Talk:Cave survey

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Caves

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New article

So I've created this new article, separate from the Speleology article, using the contents of the cave cartography section on that page as the basis. Added more detailed information on the BCRA grades and a section on hydrolevelling amongst other stuff. Currently lacking in citations, please feel free to contribute if you find some good sources. Fattonyni (talk) 00:05, 2 May 2009 (UTC)[reply]

No mention of tape sag or other correction factors.

I'm seeing accuracy of measurement in the article mentioned in approximately 1-2 cm range. But no mention whatsoever of tape sag or other "standard" corrections that land surveyors use. (Mostly because it is a rare cave survey that makes any of the obvious corrections.)

As an example, take tape sag:

Gravity pulls the tape into a catenary curve, it is not a straight line. The harder you pull the closer to straight it is, but it is always taking a path longer than the straight line distance. Older land surveying texts give the "standard" corrections for sag, which depend on

1) the tightness it is being pulled (A land surveyor's standard 15 lb pull seems unknown in cave survey circles)

2) the length of the shot

3) the weight of the tape per unit distance.

4) the stretch of the tape (pretty much negligible for the accuracy of a standard cave survey).

5) the temperature (pretty much negligible change in length with temperature for a fiberglass tape for the accuracy of a standard cave survey)

Item's one and two are major effects, and together are often notably larger effects on a single shot than the claimed accuracy of some cave survey length measurements.

Good older land surveying texts (before electronic distance measurements) often cover this in great detail.

For cave surveyors who tell me "It makes no difference", I suggest the simple experiment:

Find a level wall with a flat top.

Measure between two points on the top of the wall (say 10 or so meters apart), with the tape laid flat on the top of the wall. (This is your control measurement). Measure on the face of the wall between the same two points. Note the difference. This is the difference that tap sag makes. — Preceding unsigned comment added by 155.98.206.65 (talk) 14:17, 27 June 2013 (UTC)[reply]