November 19, 2007
GIS Applied to Chaco Canyon, New Mexico
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GIS Applied to Chaco Canyon, New Mexico
by Susan Smith
Featured keynote speaker at the Southwest ESRI User Group Conference held in Santa Fe, NM this year at the La Fonda Hotel was Anna Sofaer, the award winning director of the documentary, Mystery of Chaco Canyon. Chaco Canyon is an historical site known for its largely intact massive ceremonial structures. It is held sacred by the Pueblo Indians, who believe they are the descendants of the ancient inhabitants of the Canyon. An archeoastronomer, Sofaer is the founder of The Solstice Project and the author of the recently published book called Chaco Astronomy.
Sofaer was joined by speakers Bill Stone, national advisor for the State of New Mexico and the National Geodetic Survey, and Rich Friedman, GIS supervisor, for the City of Farmington, also working for the Project.
Sofaer became acquainted with Chaco Canyon a week after summer solstice in 1977 when she joined a group of rock art recorders led by climber Jay Crowdy. Fajada (“Banded Cliff”) Butte, a butte that rises 380 feet above the valley floor in an almost perfect north-south, east-west axis, stands in a two-mile wide gap between two mesas which form the south wall of Chaco Canyon. Upon climbing the butte, which in itself is a feat not to be attempted without rock climbing equipment, Sofaer and the other recorders noted the views of Chaco Canyon and also ruins of approximately 20-30 dwellings.
More importantly, they discovered at the top of Fajada Butte a large spiral petroglyph that at the end of the day, was recorded with a dagger of light from the sun piercing its center. This marked the highest point of the sun during the day and the year, and the marking involved both the lunar and solar aspect.
How ancient people were able to ascend the butte is just one aspect of a larger mystery. In recent years, evidence of a large ramp built on the southwest face of the butte suggests there was once an easier way up. The ramp was believed to have been constructed in three sections. The 3-slab so-called
Sun Dagger site is located at the foot of the uppermost 40 foot high cliff band.
According to The Solstice Project website, “The three sandstone slabs are 2 to 3 meters high, 0.7 to 1 meter wide, and 20 to 50 centimeter thick. They are estimated to weigh around 2 metric tons (2000 kg). Geologic evidence indicates that the slabs were originally part of a single large block which broke off from the cliff face to the left of their present position and toppled over, fracturing along two weaker bedding planes upon impact. The three slabs do not touch, being separated from each other by narrow gaps about 10 centimeters wide.”
Stacked almost vertically against the cliff face, the slabs form the side wall and roof of an “elongated triangular niche” which remains shaded except around midday. At this time, sunlight shines through the slits between the slabs and illuminates the cliff wall for about 25 minutes near summer solstice or up to three hours near winter solstice.
Sofaer said no other ancient astronomers have devised such a complex system of shadow and light. In 1978, Sofaer formed The Solstice Project. On the basis of photogrammetry and photographs, the Solstice Project procured funding from Congress to begin their research. Sofaer assembled a team of archeologists, astronomers, geodesists, architects, and many others to launch ongoing mapping and surveying efforts to record the fragile slabs and spirals, as well as extensive photo documentation of the site’s solar and lunar markings. These efforts finally culminated in the precise digital restoration of the Sun Dagger in 2006.
The Solstice Project is one of many research groups that have studied the canyon since the 1800s, yet it is probably the only such group that has taken advantage of satellite, photogrammetric and 3D modeling technology to learn more about the Chacoan culture.
The computer graphic model of the Sun Dagger took 20 years of work. The need for precise alignment of the site spurred Bill Stone to establish a geodetic control at a station on Fajada. This was then tied into a new online positioning user service called OPUS, a CORS network with stations in Aztec and Albuquerque, and to four GPS continuously operating reference stations.
Pueblo Bonito, the largest structure in Chaco Canyon, is believed to have been occupied from the 800s to the 1200s.
Architects can reconstruct digitally what they built by what is left: there were seven main buildings and some smaller ones with over 700 rooms that originally reached four stories high. They estimate the construction took 250 years to complete and was completed in stages. Some of the rooms were deliberately sealed shut which suggests they were not used for living space.
Adding to the mystery is the fact that building materials had to be transported from a great distance away. Thousands of tons of sandstone were hauled, and timbers were hauled at least 250 miles into the canyon. Some researchers feel food had to be imported as it was a difficult place to grow crops, and pottery shards suggest materials came from distant sites.
Life in Chaco Canyon
Those who study Chaco believe the migrations began over centuries, that the people who became the Chaco were looking for a place that was to be the center of their world.
But why these ancient people chose to construct the center of their world in a desolate geography where the temperatures are extreme: ranging 110 degrees F to high winds of well below – no one really knows.
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-- Susan Smith, GISCafe.com Managing Editor.