Surveying an Ancient Highway

The Great Inca Road in South America is one of the largest roadway systems ever built. Surveyors in Chile are working to preserve it as a part of the world’s global heritage.

By John Stenmark, LS

For nearly 300 years, the Inca civilization was the largest society in South America. From its origins in Cusco, Peru, the Inca Empire grew to cover the western portion of the continent, extending more than 4,000 km (2,500 miles) from central Chile northward to Colombia. To the east, the empire included portions of Peru, Bolivia, and Argentina.

As in earlier empires in Rome and Egypt that covered large geographic areas, roads and transportation systems were major contributors to the Inca success. The Inca roadway system—collectively known as the Qhapaq Ñan (Great Inca Road)—was made up of more than 40,000 km (25,000 miles) of primary and secondary roads as well as countless smaller routes. The Qhapaq Ñan connected commercial and population centers with agricultural regions, ports, and religious sites. Today, efforts are underway to catalog and preserve the remnants of the historic roadways.

Because of the roadway system’s age and cultural significance, the governments of six countries (Argentina, Bolivia, Chile, Colombia, Ecuador, and Peru) in 2002 requested the United Nations Educational, Scientific and Cultural Organization (UNESCO) to designate the Qhapaq Ñan as a World Heritage Site. When granted, the designation will recognize the Qhapaq Ñan as having outstanding value as part of the world’s natural and cultural heritage. In addition to encouraging preservation, the designation helps a site’s local economy by stimulating publicity and tourism. Currently there are approximately 890 UNESCO World Heritage Sites. Most of the sites are relatively compact and limited in size to individual communities, buildings, and features. So when a proposed site covers thousands of kilometers and millions of hectares, it gains a lot of attention.

A Demanding Project

UNESCO guidelines state that any request for consideration as a World Heritage Site must include data that is “as exhaustive as possible, making sure the necessary documentation and maps are included.” For the Qhapaq Ñan to meet the requirement would take an enormous effort in surveying. In Chile, the Ministry of the Interior contacted Geometrica, an Antofagasta company for surveying, engineering, and architecture, to provide surveying and mapping services on the Qhapaq Ñan. The work would encompass three regions in Chile: Arica and Parincota; Antofagasta; and Atacama.

The project’s objective was to collect survey information of sections of the Qhapaq Ñan proposed as World Heritage Sites. In addition to mapping and georeferencing the alignments of the roadways, the project needed to identify and locate the numerous archaeological features along the road. The project deliverables included detailed data and maps in CAD, GIS, and hard-copy formats. The requirements called for all coordinates to be tied to the Chilean National Geodetic Reference System (NGRS), with accuracy in plan and profile of 20 mm (0.07 ft).

In planning the work, Geometrica examined the terrain and vegetation in the project area. Much of the work would be done at high altitude (2,500 m or 8,200 ft) and in areas with little or no tree cover. According to Geometrica senior engineer Luis Pino Bavestrello, the work was an ideal project for real-time kinematic (RTK) surveying using GPS. “We have compared the RTK GPS method with work carried out using a total station,” Bavestrello said, “and the GPS clearly reduced the time required for the work. This is important, since much of the work was performed in an extreme climate. Additionally, there are many details to be collected in archaeological sites. To do that work with a total station would require extra instruments and setups to maintain a complete line of sight.”

Static Control and RTK Surveying

In Chile, the NGRS geodetic framework is managed by the Chilean Military Geographic Institute (Instituto Geográfico Militar or IGM). IGM maintains more than 300 geodetic control points across the country. The NGRS is connected to the SIRGAS (Sistema de Referencia Geocéntrico para las Américas, or Geocentric Reference System for the Americas) datum. By conducting their work in the SIRGAS frame, Geometrica made it possible to connect their portions of the Qhapaq Ñan to surveys in other regions and countries.

To connect the surveys to the national grid, the Geometrica team used static GPS methods to establish 20 new control points tied to IGM control stations. They processed and adjusted the data using Trimble Geomatics Office software. For elevations, Geometrica used IGM benchmarks tied to Mean Sea Level.

Geometrica assigned four individual surveyors to conduct the RTK work for the project. Each surveyor was equipped with either a Trimble R6 GPS or R8 GNSS system and a TSC2 controller running Trimble Survey Controller software. The surveyors used a Trimble 5700 GPS Receiver as the base station for their RTK work. Cellular phones are not practical in the remote locations, so the team used a Pacific Crest radio modem to broadcast RTK corrections. Cesar Morales Duran, land survey engineer with Geometrica, said that the configuration worked well. “The 35-watt signal had sufficient range, so there was no need to set up repeater stations,” said Duran. “The GPS rovers did not have any problems to receive signals from either the radio modem or the satellites. This allowed us to work confidently and rapidly.”

The RTK approach was helpful in preserving the integrity of the archaeological sites. Unlike total stations, which would have required numerous setups at each location, the teams could place the GPS base stations well away from the valuable, vulnerable relics and have only a single person with a RTK pole on the site. “RTK allowed us to use fewer workers in the field, which reduced foot traffic on the sites,” said Bavestrello. “Each surveyor could work as a one-person team and with knowledge of the points required in the survey. It was an excellent optimization of resources and time.”

