Prism Constant Basics

Redundancy, redundancy redundancy, (oh, and did I say redundancy)? I would set on each point and shoot to both the other points. Ie: set on on B and shoot A & C, then set up on A and shoot B & C, then set up on C and shoot A & B. take the mean of each common redundant distances. Also, so as to not introduce tribrach error, you might keep the same tripod/tribrachs over each point, and clip the instrument into each. That might not be possible if your spare tribrachs can't accomodate that, or if you aren't shooting to a prism clipped into a tribrach.

0.01 foot wouldn't be an issue, but that always depends on the precision of the work you are doing, and the type of instruments you are using. In theory, it should be within the error estimate of the edm. Don't forget to take temp and pressure if you need to dial in that ppm error.

> Aloha, I know this question was asked several times in past. I searched and read all that I found. However, there are couple things I want a bit more clarification.
>
> I hope you'll don't mind me asking this!
>
>
> Following the simple formula Prism constant = AC-AB-BC
> 1. I would set up my station on B then shoot point A and C
> 2. Then I would setup on point A and shoot point C
> Is this correct?
>
> I notice every time I breakdown and setup the instrument, I tend to introduce a little error i.e .0.01' or so. Will that be an issue?
>
> As always words can't never express the full extend of my gratitude to each and everyone of you! Mahalo!
>
>

I would think so inasmuch as 0.01' = 3.048 mm and you are trying to estimate that prism constant at 30 mm or whatever it works out to be.

Since you have access to this info, I'd recommend multiple observations and then either averaging AC, AB, and BC so that you can minimize this or go full on and model the standard errors of setting the instrument up and those distances via stats. Either way, I wouldn't treat a one trip to this and assume everything is error free.

Good question and really, Kent can help mucho on this one.

I would also check all prisms to make sure you are getting the same distance from each. Set on B and put each and every prism at A. They should obviously be the same but there could be an n odd-ball prism in the bunch.

Check your tribrach calibrations before you measure the prisms. See an [msg=155602]old thread[/msg] that was recently resurrected for alternative methods to do that.

0.01' I would not worry about that.

What is the tolerance of your total station and range pole? Is the range pole plumb? Most newer total stations have a setting where you give the temperature. You have a lot of equipment that has to be prefect to shoot flat, the total station, range pole, range pole bubble, the guy holding it, the guy setting up on the point.

To measure everything to 0.01' Tear down the instrument and call it a day

> Redundancy, redundancy redundancy, (oh, and did I say redundancy)? I would set on each point and shoot to both the other points. Ie: set on on B and shoot A & C, then set up on A and shoot B & C, then set up on C and shoot A & B. take the mean of each common redundant distances. Also, so as to not introduce tribrach error, you might keep the same tripod/tribrachs over each point, and clip the instrument into each. That might not be possible if your spare tribrachs can't accomodate that, or if you aren't shooting to a prism clipped into a tribrach.
>
> 0.01 foot wouldn't be an issue, but that always depends on the precision of the work you are doing, and the type of instruments you are using. In theory, it should be within the error estimate of the edm. Don't forget to take temp and pressure if you need to dial in that ppm error.

Aloha, Tom: I think you missed to list one more redundancy! 😀 I got the point! Thanks! 😉

I have two tribrachs and two tripods. I can use one for gun and the other for prism. But I will need to breakdown one setup for the third point. I could also setup the prism on the range pole with a bipod.

I take a lot care and always very meticulous when I setup my instrument. It bugs me when I get repeated errors--even if it is within a small range. I just bought a gadget from amazon that gives me the pressure reading which I religiously enter every time I setup in addition to temperature.

Thanks again!

>
> I would think so inasmuch as 0.01' = 3.048 mm and you are trying to estimate that prism constant at 30 mm or whatever it works out to be.
>
> Since you have access to this info, I'd recommend multiple observations and then either averaging AC, AB, and BC so that you can minimize this or go full on and model the standard errors of setting the instrument up and those distances via stats. Either way, I wouldn't treat a one trip to this and assume everything is error free.
>
> Good question and really, Kent can help mucho on this one.

Thanks Kris!

