Mapping the Canyons of the Ancients National Monument
Crow Canyon Archaeology Center and the Canyons of the Ancients National Monument looked to map Sand Canyon an ancestral Pueblo site. They knew traditional mapping would not suffice. Using UAV (Unmanned Aerial Vehicle) LiDAR archaeology technology, they received impressive results. They also made some new discoveries.
Managed by the Bureau of Land Management, the culturally rich Canyons of the Ancients National Monument in Colorado, USA contain a wealth of historical and environmental resources. Now a recreational hotspot, in A.D. 1240, the Pueblo community occupied the area. They constructed over 70 villages with approximately 30,000 residents. In Sand Canyon alone over 90 subterranean structures, known as ‘kivas’, were used by families as dwellings. Hundreds of years later, the preservation team were looking for a way to accurately survey this site of archaeological importance. It had been over 20 years since a survey of this calibre had been undertaken. They needed new ways to visualise the area.
In partnership with local UAV service experts, Caddis Aerial, Routescene Inc were chosen to create a high-resolution terrain model of the Sand Canyon Pueblo site. They used their UAV LiDAR 3D mapping solution. This turnkey solution was identified as the perfect system for penetrating the dense vegetation and producing high-resolution data.
Fundamental to the Routescene system is a carefully thought through 6 step workflow which spans the entire utilisation of the solution. From survey and project planning, data acquisition, data processing to the final outputs or “actionable information”. This workflow is a set of orchestrated and repeatable procedures and processes. Every survey and subsequent data analysis is undertaken in a systematic, streamlined way; ensuring the best possible outcomes each time.
Surveying the area with UAV based LiDAR
The team started the flight planning process by considering the objectives of the survey and the diverse terrain of the Sand Canyon area. In the case of this survey, the aim was to get as high a resolution terrain model as possible. This included digitally removing trees and shrubs. The secondary objective was to ensure a high degree of accuracy. In particular relative accuracy with respect to the topographic survey conducted many years previously.
The UAV used was the DJI M600 Pro. This drone was chosen for its stability and high lift capacity. The UAV can sustain a flight duration of 15-20 minutes, dependent on environmental factors. Data is collected from an area of at least 400 x 400m in a single flight. Therefore, surveying 2+ km sq per day was a realistic and achievable aim. Designed for use on UAV’s, Routescene’s LidarPod is the hardware element of their UAV LiDAR solution. It includes a carefully selected array of sensors to ensure that the solution is fit for purpose across a range of different survey and mapping applications. The Velodyne HDL32 is the workhorse of the modern UAV mapping industry. Providing a scan rate of up to 1.4 million points per second from 32 different lasers angled in a 40 degrees field of view. The LidarPod has a radio modem built in to enable command and control. More importantly allowing operations, complete with real-time QA monitoring, over a distance of more than 2km.
Sufficient line of sight for flying the UAV
The team chose a take-off point on the main dirt road at the north of the survey area. This provided with sufficient line of sight so the team could observe the UAV at the outer edges of the survey. The terrain and the direction of the slope of the area had to be considered when deciding in what direction to fly the lines. In this case, the decision was made to fly the lines north-south at a constant altitude. The UAV was flown at 40m altitude relative to the take-off point, which meant that due to the undulating terrain, the altitude at the west and east boundaries was 20m. In the south of the area, the altitude was 60m due to steep cliffs.
The team planned the flight so that the outer boundaries were covered first. Then they worked towards the middle where the take-off point was located. This ensured that when the UAV battery reached the point at which to abort the mission, the time to fly home was as short as possible.
The aim was to ensure a very high DTM (Digital Terrain Model) resolution. This was achieved by flying as low as safely possible, flying at a speed of 5m/s and setting the overlap between adjacent flight lines at 100%. This meant each piece of land was surveyed twice.
This flight planning not only created a higher data density but also increased the probability of the laser points hitting the ground. The use of a multi-laser LiDAR scanner within the LidarPod ensures that the lasers can hit the ground from all angles. This significantly improves the vegetation penetration capability of the system.
Once the UAV was ready to go, the mission plan was uploaded to the UAV from DJI’s GS Pro app. The M600 Pro was flown manually to the start of the mission and the mission flight plan executed from within the GS Pro App.
Ground targets – a must for drone based LiDAR surveys
When the team was deployed in October 2018, the first day was spent establishing ground control and undertaking a reconnaissance of the site.
Routescene has a strong ethos of Quality Assurance (QA). With foundations in basic land survey training built upon by many years working in the offshore industry where QA\QC was of paramount importance. As part of the standard QA process, Routescene advocate every survey has ground control established. Accurately surveying in Ground Control Points (GCP) and placing Routescene LiDAR targets on those points.
