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EDM observation in the zenith direction
Posted by john-hamilton on August 7, 2017 at 2:22 pmI used a Wild NL last week to transfer positions down two shafts (40′ down to an air gallery tunnel). The NL has a compensator, and it works very well. The cross hair did not move more than 0.5 mm when rotated. We had previously done this with a Wild T2 with 90?ø eyepieces from the bottom up, this method from the top down seemed much easier and more accurate.
In 2008 we transferred elevations down the shafts by suspending a 30 m tape vertically, and putting an 18 lb weight at the bottom, then reading the tape with a level at the top and bottom. I am thinking another way to do this would be to use a 1 mm EDM at the bottom to shoot upward in the zenith direction.
Does anyone know of a device that would enable a prism to be sighted from below and then rotate the prism 90?ø to be able to shoot it in from the side (i.e. a trig tie)? in other words, the prism is facing down for the shot from below, then it is facing horizontal for a shot from the surface network.
john-hamilton replied 6 years, 8 months ago 11 Members · 29 Replies- 29 Replies
A 360 prism with the axis fixed horizontally rather than vertically might do the trick
With a normal prism you’ll probably need a bit of sticky tape to hold it in place for the “horizontal” shot, otherwise gravity will rotate it to face downwards again.
Berntsen Mini Prism – 25.4 mm – Swiveling 360 Degree
Lee: I think that Berntsen will work, we could put it on the ceiling above the shaft.
Thanks
John,
Many years ago I had a similar task of measuring vertical distances. I needed to measure a stope with several landings that cork-screwed its way up for approx. 150 ft. I used a Wild GAK for my bearing basis as I moved up the stope. As for the distances, I found a couple of Wild prism holders that were designed to work with the Wild/Leica DI3000 distomat. The prism holders had set screws that allowed the prism to slide up and down at fixed distances above the target HI. I could rotate the prism assembly through nearly 90?ø so that it could be pointed down. The slide feature of the prism holder had enough travel that the prism would clear the tripod and tribrach.
I don’t have any photos of the equipment, but if you have an interest I’ll take some photos later this week. The prisms and my other underground survey gear are in storage now.
Thanks, Gene.
What Gene is referring to is the Wild GPH-1, GPH-3, or similar- if mounted to a tribrach that is placed in the vertical plane (90?ø to the regular horizontal); the prism could rotate between horizontal and vertical while retaining it’s primary centroid position.
Some pictures of what I’m describing are:[GALLERY=media, 47]GPH-3 case by NDrummond posted Aug 7, 2017 at 8:48 PM[/GALLERY]
[GALLERY=media, 46]GPH-3 assembly by NDrummond posted Aug 7, 2017 at 8:48 PM[/GALLERY]
[GALLERY=media, 45]GPH-1 prism assembly by NDrummond posted Aug 7, 2017 at 8:48 PM[/GALLERY]Some links to what I’m describing: (I’m not the seller of any of these)
http://www.ebay.com/itm/201331000380
http://www.ebay.com/itm/112439858019
https://www.amazon.com/Leica-Triple-Reflector-Holder-Target/dp/B00U0860J2John Hamilton, post: 440453, member: 640 wrote: Does anyone know of a device that would enable a prism to be sighted from below and then rotate the prism 90?ø to be able to shoot it in from the side (i.e. a trig tie)? in other words, the prism is facing down for the shot from below, then it is facing horizontal for a shot from the surface network.
Why would not a GMP 101 mounted on a horizontal or diagonally aligned spigot do the trick? This mini prism or any of the full sized nodal leica prisms would do it. Turn it to face in any direction while mounted on the spigot and the apparent centre of the prism will always occupy the same position in space. There wouldn’t be many positions around a sphere from which you couldn’t see the prism. The spigot could be affixed to a sufficiently heavy base or epoxied in place, keeping in mind that I don’t know where you are working.
Some pics…
The dam has two towers separated by a spillway. The shafts are in each tower and go down to an air gallery that runs under the spillway. In the “old” days the alignment line was on top of the spillway, but safety issues made them remove that and put it in the air gallery.Picture of the dam, built in the 1930’s:
Here is a picture from one tower to the other. We have a GPS point on the roof of each tower, on the landing at the top of the steps.
Scanning the spillway with the old GX scanner in 2008:Here is the inside of the air gallery tunnel, we run levels and measure alignment on pins in the floor, 6 times last year and 6 times this year.
One of the shafts:
We scanned the shafts and the air gallery last year with the TX8:
Looking up the shaft with spherical scanning targets at the top:
Plexiglass sheet we used at the top when plumbing from below with T2:
chris mills, post: 440462, member: 6244 wrote: With a normal prism you’ll probably need a bit of sticky tape to hold it in place for the “horizontal” shot, otherwise gravity will rotate it to face downwards again.
Standard Leica round prisms are pretty much balanced so shouldn’t be drooping.
squowse, post: 440715, member: 7109 wrote: Standard Leica round prisms are pretty much balanced so shouldn’t be drooping.
the standard leica prisms are front heavy and specifically employ friction to keep them from drooping. they aren’t balanced, but obviously won’t droop if they are functioning properly.
the gpr111 has rubber o-rings on each side of the prism assembly screws to provide friction to hold whatever vertical orientation you set the prism to. the gpr113 has a brass colored spring wedged in the pivot between the bracket and prism assembly. if you you see a gpr113 with some flagging tape hanging out from between the bracket and prism assembly you know that holding spring is missing/spent.
Conrad, post: 440896, member: 6642 wrote: the standard leica prisms are front heavy and specifically employ friction to keep them from drooping. they aren’t balanced, but obviously won’t droop if they are functioning properly.
