How about establishing some targets on the rock face across the road and resecting instrument positions as needed??ÿ
The problem I see with using RL on this project is most of the time I'd be looking at a pretty oblique angle on the targets since it's a corridor. Obviously I want to be perpendicular to targets when shooting RL and that just won't happen on a long, narrow project like this. The vast majority of the site is 20m wide, the one image of the big open area with the ravine is the exception.
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The Trimble's were using don't seem to be as accurate as Leica's in RL as well. I believe they have a wider beam?
The answer to that is 2 face readings when shooting reflectorless, and also updating the atmospheric corrections.
I think you misunderstood what I was getting at. RL isn't as accurate when your angle diverges from perpendicular regardless of how many times you shoot it.
My fist thought was similar to @norman-oklahoma; setting control into the rock face, but maybe fabricating a mount for 360 prisms setting them a few hundred yards apart.?ÿ Then you could resect and check in from any angle.?ÿ I doubt I'd trust reflectorless.
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Just curious, are you setting control on SPCS grid?
Yea something along those lines would work well.
No, this is in BC, Canada. Were in a ground based system scaled from UTM.
Assuming you don't want to access the targets very often then using 360 prisms is going to be very expensive. A much cheaper option (and an insurance against some of the targets getting damaged/obscured/destroyed) would be to make up a load of targets from channel strip and fix them every 50-100 metres. That way you will nearly always be able to set opposite one, hence shoot square - the next one will always be nearly at 90 in either dierection, so again shooting square.
Use 50mm x 50mm channel, cut 80mm long and 25mm. square targets. Stick on targets centrally to all three faces at top of channel. ID number on all three faces at bottom. That way each target will be 25mm. square from centre of channel, so you can use a 25mm. prism offset for all reference shots (remember to change it back when working on site shots!). When you initially survey them the mean of all three observations gives the "internal centre point" of the channel.
Drill rock face for an anchor and bolt on target through suitable hole just below the centre face target. Quick and easy to make and install - big enough for the construction guys to see and avoid. When construction moves on and an area is finished you can even unbolt them and reuse.
The limitation is that targets need to be fairly close to your working height in order to give an exact height reference, but if you are happy to be within a few mm. then they could be set a lot higher.
Just going out on a limb, but I'm guessing that the price of precise, permanent control is nothing on a 5 km project like this.
Assuming you don't want to access the targets very often then using 360 prisms is going to be very expensive. A much cheaper option (and an insurance against some of the targets getting damaged/obscured/destroyed) would be to make up a load of targets from channel strip and fix them every 50-100 metres. That way you will nearly always be able to set opposite one, hence shoot square - the next one will always be nearly at 90 in either dierection, so again shooting square.
Use 50mm x 50mm channel, cut 80mm long and 25mm. square targets. Stick on targets centrally to all three faces at top of channel. ID number on all three faces at bottom. That way each target will be 25mm. square from centre of channel, so you can use a 25mm. prism offset for all reference shots (remember to change it back when working on site shots!). When you initially survey them the mean of all three observations gives the "internal centre point" of the channel.
Drill rock face for an anchor and bolt on target through suitable hole just below the centre face target. Quick and easy to make and install - big enough for the construction guys to see and avoid. When construction moves on and an area is finished you can even unbolt them and reuse.
The limitation is that targets need to be fairly close to your working height in order to give an exact height reference, but if you are happy to be within a few mm. then they could be set a lot higher.
I'm having a tough time picturing this
Just going out on a limb, but I'm guessing that the price of precise, permanent control is nothing on a 5 km project like this.
You'd think.
It's a 600 million dollar project over 4 years and there wasn't permanent pillars set throughout the site. I tried bringing it up and getting some set to no avail. Although there really anywhere on site that won't get worked over by the end but If I were on it from the start the least I would have done was set permanent monuments on either end.
Just curious.?ÿ How much elevation change over the route?
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Just curious.?ÿ How much elevation change over the route?