Managing Information

To coordinate the work to be performed by the four surveyors, they divided each work site into sections, with the area to be covered by each surveyor entered in the data collector. This let the surveyors concentrate on relatively limited areas while ensuring complete coverage of the archaeological sites and roadways. In order to provide a check on the consistency of the data collected by the different surveyors, some coverage areas purposely overlapped. Archaeologists with special knowledge of the Inca road system accompanied the teams, guiding the surveyors by pointing out the existing archaeological elements along the Qhapaq Ñan.

The teams were careful to ensure accuracy of the locations. They collected data at five-second intervals and set up filters in Trimble Survey Controller to prevent storing data outside the required accuracy range. By setting additional control points, they kept the average baseline length for RTK to around 2 km (1.2 mi). The combination of short baselines, tight filters, and well-planned procedures eliminated errors.

Duran explained the approach to the detail surveys. “For data collection, we created a library of attributes using the Feature and Attribute Editor in the software,” he said. “We took photos of the important features and cross referenced them to the measured points.” The surveyors collected position and attribute information on features including the roadbed, cairns, mortars, tombs, animal pens, petroglyphs, copper concentrations, pottery, structures, and rock alignments. The archaeologists supplemented the position data with detailed information that will be used in a GIS for the project.

In the field, the surveyors worked in shifts of ten consecutive ten-hour days. At the end of each day, the surveyors met to download their data into Trimble Geomatics Office running on a laptop computer. They made backup copies of the data on external disks and crosschecked the work against written notes. Finally, they reviewed the project progress and planned the work for the following day.

Working in the remote areas posed interesting challenges. In unpopulated regions, the crews carried food and water to last for the ten-day shifts. When the surveyors were working near villages, local residents helped to provide food and lodging. But there were exceptions. On one occasion, the locals fired their guns in an attempt to drive the surveyors away from the land. The team’s architect was slightly wounded, and local police intervened to enable the surveyors to complete their assigned work.

Duran described another experience in the backcountry. “A benefit of using GPS for surveying is that it can help with navigation when you’re lost,” he said. “On one occasion while [I was] surveying, it got dark very quickly and I did not have enough time to make it back to the base. In the dark and with no visual references, I became disoriented. With my GPS system, I used the software to set out the location of the GPS base station that was 12 km (7.5 miles) away. I followed the directions that the system gave—through quite rugged terrain—and safely returned to our base.”

Ready for More

It took four months for the Geometrica surveyors to complete the field work for the first part of the project. The teams covered six sections of roadway totaling more than 25 km (15 miles) and created detailed maps of 16 archaeological sites with a combined area of approximately 132 hectares (326 acres). They collected thousands of points and hundreds of photographs, all tied to the SIRGAS grid and within the project accuracy requirements.

In the office, Geometrica processed the field data and exported it to AutoCAD Civil 3D. They prepared plan view maps of the Qhapaq Ñan at 1:1000 scale with contour intervals of 0.5 m (1.6 ft). They also prepared maps of the archaeological ruins at 1:200 scale, and architectural plans and profiles at 1:50 with contour intervals of 0.25 m (0.8 ft). The maps were output to hardcopy as well as DWG and PDF formats. To provide information for the GIS, they used TGO to output the survey data to a shapefile. It was then imported into ESRI ArcGIS software, which attached attributes, symbols, and other information required by UNESCO and the Ministry of the Interior.

With the first phase of the project complete, Geometrica is looking ahead to extending the maps and database for the Qhapaq Ñan. “Thanks to the productivity of the GPS equipment, we managed not only to geo-reference the roads but also to survey the existing archaeological sites in different villages that were connected by the Main Andean Trail,” said Duran. “Compared to our experience on previous jobs, we were able to reduce the survey time by more than 50 percent.”

The Qhapaq Ñan

Around 1250 AD, the Inca expanded from their capital city of Cusco and began to conquer and occupy a territory that would ultimately exceed 1 million square kilometers (390,000 square miles). Over time, they developed a network of roads and trails to move people and goods throughout the empire. By the time the Spanish conquerors arrived in 1532, the Inca Road had evolved to be a marvel of engineering accomplishment. At its height, it was a system with more than 40,000 km (25,000 miles) of improved roadways, viaducts, bridges, and structures.

The Inca Road was made up of two main trails. The coastal route connected Tumbes, Peru to Talca in Chile. Farther east, the highlands (or Andean) route ran from Quito in Ecuador southward to Mendoza, Argentina. From these arteries, hundreds of shorter routes connected to other cities and commercial or agricultural regions.

The materials and conditions of the roads varied with the terrain. Wet areas included bridges, culverts, and causeways. In the arid mountains, builders traversed steep terrain with switchbacks or stairways. Road surfaces were commonly dirt, although stone pavers were used in many places. Many stretches of road were simple pathways less than 4 meters (12 feet) wide, but in some areas the trails were up to 20 meters (66 feet) across. Way station inns called tambu were built at intervals of approximately 20 km (12 miles) along major thoroughfares. The tambu provided supplies and shelter for soldiers and government staff traveling on the road.

In the centuries since the Spanish conquest, much of the Inca Road has been destroyed. Modern construction has done most of the damage in developed regions, and erosion has erased the road in its remote areas. In many locations, evidence of the road is difficult to find at all. By designating the Inca Road as a World Heritage Site, UNESCO and other organizations can recognize and preserve the road as a testament to the enterprise and technology of the Inca society.

John Stenmark, LS, is a writer and consultant working in the AEC and technical industries. He has over 20 years experience in applying advanced technology to surveying and related disciplines.

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