Whilst traversing through hilly wooded terrain, I've sometimes not had much of an option for a good long backsight. How close is "too close" for a backsight?
Haywood
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Depends on how it was set. If using forced centering then distance is not as critical as when you are setting up over points using an optical plummet or plumb bob.
It also depends on the ratio of FS distance to BS distance. if very unbalanced, then any error is magnified.
One solution is to place an azimuth in the traverse, either GPS (pairs), astronomic, or by gyro. Also helps to have common objects sighted throughout the traverse (radio towers, tanks, steeples, etc)
Least squares software is a very powerful tool to use to add additional info to a traverse and also to run simulations to see what accuracy you should be getting.
In the mid 1970s a field party brought in a traverse closure where the BS was less than 4ft to a FS less than 20ft.
It had perfect angular closure, we laughed and cried inside while trying not to burst it out in front of the old codger........
He was known for his pencil whippin' ways. o.O
When the shots are really short, traverse with three tripods and tribrachs with targets that are in good adjustment and as alike as possible. Matched sets if possible.
Swap out the instrument and the targets to avoid any setup differences and that will keep setups to once per hub.
It will not cure all problems and hopefully lessen them.
A Harris, post: 360882, member: 81 wrote: In the mid 1970s a field party brought in a traverse closure where the BS was less than 4ft to a FS less than 20ft.
It had perfect angular closure, we laughed and cried inside while trying not to burst it out in front of the old codger........
He was known for his pencil whippin' ways. o.O
When the shots are really short, traverse with three tripods and tribrachs with targets that are in good adjustment and as alike as possible. Matched sets if possible.
Swap out the instrument and the targets to avoid any setup differences and that will keep setups to once per hub.
It will not cure all problems and hopefully lessen them.
I had a guy tell me that whenever he had a short BS situation, he would switch the BS and FS around to avoid having a short backsight (thereby resulting in a short foresight, but nobody ever warned him about that). Yea, that will help.
A rough guide is
angle error = angle measurement error + arctan(centering / sight distance)
If you can center to 0.01 ft and shoot 200 ft the centering contributes 10 seconds to the angle error.
You can probably take square root sum of squares to combine centering of instrument, centering of target, and angle measurement error.
Forced centering makes the centering errors fixed for that set of measurements, so you get an almost perfect measurement of points that are somewhere near but not exactly on the point of interest. You still have errors but they don't accumulate so much throughout the traverse.
Sometimes in rough, heavily vegetated terrain you just have to take what you can get. Sometimes that's 40-50 feet. Either that, or you can abandon a station or 3 and try a different route. Or run 2 routes thru the difficult stretch, meeting up on the far side. The short shot situation is often compounded with steep sight lines and poor set-up conditions. Your least squares software should automatically increase the allowable error for those angles to/from short lines. One trick for manual traverse calcs is to order your input such that the traverse ends with with the short shot. It doesn't help the angular misclosure but might help the coordinate misclosure.
I occasionally get such scenario and if possible place a backsight (for next station) as far back as possible from my current station and log as a point.
I then use that as my backsight.
I carry reflective tape stuck to a piece 16 gauge aluminium, about 40 mmå?. These can be placed higher up in a branch or tree as far as possible away from my foresight, next station.
Often I can significantly increase my backsight that way.
Obviously care needs to be taken but I treat them like any control shots.
I can attest to the results.
As for distance. I don't like backsight/ foresights less than 25 metres, but obviously times when that's not possible.
On rolling ridges if say you can only manage stations 10-15 metres apart, then often you can read out to a distant point that is visible from both stations along the ridge. Might need a bit of scrub cutting, but it can significantly increase strength of readings.
Same as previous example, but these are at right angles å± to direction of traverse
You seldom end up with a short backsight without first having a short foresight. So think ahead.
Turn a minimum 2D&2R (i.e. exceed your normal) to the foresight, repeat at the next setup.
Just because you need a short foresight traverse point to get required sideshot/s, does not mean it has to be part of the main traverse. Continue a second foresight past the first as the main traverse point and they may not even be intervisible. If they are intervisible, you have a triangular tie set.
