The difference between a nodal prism and non-nodal is just the prism constant used.
When the prism is mounted on a pole or tribrach, it then needs to be positioned in the housing correctly.
If you re just fixing prisms permanently then you can just use the nodal prism constant. Nothing else is required.
I found these quite good value, we got them with an L-bracket that was quite versatile. Also a clip on rain shelter
https://www.sccssurvey.co.uk/leica-gpr112-monitoring-prism.html
https://www.sccssurvey.co.uk/leica-ght112-mounting-set.html
nodal constant on these would be -6 on Leica equipment. -40 on others
We just got done monitoring a bridge abutment while the slope protection was removed and a soil nail wall was put in place.?ÿ We use the same instrument but used adhesive backed reflective targets.?ÿ We had 18 points to monitor 3 times a day, 7 days a week, for almost 4 months.?ÿ Worked out extremely well.
Were you shooting the reflective targets robotically?
Not possible at present to do this - not reliably at least.
I would used fixed prisms for control as well so you do not have to set up tripods every visit. If properly executed resection from fixed points outside the area of subsidence is almost always the best approach when working in a dynamic environment.
A variety of targets for this kind of work are used underground. Ordering in the states can be a bit of a chore.
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Several years ago I was monitoring some thrust borings under a railway, similar situation where the ground was not very stable so permanent monitoring set-ups could not be used. I decided that permanent fixed targets would be needed and to resect off those.
Experiments with some mini glass prisms showed that they would give consistent results (although not necessarily correct results) as long as the incident angle didn't change very much. The targets to monitor were stick-ons fixed to the rail web with epoxy, so the instrument set-up positions could vary, but not by a great amount.
Thus the expected set-up positions would always have a similar angle onto the targets (for any set-up location. The mini prisms were mounted on four structures within around 100 metres of the site, angled so they were visible from all likely set-up positions. Before the job commenced tripods were set at the five probable observation positions and a rigorous coordination of those positions and all the targets was carried out.
BUT... the trick was to treat the mini targets as "separate" points from each of the main set-up positions, so each ended up with a set of local coordinates relating only to the set-up position being used. There were fairly small differences due, as already said by others, to the different viewing angles from each set-up. However, as a unique set of mini target coordinates was used for each set-up location, then any inherent error in the viewing angles was always the same, hence the "false" coordinates gave the correct answers for that position (not strictly true but it works for practical purposes).
As additional checks, each set-up position observed an overlap of the stick-on targets so it could be seen that adjacent set-ups were showing the same movement on targets which had been observed from both. The requirement was to detect mm. movement
In practice I found that the instrument could be moved up to 5 metres from the nominal position without having any effect on the resection. The mini-prisms used had a tiltable head mount (which I fixed solid) and screwed onto a short length of 3x3x0.25 aluminium angle bolted to the structure walls (so it could be removed at the end of the work).
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To reliably measure movement of 0.001ft (0.3mm to me!) would realistically require precise (digi these days probably) level runs.
The refraction involved in total station monitoring is almost impossible to adjust out to this level of accuracy.
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Leica circular prisms mounted on spigots are also pretty good (near nodal) and can be rotated to point in any direction whilst keeping the same point.
Works if you need lots of stations/monitoring points and only have a few prisms.
Leica circular prisms mounted on spigots are also pretty good (near nodal) and can be rotated to point in any direction whilst keeping the same point.
Works if you need lots of stations/monitoring points and only have a few prisms.
To reliably measure movement of 0.001ft (0.3mm to me!) would realistically require precise (digi these days probably) level runs.
The refraction involved in total station monitoring is almost impossible to adjust out to this level of accuracy.
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That only gives you vertical movement - digital levels tend not to be very good at measuring horizontal displacement.
Regardless of what the customer asks for, you can only monitor to the best accuracy obtainable and explain why anything better is not possible within the budget.
Have you not tried turning it on it's side 😉
I would used fixed prisms for control as well so you do not have to set up tripods every visit. If properly executed resection from fixed points outside the area of subsidence is almost always the best approach when working in a dynamic environment.
A variety of targets for this kind of work are used underground. Ordering in the states can be a bit of a chore.
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Have used Goecke several times.?ÿ They are great to deal with, it is just hard to pay them from the US.?ÿ The only way is to wire payment, or perhaps send a cashiers check.?ÿ This means a trip to the bank and a $50+ fee to the man.?ÿ You have to love the US banking system.