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Laser Scanning at Machu Picchu

 

Laser scanning at Machu Picchu
 

During the summer of 2005 researchers from CAST, in collaboration with the University of Pennsylvania Museum of Archaeology and Anthropology, conducted a high-density survey (HDS) of the Inca ruins of Machu Picchu and Huayna Picchu using the Optech 3D laser scanner. The team returned  again in 2009 for the Computer Modeling of Heritage Resources Field Program in collaboration with Cotsen Institute for Archaeology at the University of California, Los Angles (UCLA) for additional surveying.  Constructed at an elevation of nearly 8,000 feet (2,440 meters) above sea level in the mid 1400s, Machu Picchu initially served as a place of worship and as a royal retreat for the Inca ruler Pachacuti and his guests. This UNESCO World Heritage site is now Peru's largest tourist destination, attracting nearly a half a million tourists each year.

Machu Picchu was a challenge to HDS for several reasons.  As mentioned above, thousands of tourists visit Machu Picchu daily.  Therefore, it was decided by the project team that it would be best to survey the site at night.  While the team targeted scan setup locations during the day, scanning at night posed several challenges including how to determine the scanner field of view and how to ensure sufficient overlap within the data.  As the team scanned the site over a period of four nights, methods for overcoming the challenges of "night" scanning were determined.

The layout of Machu Picchu is very unique.  The site is located on a steep mountainside in the Andes and is comprised of numerous stone structures and terraces.  The structures are located so close together that it would have been difficult to use the Optech scanner, which requires a minimum distance of three meters from a scan target, to scan the entirety of all of the structures in the site.  A short range unit could have been used but this would require a massive number of scans and could easily require months of collection and processing.  Therefore, the majority of 3D data collected at Machu Picchu was acquired from several distinct vantage points that provided good "panoramic" perspectives  of the site.  The resulting dataset does not have the resolution necessary to resolve fine structural details but rather shows the general layout of the site at an approximate three centimeter resolution.    

The Optech laser scanner was particularly suited for scanning Machu Picchu because it uses infrared technology which makes it possible to scan at night and it has a scan range of up to one kilometer. We believe that the Machu Picchu project was the first of its kind performed by the team where scanning took place in the dark.  Scanning typically occurred between 4:30 pm and 10 pm each evening and each scan took about an hour with a typical scan resolution between three and five centimeters.  Normally the scanner field of view (FOV), the area to be scanned, can be determined using a digital camera that is part of the scanner.  However, when scanning in the dark, the camera does not have much use.   Instead it was determined that first running a quick, very coarse scan provided a nice outline of the features in the scanner’s FOV.  The "pre-scan "resolution was set high (often greater than 100 cm) making the average scan acquisition time less than a minute.  In viewing the pre-scan, the scanner angle and position could appropriately be adjusted.  This technique was also useful for determining the correct amount of rotation between two successive scans. 

Machu Picchu Scan Locations
Map showing scan positions and scanned areas

The image above illustrates the six principal scan locations within the site of Machu Picchu . Scans from positions 1 through 4  were acquired at a 3cm resolution while scans from position 5 were acquired at a much courser resolution of 20 cm due to distance and time constraints.  Scan position 5 was located northwest of the site on top of the smaller mountain next to Huayna Picchu.  Scan position 5 provided a nice perspective for scanning the agricultural terraces and the northwest side of the sacred hill and plaza areaScan position 6 represents the scanning of a central temple and scans of this structure were acquired at a higher resolution of 5mm.

 

 


Scan Position 5 located on small mountain to the Northwest of the site. 

Scanning Huayna Picchu
Scanning Huayna Picchu from Scan Position 5
   

                                                   
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In addition to scanning the ruins of Machu Picchu, scans were also acquired of the ruins on top of Huayna Picchu, the steep mountain to the north of Machu Picchu were additional Inca ruins are located. The image above depicts the scanning of Huayna Picchu which was an estimated 500 meters away from scan location 5. The 3-dimensional recording of this otherwise difficult to map structure was made possible given the long acquisition range of the Optech scan unit. Scans of Huayna Picchu were acquired at a 3 cm resolution and took approximately 2 hours to obtain.

 
2D map showing planimetric view of Machu Picchu


Top-down, planimetric  view of 3D data produced from survey with the Optech laser scanner

A total of four nights were spent scanning at Machu Picchu. Twenty five scans were obtained resulting in a collection of over eighty million data points. Data representing the entire layout of the site, Huayna Picchu, and close-ups of unique areas within Machu Picchu can be downloaded along with a free data viewer at the Machu 3D Data section of the InVirMet Data Repository