About using reference spheres while scanning

Caving and certainly scanning caves can be a risky business. So the shorter the time spent in the cave the better. In the picture we see daredevils Manfred and Benjamin traversing an ancient rattan bridge over a 70 m deep abyss (Kuris entrance, Simud Puteh). This rattan ropework is still used by local nesters for collecting the valuable edible swiftlet nests.

A time-management analysis of the scanning process underground revealed that most of the time was used for transportation of the equipment and setting up the reference spheres. Placing these spheres can indeed be tedious and optimal placement often involved significant risk to personnel.

Another difficulty with the spheres is trying to keep them clean in the muddy cave environment. In the Gomantong Caves we had the extra difficulty of bird and bat droppings splattering the spheres. Keeping these expensive spheres clean proved to be a daunting task. As an alternative we soon used cheap polystyrene spheres which could easily be abused.

Furthermore, complex caves may require many spheres, adding a layer of complexity to transportation arrangements and required workforce.

The lesson we learned from field experience is that reference spheres are really not needed. They are used for semi-automatic registration of the point clouds, with specialized software. However, alternative registering processes are available that does not require reference spheres.

Registering

The process of joining all the point clouds scanned with reference spheres into one big point cloud is called target-base registering. Specialized software uses these spheres to semi-automatically find matches between the different scans, speeding up the process of registering. This is not so demanding on CPU resources (match the spheres and the rest will follow) and can be done with a powerful laptop, and possibly even during the fieldwork.

When no reference spheres are used, the scans are processed instead with cloud-to-cloud registering. This works by manually positioning the scans as close as possible. This has to be checked in the horizontal and vertical planes. The computer will then try to do the final match. This is very demanding on CPU resources and needs long processing times, but the results are very good with low mean target tensions.

To use cloud-to-cloud registering a powerful computer is needed with many and fast cores, a lot of RAM (64 GB is a good start) and fast SSD disks. A laptop would not normally meet these requirements. So registering will not be possible during the fieldwork and can only be done afterwards. This also means that registering is best done by a person who knows the cave well. This insight into the cave will make solving the 3D jigsaw-puzzle during registration a lot easier.