This is a special type of Leica prism holder.
It has a very specific function, in that, it not only precisely locates the gauge line but it also provides the required top of rail elevation.
It also helps to have a section of running rail laying around the basement.
P.S. The cylindrical parts are, in fact, magnets.
So was the slot cut just so a spherical end mill could be introduced without cutting through at the sides? I would have thought that it'd be simpler just to mill all the way across.
Neat tool, in any case, if you do much rail location. Was it made by Port machinists? If it was done by a job shop, it'd probably cost several hundred dollars as a one-off.
> Neat tool, in any case, if you do much rail location.
Actually, if you do much rail location, I'd think you'd want a way to elevate the prism and target above the rail and track bed. An angled shoe that would replace a point on a prism pole would work just fine, I'd think.
Kent,.. Leica have a standard 1 metre extension pole that
you would then mount onto the nipple. The prism then attached on top top of the pole.
RADU
Kent,.. Leica have a standard 1 metre extension pole that
With the arrangement of the magnets, and the rounded nature of the rail steel, does the horizontal part contact the top of the rail sufficiently to ensure a perpendicular/plumb alignment?
Kent,.. Leica have a standard 1 metre extension pole that
> you would then mount onto the nipple. The prism then attached on top top of the pole.
Richard, I was thinking of a foot design that would screw onto a conventional prism pole so that a person could run the rod at standing height, as one would probably want for robotic operations.
The shorter the distance from the measured point to the prism the better for accurate work as there is less error in leveling. Railroads have gentle grades, with the instrument at 5-6 feet up I'd think you would have clear sight as far as you want to measure. With bad atmosphere there might be issues but just take the measurements at the right time of day.
It looks like a good tool to me, adding height would just screw it up. Get a bunch, have one guy setting them out while another is taking the shots.
I've done some industrial surveying in years past, we rarely used a prism pole. Always tried to be right on the point with a mini prism or mag mount. With reflectorless you might just paste a dot or something with no rod height whatsoever (direct scan).
> The shorter the distance from the measured point to the prism the better for accurate work as there is less error in leveling. Railroads have gentle grades, with the instrument at 5-6 feet up I'd think you would have clear sight as far as you want to measure. With bad atmosphere there might be issues but just take the measurements at the right time of day.
I'd suggest that some surveyor actually try sighting a prism barely a foot above the track bed on a railroad track on just about any day over ranges of hundreds of feet and compare it to the results obtainable to a prism/target at a standard prism pole height of nominally 5 ft. The refraction on a line grazing the track bed would be ferocious and the results unimpressive.
> It looks like a good tool to me, adding height would just screw it up.
Actually, raising the target height would unquestionably improve the accuracy of the vertical angle measurements and hence the heighting. If an assistant can't plumb a prism pole within half a hundredth, something is wrong that needs to be attended to. Making said assistant work at rail/ballast height would be a novel punishment, but ultimately not productive.
My guess is that Scott's intended use is in NYC subway tunnels where heat waves aren't a problem. The question about prism height is a fair one from an ergonomic and production perspective, but I expect there's a reason he had it built low.
> My guess is that Scott's intended use is in NYC subway tunnels where heat waves aren't a problem. The question about prism height is a fair one from an ergonomic and production perspective, but I expect there's a reason he had it built low.
I suspect the main reason was that he wasn't going to be the guy actually using it. Just a simple operation like pointing the prism toward the instrument is one heckuva lot easier at 5 ft. than at deck level, where for each target setup the instrument operator has to give directions on prism orientation to the guy or gal sitting on the track.
I don't think mapping a RR from hundreds of feet away through a land parcel is the intended use of this tool. They are trying to precisely locate rails either for design, asbuilt or pass inspection. They are not going to be hundreds of feet away shooting with a prism pole, at least I wouldn't do it that way. You are going to try and control every possible source of error you can by keeping it close and tight. 2-3 mm might be all the error allowed.
Why use magnets if you are going to have a human operator. They are going to clip these on the rails and leave them for awhile. Making them tall just increases the chance of disturbance. I worked a big industrial job for 18 months. It's a whole different world from land surveying.
> I don't think mapping a RR from hundreds of feet away through a land parcel is the intended use of this tool. They are trying to precisely locate rails either for design, asbuilt or pass inspection. They are not going to be hundreds of feet away shooting with a prism pole, at least I wouldn't do it that way. You are going to try and control every possible source of error you can by keeping it close and tight. 2-3 mm might be all the error allowed.
Well, I think Jim has the right answer: that is is for subway track where the setting out distances would probably be short. For general railroad work, though, usually the track bed is going to be above grade to either side of the track. So if you are set up off the tracks, the clearance of the line of sight to the track bed will be closer to a graze if the prism is right on the rail. Maintaining accuracies of 0.01 ft. over distances of 300 ft. doesn't strike me as heavy lifting if you can avoid grazing lines.
Like any tool, it's effectiveness varies along with the application and the skill of the user.
I can see where this would be really handy in some instances.
I can also see where it might be totally useless in others.
Your toolkit seems no ending at times
That probably beats my lath jig that I make to locate the CL each time we have a RR R/W to locate and the only monuments are the rails themselves.
Believe me, the rails are never so close to design hereabouts that anything is exactly or even closly as plans say as opposed to how well the are probably aligned in the city.
