Sunday, 20 July 2014

Surveying

By Steven Tucker 20 July 2014

If there is something that I am grateful for, it’s the hard work done in the past by cavers in surveying, especially caves like Apocalypse, Wonderfontein, Crystal, Chaos, West Driefontein, Cango, Arnhem, Thabazimbi, Sterkfontein and Bats, ten of the longest caves in Southern Africa. Between the aforementioned there is conservatively sixty five kilometers of surveyed passages. The amount of work put into those is phenomenal.



Now imagine the effort it really took. You are in the deep parts of Wonderfontein; it’s a flat out crawl in which you can’t even turn your head and it’s half filled with water and a muddy floor. It’s a struggle keeping your mouth above the water to breath never mind trying to survey this piece of horror. But you are at your survey station and your next one is selected. You need a few pieces of information to get the survey done. Distance from yourself to the next station with a tape measure, but the tape measure needs to be dragged through the mud to get there and soon it can barely be read and you don’t have a single clean spot to wipe it off on. Next you need the inclination between the stations, so you try and get your head in such an angle that you can actually see the next station and use the inclinometer. Your third measurement is a compass direction, even worst to get as this is affected by the light on your head, now you have to try and get your light away from the compass, but still get your head close enough to it to read the direction and remember it’s so tight you can barely turn your head. Then you require a height measurement, easy; it’s called way to low! But you measure it for the sake of accuracy. Next are the measurements to each wall to get the width of the passage. It gets even tighter there, so you play rock paper scissors with your caving buddy to see who is going to drag the tape measure to the edge of this wide and flat passage. You win and when he gives you these last measurements all of it needs to be written down and a sketch drawn. Can a piece of paper survive this environment? Barely, but as mud caked as it may be, you manage to keep it intact. Getting the pencil to write is another story, the only spot to wipe the point clean is your tongue and caving mud is not a very palatable.


Measurements taken inside a cave 


Fortunately not all of the passages surveyed are this difficult, but to reduce errors due to slack in the tape measure the maximum distance between survey stations was generally only ten meters. With the above caves coming in at sixty five kilometers that means 6,500 measurements between stations at the very least as well as another 26,000 distance measurements to get your left, right, up, down for passage dimensions. Each of those 6,500 measurements is a three dimensional shot as it takes into account the actual distance between two points, however the map you take into a cave is two dimensional, either showing the horizontal plan of the cave or the vertical profile of the cave.

One of your measurements was 10 meters at an inclination of -45 degrees in the direction 275 degrees magnetic. You use trigonometry to calculate that the horizontal distance of the above shot is 7 meters. Trig also shows that the vertical distance of that shot is 7 meters. Then your compass direction also needs to be modified as you want to show true north on the survey, not magnetic, but this is simply deducting 17 or 18 degrees. Now you need to draw this onto another piece of paper. You choose a scale of 1 meter equals 1 millimeter and using a protractor and ruler you draw a single 7 millimeter line on the page that will be your horizontal survey paper. Next you take the left and right measurements and indicate where the walls of this survey station are. You also draw another 7 millimeter line on another paper that will be your vertical survey and indicate on it where your roof and floor height is in relation to the survey station. Now to get up to the sixty five kilometers that make up ten of South Africa’s longest caves, you go through the above procedure another 6,499 times. 

Measurements and rough map drawn inside a cave


When all your lines are drawn on both the pages and wall widths and roof heights are indicated, you can finally start to draw in your walls. Based on your sketches in the cave and notes made you can now trace the outline of the cave passages and indicate any important features. Unfortunately mud has covered portions of your sketches, but you try and scrape this away and remember and guestimate what certain sections look like. When this is done, you put a blank piece of paper over it and retrace all of the walls you just drew in order to get a survey of the cave walls and passages without the lines running through the middle as these can make things confusing and quite simply, they don’t look nice on a final survey. Having completed the tracing and adding a few final features you just remembered, you add up all those measurements, and add up the vertical measurements on the vertical profile sketch and add these in the corner of your survey so that everyone will know exactly how long and deep this cave really is. As a final gesture you proudly sign your name at the bottom, because what you have just completed is truly a work of art.

For all those whose names appear on surveys of South African caves, thank you for your hard work!

3 comments:

  1. well, yes, it was hard work at times, but we did it for fun.

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  2. It's really hard work. I appreciate your efforts that you have done it on survey. I like your post and it will be very important for surveyor. They will get some idea from this. Thanks!!

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