Alternately a shot to a boundary monument may be obtainable from before and after the short sight, use that as a tie.
Set a second short foresight traverse point near the first. It may be so close that you can only tape the distance between them, but from the next traverse point you will have a backsight check point to tighten the geometry. If the distance is really short you may want to occupy both points.
Lastly do a cross tie traverse to tighten the geometry.
Paul in PA
it's very helpful to actually sight what you are looking at instead of sighting a secondary object such as a tribrach. If you can sight ahead and back to the actual point, then do it, since you are using short turns this shouldn't be too hard to arrange, this of course means that your tribrach for the instrument needs to be as close to perfect as you can get it...........................
When running large traverses we used the same principal, actually putting sights on the point, this would allow you to look at the point instead of something hovered over the point, the same thing can be accomplished on short sights, sometimes you can see the dimple, or maybe a tack in the dimple. Also this allows you to check your tribrachs, if they don't line up, then either they are off or the instrument needs some adjustment
I think 50' is too short.
It just depends on what you are doing. You want to try to keep your legs as long as possible. Short Backsight or foresight. You introduce a short leg into your closed traverse, it doesn't matter. As someone pointed out, just backsighting the long "foresight" and foresighting the short "backsight" does nothing extra, except waste time.
One guy told me once that he fixed on some "spot" on the short sight target, so he could refind that spot everytime he sighted it. It made the average angle look better within tolerance for the multiple sets. Bad idea. You want to find the center of the target the best you can each time. The spread will help you get a better average of what the "true" angle is. As everyone know, who has done a lot of traversing, there are always adverse conditions you have to deal with. There is not always a solution to get a long sight, or there area always awkward setups you have to do.
Haywood, post: 360876, member: 9957 wrote: Whilst traversing through hilly wooded terrain, I've sometimes not had much of an option for a good long backsight. How close is "too close" for a backsight?
Haywood
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SOP is anything under 200', is turned from and to a bob string.
I have a laser plummet so setups are fast and simple. I was just wondering what would be too close, but as long as it closes or falls within my tolerances, I am good.
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I would be just as concerned about a very significant change in elevation as I would the short distance to my BS and take whatever steps I could to avoid hanging my entire traverse on that one short and/or steep leg.
Just because I'm paranoid, doesn't mean they aren't out to get me.
short legs in a traverse are not much of a problem if using forced centring.
what you do with them during and afterwards (eg topo and setout) could be a problem -
if you were to come from a short to a long then your centring errors would be extrapolated.
Of course any traverse is shot D&R, even more important over up and down terrain. Also you always carry an elevation on your traverse points, whether needed or not on field points.
Paul in PA
Thanks for all the input guys. Much appreciated.
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Haywood, post: 360876, member: 9957 wrote: Whilst traversing through hilly wooded terrain, I've sometimes not had much of an option for a good long backsight. How close is "too close" for a backsight?
Haywood
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you need to know how good your gear is. forced centering won't help much if your prism optical centre doesn't sit plumb on the spigot (for a leica arrangement) or the prism isn't centered in it's holder, which doesn't sit centered in the carrier. badly build/adjusted/centered gear could turn a 100m backsight to poop. good gear could make a 10m backsight good enough.
so the answer is: it depends on how good your gear is, and you won't know how good it is until you test it.
Once, I had a situation, where we shot from a valley, to a ridge. Moved up to the ridge. Short shot, to ridge top. I walked down the ridge some 150 feet, set a COT (cut off tree, with finish nail, and reflector tape) this became the traverse station. Next shot was other side of ridge, where you could not see from the first ridge shot. We continued traversing over the mountain.
But, we did not have a short backsite.
N
Use a plumb bob string for short shots. If your using a rod instead of a tripod, sight as low as possible for angles and shoot the distance twice with the bubble rotated 180 each time to help with the bubble error. Leave a good backsight so you can double your angle and have less time for instrument drift during your setup, also it helps if you or the rodman accidentally kick a leg