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Although this site was discovered almost a decade ago 
most efforts to create a spatial model of it failed. Five 
different sets of photogrammetric images have been taken 
over the years, but until recently they either missed control 
information, camera parameters or did not provide 
sufficient coverage. When the Department of Geodetic 
Science and Surveying and the Center for Mapping of The 
Ohio State University got involved in 1991, the National 
Park Service had closed the Sundagger Site completely, so 
that nobody is permitted access to the slabs. This was 
necessary as the popularity of the site brought many visitors 
to Chaco Canyon and resulted in severe erosion and 
vandalism around the slabs that threatened to permanently 
destroy the site. Therefore, we did not have any means of 
acquiring new imagery or establishing control. We had to 
fully rely on existing data. 
In this paper we discuss the procedures that were 
applied to combine various sets of photographs of the 
Sundagger Site. We explain how we created a spatial 
surface model, and simulated shadow-casting by computer 
animation. In the next chapter we give the reader some 
more detailed information about the Sundagger Site. 
2. SOME FACTS ON THE SUNDAGGER SITE 
The Sundagger Site is located close to the top of Fajada 
Butte in Chaco Canyon, New Mexico (Longitude: W 1079 
54' 33", Latitude: 369 01' 08") (figure 2). Although the 
existence of the site had been known for a long time, it did 
not become the focus of scientific investigation until 1979 
when its interesting shadow-casting phenomena were 
described by Anna Sofaer, president of the Solstice Project 
(Sofaer et al., 1979). It consists of three flat slabs, each 
about 2.5 m high, which are leaning against a rock-wall. 
Through the narrow gaps between the slabs sunlight can 
penetrate at certain times of the day. On the wall in the 
back of the slabs there are two spirals. The larger one 
consists of ten rings which are about equally spaced. At the 
summer solstice the shadows cast by the slabs form a 
dagger, which moves from the top to the bottom of the 
spiral. This dagger changes its location every day until it 
arrives at the edges of the spirals at the winter solstice. At 
this time two Sundaggers appear at the left and right edges 
of the spiral. Additionally, one edge of the right-most slab 
casts shadows onto the spiral at the major and minor lunar 
standstills. This finding promoted the assumption that the 
site was used both as a solar and a lunar calendar by the 
Anasazi Indians, which is unique in North America. Figure 
3 shows the solar and lunar markings observed at the spiral. 
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3. PHOTOGRAMMETRIC RECONSTRUCTION 
Five sets of photogrammetric images were taken 
between 1979 and 1987, each with different, mostly non or 
semi-metric cameras. Each photo-set had individual 
control-points, however, for some the coordinates were 
missing. An astronomic azimuth was measured for at least 
one of the image sets. Table 1 shows a comparison of 
photography taken at various epochs. Due to the quality of 
the images and the availability of control points we selected 
the 1983 photographs and added a few images from 1979 to 
cover a larger area of the rock formation. 
The photos were taken with a metric stereo-camera (40 
cm base). The focal lengths of the two cameras and all 
control points were made available to us. We found out 
that these images were used by Aero-Metric, Inc. of 
Sheboygan, Wisconsin, to create maps of the slabs. About 
9000 points representing slabs and spiral were already 
available in a local, 3-dimensional coordinate system. 
These points were never displayed in a proper spatial model 
and thus deemed useless for archaeologists. To check the 
potential accuracy of these image-pairs we ran a bundle 
triangulation using the control points and tie-points 
available. It turned out that the points on the Sundagger 
Site can be located to about 3 mm. In addition to the points 
collected by Aero-Metric, we captured a number of 
breaklines, as well as the whole back wall from the stereo- 
pairs. We also added two photos of the 1979 data-set, in 
order to cover the rock to the left of the slabs. This is the 
back wall. It is important for the simulation of the creation 
of the Sundagger Site. 
4. GENERATION OF A SPATIAL SURFACE 
MODEL 
The large number of 3-dimensional points collected in 
the stereo-plotter cannot be visualized on a 2-dimensional 
map. The high point density makes it virtually impossible 
to distinguish any of the features of the rock surfaces. 
Therefore, all points belonging to a slab or to the back-wall 
were connected by triangles to create a surface model. 
Unlike TIN structures used for digital elevation models 
(DEMs), which are based on elevations plotted on top of a 
plane and therefore, create a unique network of triangles, 
our network must represent multiple surfaces for any point 
of the X, Y-plane. This means that our surface model has to 
cover both top and bottom of any object. 
      
  
  
  
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Figure 2: Map of Chaco Canyon, New Mexico, showing Fajada Butte which holds the 
Sun-dagger Site. 
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