Merrick Extends Life of LiDAR Sensor by Modifying Flight Operations

Leica ALS40 Contributes to Colombian Market and History

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In 2006, Merrick & Company deployed its five-year-old Leica ALS40 airborne laser scanner to South America for a large coastal mapping project. The Colorado firm saw enormous potential for the LiDAR system to complete a variety of terrain mapping work in Colombia and nearby countries. By devising new field operating techniques specifically geared toward overcoming difficult weather and terrain conditions in the region, Merrick kept the sensor busy for almost another five years, ultimately doubling its productive life in the harshest of circumstances.

For almost 60 years, Merrick & Company has been developing and adapting cutting-edge innovations and applying them as solutions in disciplines as diverse as renewable and nuclear energy, national security, sustainable infrastructure and geospatial technologies. Some of the Greenwood Village, Colorado, firm’s deepest roots reside in aerial mapping and land surveying where the company has been a major player for over five decades.

Merrick recognized the value of airborne LiDAR early on and was routinely performing terrain mapping projects by the late 1990s. After sub-contracting the LiDAR acquisition portion to other companies for several years, Merrick purchased its first system in 2001, a Leica ALS40. That particular laser scanner was just the tenth system to roll off the assembly line at Leica Geosystems, the beginning of a long line of ALS models.

"We realized if we wanted to continue the Merrick mapping legacy, we needed to purchase a LiDAR…because we felt strongly that this was the direction where the traditional photogrammetry business was going," said Brian Raber, Vice President of Merrick’s GeoSpatial Solutions market sector team.

Over the next 10 years operating the ALS40, Merrick flew hundreds of projects with the system co-mounted next to a digital camera in a twin-engine aircraft. In some cases, the LiDAR provided digital elevation model (DEM) data to orthorectify the imagery, but for the majority of projects, the laser point cloud became the primary acquisition tool for the generation of digital surface models (DSM) and terrain models (DTM) as key deliverables.

While most of the LiDAR projects were carried out in the United States and Canada (including Hawaii), Merrick deployed the sensor to Colombia in 2005 for the proof-of-concept phase of what would become a multi-year (multi-million dollar) mapping program of that country’s entire 2,900-kilometer (1,800 miles) Caribbean coastline plus those high priority segments and sea ports of the Pacific coastline.

Under contract to the Colombian Maritime Directorate of the Ministry of Defense (DIMAR), the project was the first detailed mapping of tidal lands, riverine environment and maritime beaches in the coastal areas providing critical data sets for environmental preservation, flood and tsunami modeling as well as navigation charting efforts. The pilot phase proved to DIMAR that the airborne laser scanning technology was the right solution for highly accurate coastline mapping even in densely vegetated jungle conditions. At the same time, the Merrick team was learning that airborne operations of a LiDAR sensor in the equatorial region of South America would be different from similar projects in the United States. Local conditions required them to devise an entirely new concept of operations (CONOPS) for almost every phase of activities from flight planning to aerial acquisition.

Heading to Colombia
"Missions in Colombia are difficult because of the rapidly changing weather patterns," said Raber, explaining that LiDAR can’t penetrate the clouds which seem to hang over most of country much of the day. Flying under the clouds was the only option, but even that had risks due to steep mountainous terrain and the potential for gun fire from below.

At the time the DIMAR project was kicking off, the Leica ALS50 was being introduced as an upgrade for the Leica ALS40, and Merrick was among the first in line to order the more advanced system. Where the ALS40 offered a 15-25 kHz laser pulse rate, the newer scanner boasted a maximum pulse of 52 kHz, more than double that of its predecessor. (In comparison, the ALS70 has a 500 kHz pulse rate.)

"More powerful LiDAR sensors are appealing because they enable operators to hit the ground with an equal or greater number of pulses while flying at higher altitudes, collecting more points in a shorter period of time," said Ron Roth, Airborne LiDAR Product Manager for Leica Geosystems. "Higher power improves the economics of many LiDAR applications."

In Colombia, however, the higher altitude advantage of a faster laser was neutralized by the low-cloud conditions. Flying at the 10,000-foot AGL (above ground level) altitudes typical in U.S. projects was almost never an option in the equatorial region of South America. To complete most LiDAR collections, Merrick found itself routinely flying its DIMAR missions below the clouds at altitudes of 3,000-feet AGL.

"The lower pulse rate system was perfect for flying closer to the ground," said Merrick’s Raber. "Was the ALS40 the most advanced technology available at the time? No, but it was the most appropriate, and we extended the operating life of this investment flying in Colombia."

Had it not won the DIMAR contract, Merrick may have sold or traded in the older laser scanner when it purchased the new Leica ALS50 to serve as the workhorse of its North American terrain mapping activities. But having experienced minimal maintenance issues with the Leica ALS40, the company saw an opportunity to gain additional productivity and build a new business around it in another part of the world.

"The system’s reliability was a key part of our decision to extend its operating life by moving it to South America," said Raber.

Merrick dispatched the LiDAR unit on permanent deployment to Bogota where the sensor was mounted next to a custom-built small-format digital camera in a leased aircraft for simultaneous elevation and imagery data collection.

