Closing Loops and resurvey

[alanf]

I am keeping raw survey data structured so that compass does not automatically close loops. This was excellent advice I received from Jim Coke, to prevent the data and errors from being distorted.

When the line survey becomes a cartographic survey we will worry about the error correction. By using this approach its very easy to visualize the degree of error. Compass gives error reports on closed loops, but its definitely better to see the data exactly as you surveyed it. I then measure closure errors with the measure tool.

The question here is what degree of error do surveyors here consider acceptable before feeling the need to resurvey? All input very much appreciated.

[Kelly Jessop]

Good question,I think it is relative to the size of the loop. Are you pulling tape or counting knots?

[CaveBuddy95]

If I may, as Alan's partner in crime (during survey only!) we are pulling tape on most passages. Some sections are knotted lines.

[apitkin]

I am keeping raw survey data structured so that compass does not automatically close loops. This was excellent advice I received from Jim Coke, to prevent the data and errors from being distorted.

When the line survey becomes a cartographic survey we will worry about the error correction. By using this approach its very easy to visualize the degree of error. Compass gives error reports on closed loops, but its definitely better to see the data exactly as you surveyed it. I then measure closure errors with the measure tool.

The question here is what degree of error do surveyors here consider acceptable before feeling the need to resurvey? All input very much appreciated.

Good advice from Jim, who may be the most experienced underwater cave cartographer around.

As Kelly says, it depends on the size of the loop, but also a couple of other things, particularly how much effort it took to get the survey in the first place. If you don't need your map to be highly accurate to (for example) give divers a good idea of the cave layout, then you can leave a larger error than if you are surveying to get a precise location (e.g. of a sinkhole in the jungle that you need to cut a trail to). If you had a miserable dive thousands of feet into the system through horribly unstable passage to get the survey, you may decide that the additional accuracy you might get with resurveying isn't worth it.

As an aside, errors in azimuth usually contribute more to loop closure errors than errors in distance, so a large error will not usually because you used knotted line rather than tape. One single incorrectly recorded digit in an azimuth, or a couple of digits transposed, can often make a huge difference. It is worth temporarily trying to close the loops in Compass (you can set up a temporary survey for this purpose that you delete later) and then running the blunder detection capability in Compass as sometimes that will show you that one single shot is much more likely to be the cause of the closure error than others. With some practice it can tell you where you may have gone wrong and therefore you can go back and resurvey a few selected areas rather than the whole loop.

Andy

[Kelly Jessop]

As an aside, errors in azimuth usually contribute more to loop closure errors than errors in distance, so a large error will not usually because you used knotted line rather than tape. One single incorrectly recorded digit in an azimuth, or a couple of digits transposed, can often make a huge difference. It is worth temporarily trying to close the loops in Compass (you can set up a temporary survey for this purpose that you delete later) and then running the blunder detection capability in Compass as sometimes that will show you that one single shot is much more likely to be the cause of the closure error than others. With some practice it can tell you where you may have gone wrong and therefore you can go back and resurvey a few selected areas rather than the whole loop.

Andy

I have found my errors from pulling tape versus knots is much better. If azimuth is an issue,then do backward shots

[Webmaster]

In surveying dry caves we've tried to keep loop closure errors under 2% horizontal, vertical is usually the less accurate measurement and that varies somewhat.

You should always try to fit the data first as-is before performing any corrections. Sometimes it's as simple as spotting an instrument reading that got reversed or otherwise misread.

[alanf]

Thanks everyone. Some great advice here. Just what I was looking for.

[bherman]

I am keeping raw survey data structured so that compass does not automatically close loops. This was excellent advice I received from Jim Coke, to prevent the data and errors from being distorted.

Sorry for reviving an old thread, but could you explain how you keep the data structured? Do you just not put the same station names at the tie ins?

[alanf]

If I close a loop at station XY1 for example, I create a station called XY1_Dummy and finish the loop at that point. That is with compass. Arianne has a nice feature that doesn't close ( adjust ) the loop but shows a dotted line to show the error on the same station name. Arianne is very very cool! I believe you can also automatically close loops if you wish to.

[bherman]

Aha, thank's for the explanation!
I have always connected the loops in Compass, simply because i thought that's how it was supposed to be done...
I'm using Ariane now on a resurvey i have started, and used the virtual lines as a means to measure the errors in the loops.

Yes, you can close the loops as well, but only if you first connect the two points as virtual stations.