In this survey, the targets placed on the GCPs were 60cm in diameter and mounted on small tripods. Covered in a highly retroreflective material, the targets give a high-intensity return which enables them to stand out. They are easy to identify in the resultant point cloud. As the targets are raised from the ground, they also enable the returns to be automatically extracted from the point cloud.
Never say never – opportunity to return
There is always a possibility that additional data needs to be collected at a later date, be it unplanned or planned. As standard practice Routescene permanently mark all GCPs using a peg driven into the ground. In the case of the Sand Canyon survey, a discrepancy was identified in the position of the control points relative to the processed point clouds. To resolve this discrepancy, the survey team revisited the site. This time in the middle of winter, located the control points under a foot of snow and resurveyed the points. After extensive analysis, it was revealed that the use of Microsoft Excel to store the GCP coordinates was proving detrimental. Excel rounds all numbers to six decimal places and, as it was storing geographical coordinates, this caused the team to lose up to 4m of accuracy. Reloading the original raw data into another format restored our accuracy. It left our ground survey team members slightly displeased. A lesson learned.
Three of the targets were used to calibrate the Routescene LidarPod for boresight misalignment. The targets are positioned in a particular way as this enables the roll, pitch and heading misalignment to be determined. The targets are overflown in a specific pattern, and in the subsequent post-processing the boresight misalignments are computed and applied to the raw point cloud. Through experience the Routescene team have observed that generally the heading misalignment is the smallest of the errors, and this can be attributed to the fact that we use a Dual heading antenna solution. The roll misalignment is the largest, and this is made up a variety of different misalignments, including the INS – LiDAR, LiDAR encoder, and laser diode misalignments. Find out more about accuracy with LiDAR surveys with drones.
Processing the data
Efficient data processing is crucial. After every mission, the data is downloaded from the LidarPod and inspected using Routescene’s proprietary LidarViewer Pro software. This software provides a framework on which to build a LiDAR processing workflow. Using the Filter Development Toolkit to develop and apply specific filters to the collected point cloud. This achieves as quickly as possible the most accurate final deliverable. More than 3.2 billion points were collected. It was important that the high resolution of the data was maintained during analysis to create a digital terrain model. The filters used in an automated sequence were sector reduction, laser ID reduction, coordinate conversion, grid creation, the purpose-built “Bare Earth tool”, a skim grid and finally a LAS export filter. This process virtually removes all the vegetation from the site. This exposes in detail the structures that the archaeologists were interested in. The resolution of the final output was an impressive 400 points/m2.
Bingo at 40%
The overall survey experience was mostly positive for the team involved. The survey was undertaken in October 2018, at that time of year the temperature was 15-20 degrees C in the early morning, increasing another 5 degrees as the sun came out. The altitude of the survey area was 7,000 ft and this altitude had to be considered in the UAV mission planning as any aircraft’s performance is reduced the higher it flies. In this project, the density altitude was also a significant consideration. Density altitude is the altitude adjusted for atmospheric conditions, such as temperature and humidity. Operating early in the morning when the air was cooler made a significant difference to the density altitude, which was calculated at 9,000-10,000ft.
The UAV flying was a challenge for many factors. Not just density altitude but also pilot experience on the craft, a newly purchased aircraft and the payload. The agreement was made to return home when there was 40% battery remaining. This is a generous safety margin but, given all the factors, was deemed to be a sensible approach. When the 40% battery capacity was reached then the current flight line was completed. Then the UAV brought into land, typically landing at 30% capacity.
The wind tended to increase mid-morning making the flying conditions even more challenging. Four separate flights were completed with no incidents. The team finished the last flight line just before the wind further strengthened to beyond operational flying limits.
Surprising results – Lidar archaeology
The results gained from the survey proved to be accurate and surprising for the preservationists involved. The Sand Canyon Pueblo was studied, mapped, and excavated between 1984 and 1995 using traditional survey techniques. The Crow Canyon Archaeological Center were excited to discover previously undocumented structures. The LiDAR archaeology images proved to be the best tool for visualising the ancient site in detail and to plan future preservation. Removing the need for painstaking ground surveys. The impressive data accelerated understanding of the Pueblo area and has allowed the client to concentrate their future work on the newly found structures.
Astonishing impact of the survey
Mark D. Varien, executive vice president of the Research Institute at Crow Canyon Archaeological Center commented, “The impact of this survey approach is truly astonishing. It illustrated how the tool could be used to record undocumented sites with unprecedented precision. It removed the need for a painstaking ground survey and the speed of delivery of such detailed results is impressive. It has accelerated our understanding – the results indicate the Pueblo was more extensive than we had previously imagined.”
Case study appeared in the press