Stereo resampling and image analysis

Once best fit solutions have been reached for the interior, relative and absolute orientations, a new and corrected stereo pair is created during the process of stereo resampling. The resulting stereo model can then be displayed in stereo on the computer screen. The computing

power of the Intergraph ImageStation allows the user to move about the stereo pair as if "flying" over the actual scene or object. It is possible to zoom in or out so as to view features at the level of detail desired. In figure 36, for example, the screen has been divided into four windows or views. The one on the upper right represents a set of beads, and the three others are zoomed in views of each individual bead. The user can define as many views as necessary, either displayed side by side or overlapped. Each can be viewed in stereo mode or not, however, only one can be roamed dynamically. An efficient way to set up views is to have one as large as the screen which can be roamed and several other smaller stereo ones which focus on various objects or features of interest. A large pointing device, called the track-ball, enables the user to control movements along the x, y and z axes, while dynamically displaying the real-life coordinates of the cursor’s current position. Therefore, one always knows the three-dimensional coordinates of the location at which the cursor is positioned. It takes a little time to learn how to "land" the cursor correctly on a surface, but we found that different operators with the same level of skills obtained very consistent results.

The CAD software Microstation and Microstation Feature Collection are integrated into the ImageStation Data Collection environment and allow the digitization in three dimension of features such as roads, an artifact’s profile, a pot’s rim or outline (see figure 37). Microstation offers a palette of predefined three-dimensional shapes, e.g. cylinders, rectangular blocks, spheres, circles, which can be used to quickly digitize the shapes of artifacts. Figure 38 shows a sphere and a cylinder displayed from four different angles. Independent rotation can be applied to each one of these views. The resulting digitization process can create a standard CAD format data file which can be exported to a large suite of other software for display or further analysis.

It is also possible to generate an elevation surface which can be converted to a DEM or to contour lines. To do so the user first defines the area to be modelled and a grid is then displayed. The user must place or "land" the cursor on selected points while viewing the pair in stereo. Once all points have been entered, the software interpolates the information and generates a surface. The results of application of this process to the surface of a large animal bone are shown in figure 39. To create this product, a one-centimeter grid was first displayed and the cursor was manually landed on each intersection creating a very coarse matrix of elevation values. A half- centimeter grid was then displayed to generate a refined version of the surface. Once a coarse surface is available to the software, it can automatically place the cursor at an interpolated depth assisting the user in generating more accurate information. This process is very tedious and time- consuming to perform manually. Third party software called MATCH-T is available for the Photostation which is designed to automatically generate elevation values for terrain and could potentially be used to create elevation surfaces of artifacts. We were unable to obtain this software during the project but hope to obtain it in the future and evaluate the results of automatic elevation matrix extraction. Based on published information, it appears likely that the method will be

Figure 36. Zooming in on an artifact’s details

Figure 38. Examples of Microstation’s ability to handle three-dimensional wireframes

effective in situations where there are complex details or texture on surfaces but may not be effective on objects with uniform surfaces with low contrast or texture.

In conclusion, the use of the Intergraph ImageStation has allowed the photogrammetric preparation of the 35 mm stereo photographs so that three-dimensional measurements could be extracted. Assessment of the accuracy obtained compared to that of manual measurements is discussed in the following section.

RESULTS

The following discussion on results is based on imagery produced by two cameras, a Minolta X-700 and a Nikon F-3. The Nikon was equipped with a 55mm Nikkor lens and the Minolta had a 28-70mm zoom lens. These two systems were used in two different photographic approaches. In one the camera was positioned horizontal to the object, as would be the case in photographing larger items (see figure 25), and in the other the camera was mounted vertically, above the object (see figure 26). This latter configuration would be common in photography of smaller items. For the most part the discussion will deal with products from the vertical camera configuration but some space will also be devoted to results produced by the horizontal one. In addition to cameras and configuration, the following also considers the impact of different control frame structures on the final results.

In the following, we will compare a suite of measurements taken on objects using manual and digital methods. For each object the same individual identified the characteristics or locations on the object for both sets of measurements. Laboratory measurements were obtained by an individual with extensive experience and included traditional measurements such as an object’s width. In other instances, a location was selected to be reflective of specialized measurements that might be taken, such as a prominence on the flaked surface of a chipped stone artifact. Normal venier calipers were used whereever possible. It should be emphasized that manual measurements were not known when the softcopy results were obtained. The same experienced archeologist, trained in the use of the Intergraph system, first obtained softcopy measurements and then took manual measurements of the same artifact features. Thus, the softcopy practitioner needed to identify the appropriate measurement location on the image of the object (widest point of the projectile point, for example) just as the archaeologists would in the laboratory.

The following tables will show the manual (vernier calipers) and digital (3-D softcopy) measurements for a variety of artifacts, and the differences for these measurements obtained by subtracting the manually derived measure from the softcopy derived one. All of the following tables show the depth (z value) measurements at the top and the horizontal (x and y) measurements at the bottom. The third dimension corresponds to the z value and for the purpose of this discussion, is referred to as depth. It should be noted that for horizontally-oriented photography it indeed corresponds to depth or width, however, for vertically-oriented photography, depth actually refers to the “height” or “elevation” of the artifact photographed (see figure 40). The units used

throughout this section are millimeters. The measurements from both manual and softcopy procedures have been expressed to hundreths of millimeters. The readout from the softcopy system was provided in these units as was the readout from the calipers. It is unlikely that such precision is truly possible for many manual or softcopy observations given the limits of the control frame

and the cameras used, but the values as recorded by the individual performing the work have been preserved here for reference purposes.

Figure 40. Vertically- and horizontally-oriented photography

In order to assess the impact of shape, size, and materials on the results a range of artifacts was utilized: a modern ceramic flower pot, a hand-blown glass pitcher, a civil war cannister shot, prehistoric chipped stone adzes, a carved ivory pendant, a small stone bowl, and a very small glass bead.