The Gomantong Caves

The making of a map

Considerations to make when using Lidar in caves

Scanning caves and processing resulting point cloud data into something useful is not a simple task and can be a costly and time consuming affair. So, before starting such an endeavour, time should be spent determing the goal of the final product. This will define the most important parameter of your scanner: the scan resolution.

What do we want to do with the data?
  • illustration
  • create a virtual cave
  • printing a 3D model
  • volume calculations
  • bat counting
  • map making
  • etc.

The goal of the project will determine the resolution needed for the scans

For example, a 3D printed model of the cave requires only low resolution scans and it makes no sense to spend additional resources scanning in full resolution. In contrast, the highest possible resolution should be used if, for example, a bat roost is to be scanned for determination of species (Azmy et al., 2012).

Figure 1: Influence of the resolution and distance from the scanner on the distance between two adjacent scan points.

The main factor to consider in order to achieve the requisite resolution is the distance between the instrument and the scanned surfaces.

For example, the distance between the Lidar and the cave walls should be closer than 20 metres when centimetre (or 10 mm) resolution is needed and the scanning is at ¼ resolution.
If the project involves detailed scans of high cave ceilings it might be impossible to get the wanted resolution even when working with full resolution. As figure 1 illustrates, it is not possible to get centimetre-scale detail if the ceilings are higher than 70 m.

 

Know your scanner

Modern scanners have the capacity for very high resolution scans, but knowledge of the characteristics and limitations of the instrument is necessary.
As an example, the Faro Focus 3D X 330 has a resolution of 40960 points over 360°, which can be translated in a resolution angle of 0.009°.
This is of course a fantastic resolution but a full scan like this takes almost an hour and produces a massive point cloud of 711 million points. These long scan times will have a serious impact on the number of scans produced per day. Full resolution settings for large projects with hundreds of scans can be impractical and will raise the costs considerably. Possible costs to take in consideration:

  • accommodation for the scan team
  • rental costs for the scanner
  • wages for the personel
  • processing time afterwards
  • etc.

So, in most cases, scanning at ¼ resolution with 10240 points on 360° or 0.036°, which results in clouds with 44 million points, will suffice.

 


Azmy S.N., Sar S.A.M., Shafie N.J., Ariffin A., Majid Z., Ismail M.N. A. and Shamsir M.S. (2012): “Counting in the dark: Non-intrusive laser scanning for population counting and identifying roosting bats.” Scientific Reports 2 (524):1-4.

Listen to the swiftlets

The Gomantong Caves have a very large population of edible nests' swiftlets. These nocturnal birds, with a body length of 14 cm, use the caves as their breeding colonies.

An unusual ability of these birds is the use of echolocation. They emit audible calls while flying and use the calls to navigate through the caves where they live. They use a relatively crude form of echolocation compared to that of bats.

Here is a sound recording taken in The North passage of Simud Puteh.