (if the compiled version is desired) to the authors at: Louise M. Senior, Department of Anthropology, Haury Building #30, University of Arizona, Tucson, AZ 85721 USA. The program may also be obtained through the Archaeological Data Archive Project, c/o CSA. Please do not request this material through American Antiquity.

For other Newsletter articles concerning pottery profiles and capacity calculations, consult the Subject index.

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