Capturing the castle with NavVis VLX

Terra Measurement isn’t your average surveying firm. They’ve made a mission not only to deliver top-level results, but also to use the latest hardware and software available to keep improving the quality and speed of their services.

The firm won a tender for a project at Dudley Castle, a hilltop fortification in West Midlands, England that is surrounded by a large and dense public zoo. Terra Measurement would capture digital documentation for the area of a scheduled monument on the site.

Time’s tight on site

The team didn’t set out to use a mobile mapping system for the survey, and had initially quoted the project with a more traditional workflow. As Beardsley explains, the plan was to capture the area — a teardrop shaped limestone escarpment 300 meters long and 220 meters wide — with total stations and GNSS units.

Work would proceed over a comfortable 40-day timeline, and the team would complete the topographical survey drawings and 3D terrain model back in the office.

But then the instruction for the project came six weeks later than expected. “It was always going to be an awkward survey,” says Beardsley, “but now the challenge was even bigger.” Terra’s team would only have two to three weeks on site “before vegetation returned on a very overgrown site” and made it virtually impossible to capture the necessary data.

And even if the team wanted to capture once vegetation returned, there was the problem of crowds. The public would soon be returning to the zoo once COVID-19 closures ended, and their presence would have slowed the total station work.

With a squeeze on time, and the remit to capture at the highest level of quality, Beardsley says the team decided to take a chance on NavVis VLX.

Virtually there

Given the firm’s dedication to new technologies and methods, it won’t surprise you to learn that Terra Measurement had already established a virtual survey workflow. And they planned to use it on the project.

“We open the data in the LSS and LSS-3D-Vision applications,” Beardsley explains. “Since the point cloud is a digital twin of the world, what we do next is essentially walk around the site with a ‘virtual survey pole.’ We digitize the details and levels every five to ten meters, just like we gather the data in reality, and use that information to generate the topographical survey.”

The catch? This methodology traditionally used point clouds from terrestrial laser scanners, not mobile mapping systems. That’s because TLS point clouds have long represented the industry standard in accuracy and fidelity — the two most important qualities in a data set when you’re performing a virtual survey back in the office.

To use a system like NavVis VLX in this workflow, Terra Measurement knew it would have to produce extremely accurate and high fidelity point clouds. So they decided to give the mobile mapper a quick stress test before trusting it on their Dudley Castle topo survey.

The team brought NavVis VLX to another nearby site they were surveying, Cromford Mills. Sometimes called the “jewel in the crown” of the Derwent Valley Mills world heritage site, the mill site is where Sir Richard Arkwright jumpstarted the industrial revolution in the 18^th century.

Terra captured the large internal courtyard, side road, and car park of the site with the mobile mapping system. “We didn’t overly control it, on purpose,” Beardsley says. “We wanted to see what it could do. But we also wanted to give the NavVis a fair chance, so we had four control points. A simple network, but far from ideal.”

After the capture, Terra compared the NavVis point cloud to a data set generated by a terrestrial scanner on its highest accuracy setting: +/- 1mm.

“What impressed me, before we even compared the data, was just looking at the NavVis VLX point cloud,” Beardsley says. “It was a whole lot cleaner and higher fidelity than we were expecting from a mobile mapping system. I thought, even with just the way this looks, the accuracy has to be there, this device has to be the real deal. We’ve been doing this for many years, and we were impressed.”

The accuracy impressed, too. After slicing up the TLS and NavVis VLX point clouds and making detailed comparisons, the team found that within 20m distance, the data was already sub-10mm accuracy — even with extremely minimal control.