It is a very simple question that I or anybody should not bring it up at all. I have been surveying for my whole life ( I lie ^_^ ) and not once this question has come to my head. However, recently I had problem with an out of plumb prism pole that I had to sight at the bottom of the pole for the H-angle. Then I switched to a prism on a tripod and started sighting at the center of the prism where all those lines meet. I found that the center of the prism did not line up with the white triangle markers on the back of the prism if I turned it away from the gun. Also for the V-angle, the markers and the orange cross bars on the prism and the center of the prism were not lined up if the prism was tilted. So here comes the question, where should I sight?
- I told my IM to sight at the white triangle for H-angle and the orange bars for the V-angle.
- I use Nikon DTM 300 and Pentax PTS V3 with after market prism and pole.
- I asked a technician at Sesco and he suggested the same.
Any suggestion and thought on these?
Just stay consistent and I believe you should be all right.
Also, I have found that if the rod is leaning slightly, you can sight the base or point for horizontal, raise the scope, and still get a good distance even if the prism is slightly off center.
In fact, you would be surprised at how far off center you can be and still get a return distance.
The problem with sighting a pole that is leaning, then shooting a distance, is that the pole could be leaning towards or away from you.
All of the little triangles and the center of the prisim should line up, and most of ours do. I know what you are saying, some don't line up perfect. If I'm close enough, I shoot the center of the glass, if farther away, I line up the the pointy triangles. Either way I try and measure to the center of the glass for both the horizontal and vertical. I'm not 100% sure why I do it that way but it seems correct in my mind.
A problem with sighting a learning pole is imagine this symbol / is the pole the bottom point is on your point but once u sight up the pole your measuring a distance that's not over the point and further from the point. Also, prisms are cut so that no matter where you shoot inside the prism all those lines connect to the same point. perhaps you can sight the tip of one of the triangles around the metal prism border.
Do not sight the apparent cross in the prism. The prism, when not pointed directly at the inst, is at an offset, because light travels faster through glass, than through air.
Always sight the target.
Nate
I wonder more about the 360° prisms for robots. The robot will find one of the prisms but unless you're really careful, it is not going to be pointed at the gun and therefore the center of the prism will not be over the center of the rod.
Nate,
you are correct bout always sighting the target what I meant was you could use those triangles or im guessing the apparent cross to find the center of the prism by using the point of one of those triangles similar to starting from a tack or the tip of a rod and then sight up to the center of the prism.
There are a lot of factors involved:
- Prism offset
- Type of Prism
- Type of target
- Type of instrument
- Distance to target
- Parallax
- Environmental conditions
- Refraction
To mention just a few.
The key to achieving accurate results is consistency. look at the requirements for first order survey work and you will see they go way beyond the achieved results. procedure is far more important.
Dugger
That is why I carry a plumb bob.
With no plumb bob, set up the prism pole,turn to and record. Rotate prism pole 180° and record a second point. The average of these two will get you thru the day.
Or, you could simply adjust your equipment first and then continue your traverse.
If your problem is that your equipment is not in alignment from point, up the pole and thru the target and with the center of the prism, then your equipment may need to be realigned or replaced with better quality goods.
One important thing with some types of systems is they they must be pointed directly at the instrument for the alignment to be correct, especially when using "0" prism offset.
Where to sight?... and Why?> NATE
What did you say???
"....because light travels faster through glass, than through air."
Does light travel fastest through glass water air or a vacuum?
In: Science [Edit categories]
1. vacuum, fastest
2. air, next fastest
3. water, next slowest
4. glass, slowest
Where to sight?... and Why?> NATE
I think the confusion is with sound. It will speed up in denser material.
Where to sight?... and Why?> NATE
Nate,
The reason for the offset is that the light ray takes an erratic path to the different back of prism surfaces to be bounced back retro-directionally. That is why at 0 offset the center of the glass is moved forward of the center of the pole. It is to account for the extra distance that the light travels around the prism.
Stephen
Where to sight?... and Why?> Everyone
Thanks a lot for all the comments. I really learn somethings new today. 🙂
> Any suggestion and thought on these?
Some of the marketed targets have pasted on emblems to form the target and most are off centered. These always have to be checked for true alignment with a prism rod that is 100% sdjusted. Other issues could be the machining of the threads of a rod with those of a prism. The machining is not always true.
The best suggestion I can come up with is to always keep the prism rod adjusted and checked through out the survey and to get to know the minor defects of your equipment. I always pair one certain prism rod with one certain prism and they never get switched to other rods or prisms I have.
Just keep consistency going, it does work.
I like to use a rod, also. After all it's connected to the ground. You will see it pointing to the monument or stake or whatever you're looking at. Also, I would recommend two bubbles at 90 degrees on your rod.
Where to sight?... and Why?> NATE
I stand corrected....
OK,
light also travels VERY slowly through my brain!
🙂
N
You guys should read the Leica White Paper on prisms.
If you ignore the sales pitch, it is pretty informative.
And will definitely clear up some the questions in this thread.
http://accessories.leica-geosystems.com/en/Original-Prisms_85273.htm
Leica also has White Papers on tripods, tribrachs, rods and survey systems.
All very informative.
Where to sight?... and Why?> NATE
Well Nate,
Now I am almost certain light will travel through my skull at a much faster rate than through the atmosphere. My skull has been described as "a perfect vacuum" many times.