Overcoming Weather and Terrain
Flying below the clouds for the DIMAR coastal and other Colombian projects, Merrick was able to achieve the mandated collection of elevation points at one meter spacing. But the weather proved so unpredictable that the aircraft remained grounded on many days, putting economic stress on the project. Target areas planned for collection on a given day were often obscured by dangerously low cloud cover, torrential rains, or heavy winds. Ultimately, Merrick devised an entirely new flight operations strategy that kept its crew busy and sensors in the air.

"We had to work multiple contracts at once to keep the system and crew utilized," explained Raber.

While working on DIMAR, the firm won two more major aerial mapping contracts in the Bogota area, along with several smaller ones in the region. One of the big assignments was from Bogota Aquaducto, the local water utility, which wanted the entire metropolitan area and its surroundings mapped, totaling 650 square kilometers. The second project was under contract to Corporacion Autonoma Regional de Cundinamarca (CAR), an environmental agency responsible for water quality. CAR tasked the firm to acquire DEMs for eight large rivers flowing into the capital city.

The Bogota projects shared the same uncertain weather patterns and low cloud conditions as the coastal areas, and they also introduced other operational challenges–extreme terrain and heavy air traffic control constraints. Bogota itself has very hilly topography and is surrounded by steep mountains, which meant flight lines had to be kept short so the pilots could change their altitude in accordance with the terrain to maintain a relatively constant height above the ground. This ensured the LiDAR pulses hit the surface in a consistent spacing, resulting in an even density of elevation points.

To accommodate weather conditions and severe terrain–not to mention an ever-suspicious air traffic control –Merrick ultimately planned each of the Colombian projects at several flying altitudes in order to be ready for whatever cloud conditions would occur. In some cases, three sets of flight plans were created based on different altitudes calculated on historical cloud heights in that particular area. Due to the low flying altitudes involved, the total number of flight lines was much greater, and their lengths much shorter, than comparable projects performed in North America.

"Having the projects planned with multiple flying altitudes, our crew would get up every day, check the weather to see where there was any chance of safe flying, and they would go," said Raber. "Sometimes they would arrive at the target area and find the expected cloud ceiling didn’t work out, so they would use the lower or higher flight plan, if possible."

He acknowledged that flying many short flight lines to cover one small area was not very efficient from an economic perspective, but the alternative was not flying at all. This strategy kept the aircraft and flight crew in the air more frequently, and it maintained a steady stream of LiDAR point cloud and digital image data flowing back to Merrick headquarters in Colorado where the processing and generation of end products were being performed.

"Typically, when you have to drop altitude, it costs you money, but we had to weigh all the other things, like [keeping] the people on the back end working and meeting the project schedule too," said Raber.

The only other significant operational modification made in South America was adjusting field of view on the Leica ALS40 to penetrate dense vegetation. When flying over tropical jungle, the field of view was narrowed to 20-30 degrees, which concentrated the laser pulses into a small swath to ensure some passed through the canopy and reflected off the ground surface. Over most other land cover types, Merrick widened the scanner’s field of view to 40 degrees or greater.

Even with the adjustments made to accommodate extreme conditions in South America, the success of the business model for Merrick’s activities there was made possible by the reliability of the Leica brand, said Juan Carlos Sanchez, director of Latin America Operations for Merrick and its subsidiaries in Mexico and Colombia. Despite what he believes were the most hostile natural environments he has ever worked in–extreme humidity and torrential rains–the LiDAR sensor continued functioning with normal maintenance for almost five years incountry, seeing the completions of the DIMAR, CAR and Aquaducto projects, among others for the U.S. government.

While Leica Geosystems continued to deliver the right replacement parts to Colombia even as the sensor approached 10 years in age, a key aspect of keeping the LiDAR in good operating condition was sticking to the field maintenance schedule and protecting the aircraft from the elements whenever it wasn’t in the air, he added. Otherwise, rain would have found its way into the cabin during one of the frequent spring deluges and damaged both the laser scanner and camera.

Merrick retired the Leica ALS40 in 2010 but has remained active in the region, continuing to serve a market for aerial mapping products that it played a major role in developing. The firm has ongoing LiDAR, hyperspectral, consulting and digital imagery projects in Colombia. And it still adheres to the same strategy of planning for multiple flight lines and making daybreak decisions as to which will be flown, depending on the weather.

Raber said the Colorado firm has adopted some of the South American operating strategy in its U.S. and Mexico activities. Although weather is more predictable in the States, Merrick’s crews now routinely consider flight plans for multiple projects and/or multiple flight plans and remain ready to divert from one to another if weather gets in the way.

"We’re more flexible and prepared now thanks to our experiences in Colombia," concluded Raber.

Author’s Note: Recently Merrick began the process of donating the Leica ALS40 to Colombia’s national mapping agency, Instituto Geogrfico Agustn Codazzi, (IGAC), which operates a technology museum to display equipment that had significant impacts for the benefit of Colombia like the DIMAR coastline project.

Kevin Corbley is president of X-Mark Media and principal of Corbley Communications Inc., a firm that provides business development and strategic communications services to high-tech organizations worldwide.

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A 3.387Mb PDF of this article as it appeared in the magazine complete with images is available by clicking HERE