SLAM scanning for tree-covered earthworks

The problem

Trees. They can frustrate even the most patient of surveyors. They block horizontal sight-lines for total stations, prevent vertical line-of-sight from your GPS to satellites, and have claimed many a 50m tape. You could commission a LiDAR (Light Detection and Ranging) flight to cover the area you want to survey, but this could be very costly and if the weather’s bad might not even get off the ground. However, there is one method of survey that loves to be surrounded by closely packed objects and can be operated by a weather-resistant geomatician: the SLAM scanner (which is also weather resistant).

Earthworks, to be scanned with SLAM scanner
Nice earthworks, shame about the trees

The solution

SLAM stands for Simultaneous Localisation and Mapping, meaning it can work out where it is and map the surrounding environment as you walk around. The most common SLAM scanner we use works by sending out 43,000 lasers per second to a range of 30m in a 360° x 270° arc in front of you and measures how far they travel before hitting something – imagine 43,000 30m tapes shooting out from where you are standing, every second, and reading each one instantly. The more objects the lasers can bounce off, the better able the scanner is to locate itself and the better the resulting mapping. So, rather counter-intuitively, the more obstructions, the better the survey.

The virtual lumberjack

The SLAM scanner, with weatherproof geomatician
The SLAM scanner, with weatherproof geomatician

The SLAM scanner produces a scaled ‘point cloud’ (see below for what this looks like) and can be georeferenced if you know the coordinates for a few of the points. If you’ve been scanning in a wooded area a lot of your points will be on trees or in the canopy above, so the next step is to remove the trees to show the lie of the land beneath. We can crop and slice the point cloud in a multitude of ways to reduce the number of redundant points and, to really fine tune it, we can pass it through various filters in the processing software to remove unwanted above-ground-level points. This gives us a DTM (Digital Terrain Model) that can be used to locate and interpret features which otherwise would be masked by trees and undergrowth. Below is an example from Dinas Powys Hillfort in the Vale of Glamorgan, which shows what the point cloud looks like from start to finish during processing, and how densely wooded the site is.

For anyone who wants to begin a deep dive into the complexities of SLAM scanning – the geoslam.com website is a good place to start.

Tom Weavill

For more examples of earthwork scanning check out our Sketchfab account.

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