That only stops them drooping when held on the normal vertical axis. When the prism support axis is rotated from vertical to horizontal the “droop” which will occur is the same movement as a side swing in the normal position. The friction on the mount is sufficient (usually) to resist wind load on the prism, but may not resist the unbalanced weight of the prism. They weren’t designed to be used in this way so there was no reason for weight to be balanced in this direction. That’s why I originally suggested using a 360 prism, which is balanced.
John,
Here are some photos of my Wild Heerbrugg GZR3 with single prism holder. I also have a triple prism holder that looks very similar. The prism can be locked at any tilted position with the two knobs on the side. This is typical Wild design. The prism holder places the prism at several heights above the target height to account for the offset between different Wild theodolites and detachable distomats in the days before integrated total stations. There is a small set screw to hold it at one of the 6 set positions. The prism holder fits in the standard Wild/Leica carrier (fits the Wild carrier I sold you many, many years ago with a tribrach, target and light in a padded case). It fits the GPR1 prism.
There is a stop that keeps the prism from rotating to a position that is exactly 90?ø down. With a small amount of work it could be machined to stop at 90?ø. What I did was calculate the angle where it stopped and add that to the normal instrument constant of 34mm for Wild/Leica prisms. Both the single and triple prism holders come with a durable prism bag (not padded) made by Wild. Feel free to call if you have any questions or interest.
chris mills, post: 441130, member: 6244 wrote: That only stops them drooping when held on the normal vertical axis. When the prism support axis is rotated from vertical to horizontal the “droop” which will occur is the same movement as a side swing in the normal position. The friction on the mount is sufficient (usually) to resist wind load on the prism, but may not resist the unbalanced weight of the prism. They weren’t designed to be used in this way so there was no reason for weight to be balanced in this direction. That’s why I originally suggested using a 360 prism, which is balanced.
Hello Chris, please correct me if I’m misunderstanding you.
when the (full size nodal) GMP111 and GMP113 are mounted on a horizontal or vertical axis the front of the glass will seek to face downwards with gravity if the means of friction is missing. When mounted on a horizontal axis and pointing downwards, the downward pull of the glass is resisting the ‘side swing’ you mention as it’is COG is below the axis of rotation. It actually seeks to point more or less straight downwards. How can I confirm this in practice? Well, I tested it with a horizontal, shaft mounted GMP113 and GMP111 with the shaft mounted in roller-bearings. They spin to face downwards.
John mentioned shooting from below, and side on. A horizontally mounted GMP113 or GMP111 is (accidentally) weighted for maintaining a downwards orientation. If the friction device is working properly then once you tilt the prism to face horizontally it can spin along its mounting axis all it wants and will always be facing horizontally.
The reason I suggested a nodal prism is they have most forgiveness for misalignment with very little sacrifice in pointing or distance accuracy, something which not all of the 360 prisms have. Once mounted on a stable spigot the reflective centre of the nodal prism remains the same point in space regardless of the orientation.
I hope this clarifies my ideas.
Could you place a retro target above the shaft and shoot it with an inverted level rod. I am guessing you could use the NL to set the target to insure you are getting a truly vertical measurement
Here is the idea I have come up with based on some of the suggestions…
I have a mag mount with 5/8 thread that is used for putting a GPS antenna on a vehicle roof. I am going to put a seco mini prism on that, and mag mount it to the metal collar at the top of the shaft, which should put it (the axis through the 5/8 stub) pretty much horizontal. I can then use the T2 with right angle eyepiece to get directly underneat it. Then I can shoot from below with the S6 nearly vertical, then rotate the prism and shoot from above (slightly sloped from horizontal). Next survey is in September, I will try it then. The ZL I used last week is not ours, it was recently bought by the Corps for a different project so I have to give it back. But we are supposed to do another shaft plumbing at the same place during the survey after the one in September.
chris mills, post: 441130, member: 6244 wrote: That only stops them drooping when held on the normal vertical axis. When the prism support axis is rotated from vertical to horizontal the “droop” which will occur is the same movement as a side swing in the normal position. The friction on the mount is sufficient (usually) to resist wind load on the prism, but may not resist the unbalanced weight of the prism. They weren’t designed to be used in this way so there was no reason for weight to be balanced in this direction. That’s why I originally suggested using a 360 prism, which is balanced.
I’ve used them on horizontal spigots with no problem. In the prism there is a little piston with a cone that clamps tight against the groove in the spigot. Seems to keep it tight enough. If you were using a topcon/nikon etc; prism in the zero constant position I could see the prism being out of balance enough to move it, but not a Leica one which I still believe is “pretty much” balanced.
John Hamilton, post: 441136, member: 640 wrote: Here is the idea I have come up with based on some of the suggestions…
I have a mag mount with 5/8 thread that is used for putting a GPS antenna on a vehicle roof. I am going to put a seco mini prism on that, and mag mount it to the metal collar at the top of the shaft, which should put it (the axis through the 5/8 stub) pretty much horizontal. I can then use the T2 with right angle eyepiece to get directly underneat it. Then I can shoot from below with the S6 nearly vertical, then rotate the prism and shoot from above (slightly sloped from horizontal). Next survey is in September, I will try it then. The ZL I used last week is not ours, it was recently bought by the Corps for a different project so I have to give it back. But we are supposed to do another shaft plumbing at the same place during the survey after the one in September.
A “station elevation” setup on Trimble Access will allow you to set the instrument elevation from one known point. The autolock removes the need for the diagonal eyepiece if you are using a prism. Built in camera is even better for this.
Or you could just use “survey basic” and use the VD to calc your elevations by hand.
So you don’t need to be directly under the point/prism necessarily.
I figured it would be more rigorous if there was no “eccentric”component, just straight vertical shot.
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