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Around 100m
@bc-surveyor I resect reflectorless from reflective sheet targets multiple times every day and none of my shots are perfectly perpendicular , however I get very good results using this method. I didn't invent it myself. It was sugested to me by a Leica tech that rebuilds these instruments and his explanations made sense to me. As long as you set your target on something flat, you can get great results even at 45 degrees?ÿ angle of incidence. Reading through the post, I think Nate's method makes a lot of sense. A very small reflective target won't need a 2 face reading , or a flat surface to be set on. The instrument ( at least Leica) returns the distance from the most reflective surface the laser hits. On ocasions I have to shoot through a chain link fence,?ÿ or a tree branch that cut my direct line of sight to the target and I still get the same results as when I have no obstructions.?ÿ
Being pedantic : you resect reflectorless from reflective sheet ...
Agree with your 45 angle and general comments, but the problem here is that of trying to hit targets from a hundred metres or more when they are facing across a narrow working strip, hence they are at around 80 degrees or more oblique to line of sight. Even if you get a signal back any irregularity in the target surface will mean you get the distance averaged from what the instrument "sees" rather than from the centre point.
All this argument about resecting to the rock face!?ÿ Put the resection control on the top of the jersey barrier then.?ÿ ?ÿ
Why cant you put the reflective targets on say a 1" dia pvc or other round pipe? Then you will always be looking at it perpendicularly no matter where you are.
https://tdr360.com/en/produit/mini-bullseye-360-degrees-surveying-target/
Canadian and $45 each
@chris-mills I meant in reflectorless mode to reflective sheet targets. My instrument has the reflective sheet shooting mode as well, but I?ÿ find the range to be relatively short.?ÿ I usually have these targets way over 100m away and my angle of incidence normally doesn't go over 30 degrees from?ÿ normal ( line to the sticker surface). I just think in this case the solution proposed by cf.67 and Lurker would be more suitable. I guess you'd have to install that reflective cylinder pretty close to plumb to get accurate results from any angle.
@ramses?ÿ
Agreed. The other problem with a cylinder is that if there is a partial obstruction to the signal you measure well down one side without realising and the offset error as a result is much greater than than from a flat target. I've had that from a 360 prism as well, although I wouldn't use those for a resection.
@chris-mills I checked SurvCe settings yesterday and I found that you can actually create a custom prism ( reflectorless type) and input a prism constant different than 0 (+34.4mm in Leica's case). So, for a 40mm diameter cylinder you'll have a +20mm (+54.4 for Leica) constant. I'm just curious what distance the instrument actually measures if the beam hits the entire visible side of the cylinder. Will it average the the distances it gets from the sides and the front oft the cyl., or it returns the shortest distance.?ÿ I believe some testing should be done using a glass prism as a base then replace it with the cyl. and compare the results.?ÿ The cyl. prism constant might be different than the radius of the tube.
Correct. For any special targets we make using reflector tape we always run them over several distances against our calibration set prism. If possible we do this before the target construction is complete, so we can shim out the target face to a round number. In any case each is marked with its prism constant.
For instance, the various targets we have made to fit exactly into bolt holes of various sizes all have a +5mm. constant for those which are observed down the line of the hole (ie target is at right angles to the shank and a 20mm. constant for those which are set to read at right angles to the flange (ie. target face is parallel to the shank)
?ÿBasic targets have a standard shank of 17mm. and then a selection of collars to fit larger size holes (biggest in this view is 50mm).
Same sort of rules should apply to all special targets - anything slightly obscuring part of the target face shouldn't make a significant change to the distance (hence why flat targets are better than curved ones) and any change error should be linear, so easy to evaluate mentally at the time.
I came across this study measuring the accuracy of Trimble's RL measurement.
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https://core.ac.uk/download/pdf/11040769.pdf
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?ÿAs you can see, as soon as angle of incidence increases past 30 degrees you start to introduce significant error.
Also, the size of target really plays a roll in accuracy too. At 100m there is a beam divergence of 80mm hz an 40mm vert (S6). The smaller reflector less targets would introduce error pretty quickly, as is shown in this study.?ÿ
The study is looking at reflectorless measurement - this isn't the same as measuring to a target. Note also that reflective material is NOT the same as prismatic material. With a perfectly reflective surface you would only get a return signal if the line of sight was absolutely perpendiclar. The reflective tape is commmonly a whole series of very mini prisms in a tape form, hence acts more like a conventional prism, albeit with a more limited range.
Warning - there comes a range when the prism ability starts to get swamped by the general (non-reflective) surround and the reading then becomes reflectorless.
And a historical footnote on prism behaviour. The VERY, VERY early Kern prisms had to be used the right way up: rotate them through 90 and you got a different distance!
