Tunnel of Mud

I went to the very end of the main cave today. This is not very far, about 24 metres as the bat flies, but it took me all day. This is because I took the total station in with me to survey the cave and the confined spaces and difficult sight lines made it a very complicated survey job.


This is my triumphant photo at the end of the day of the total station set up just outside the final chamber. I took this standing in the chamber looking at the muddy crawl back towards the entrance. A total station like this is essentially just a traditional theodolite with an on-board computer to process and store measurements. Like any theodolite it measures horizontal and vertical angles. It also uses a light beam to measure distances. By combining these two bits of information, direction and distance, the total station can calculate the position of any point that can be sighted through the telescope.

Of course, to relate those calculations to the site grid the machine needs to have been set up somewhere where it can measure the position of at least two known points. In practice this means that surveying is usually a process of constructing a web of control points all around the object that you are mapping. The more control points you put in, the more reliable your survey should be.

This is where the problems start when surveying a narrow muddy hole like most caves. First of all you have got to get the equipment down the cave with you. Then you have to set it up on its tripod and see if you can see at least two of your existing survey points. You can see the points, good, now can you get your head behind the telescope to take the reading? Once the machine is set up like this you can measure the position of everything that you can see from that place. This is often not very much. Then you need to get past the tripod without knocking it over to hammer in more control points.

These need to be visible from where the total station is currently set up and visible from where you are going to set it up next. This is the big limiting factor in how much of the cave you can do from any one station. Twice today I set up the machine in places where I thought I could see  back to my previous survey and discovered that either the roof was too low to get the equipment level or there was a previously unnoticed bit of cave wall in my line of sight. Other complications are the wet and muddy surface and the lack of light. I was terrified I was going to drop some important bit of kit into the water and lose it forever. Between this, the slippery surface, low roof and desire not to kick the tripod over I ended up moving in a strange slow motion dance with lots of kneeling and pirouettes. Imagine someone doing Tai Chi inside a small pipe while wearing steel toe-capped wellies, waterproofs and a safety helmet and you will get the general idea.


Meanwhile, outside the cave it was apparently quite a nice day. Pete has continued to take the fill out of the new cave he discovered. As you can see from this photo it is getting smaller as he moves back. He has had quite a lot of animal bone out of here but so far no other finds.

Tomorrow we will have a lot more people back on site so we should be able to get on much faster with the remaining digging.


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