> Check your tribrach calibrations before you measure the prisms. See an [msg=155602]old thread[/msg] that was recently resurrected for alternative methods to do that.

Aloha, Bill:
I followed that thread very closely and learned a lot. I ordered the Seco 2002 but still waiting for it to be shipped to Hawaii. Thanks for pointing this out...I should calibrate the tribrachs first. Aloha

> 0.01' I would not worry about that.
>
> What is the tolerance of your total station and range pole? Is the range pole plumb? Most newer total stations have a setting where you give the temperature. You have a lot of equipment that has to be prefect to shoot flat, the total station, range pole, range pole bubble, the guy holding it, the guy setting up on the point.
>
> To measure everything to 0.01' Tear down the instrument and call it a day

Aloha, Scott:

I have Trimble 5603 (3" second gun). I don't know what is the tolerance on the pole but the prism on the pole says 2mm. It is Trimble power pole. I did calibrate the vial on the pole recently using plumb bob. It was discussed on this forum a while ago how to calibrate the pole. I always enter temperature and pressure.

I do see there are a lot of variables. May be I am being too finicky:-)

Mahalo!

i would

1. check both prisms are the same model
2. set up on point A use the vertical scroller to set out point B and C on the same line
3. change prism constant to 0.000
4. shoot AB AC
5. move the instrument to B using forced centering and leaving the tribrachs where they are
6. shoot BA BC
7. calc the prisms constant taking into account if the instrument has any inbuilt adjustments e.g. the 1200

This is the perfect time to check/adjust the tri-brac cross-hairs and bubble prior to this prism offset measurement.

I would think it would be better to keep the distances short so as to eliminate environmental corrections? I mean shooting 30' you could have the PPM correction set to 0 and not be affecting anything (regardless of the accuracy requirements).

I'll be honest, I make it out to the certified baseline once a year and am happy that I agree with myself as well as the published distances. I have not worried about having a unique offset for each prism that varies by a few millimeters.

Someone please correct me if I'm wrong.

Your prism constant for that prism is 2mm and it is not made for tight work.

Just a little note: You can have an instrument constant too. The EDM is not set at the axis of the gun.

And, another note: You can and should do a "Fast and dirty check" every now and then. Just set the Total sta up, and the prism 10 ft away, and check it with a box tape.

Nate

Can you beg/borrow/steal another tripod? Using three tripods and swapping the instrument/prism each time will eliminate any centering error.

Get together with another surveyor and calibrate using three tripods.

As someone else mentioned, the Trimble +2mm 360° prism is not terribly accurate, it varies as you go around it. Try to point one of the prisms directly at the gun.

We use mostly seco mini prisms, and I have found them to be pretty good. I also have the trimble 360°, which I only use for topo. I also have a Zeiss 360° and a seco 360°, as well as some other miscellaneous (larger) prisms. We number them and keep track of the offsets.

I see that there are several CBL's in HI:

Hawaii Calibration baselines

but it looks like they have not been remeasured since the mid 80's. Best way to use those is to put a tripod over each one, so you would need 4 tripods. We use a rotating optical plummet in those situations, and I feel we are over the point to within 1 mm (with a lot of care taken).

Are you on Oahu? I have done a few surveys there and on Maui for the feds and the military, biggest problem was finding good vertical control. I guess with the new vertical datum in the 2020's that will no longer be a problem.

> Can you beg/borrow/steal another tripod? Using three tripods and swapping the instrument/prism each time will eliminate any centering error.
>
> Get together with another surveyor and calibrate using three tripods.
>
> As someone else mentioned, the Trimble +2mm 360° prism is not terribly accurate, it varies as you go around it. Try to point one of the prisms directly at the gun.
>
> We use mostly seco mini prisms, and I have found them to be pretty good. I also have the trimble 360°, which I only use for topo. I also have a Zeiss 360° and a seco 360°, as well as some other miscellaneous (larger) prisms. We number them and keep track of the offsets.
>
> I see that there are several CBL's in HI:
>
> Hawaii Calibration baselines
>
> but it looks like they have not been remeasured since the mid 80's. Best way to use those is to put a tripod over each one, so you would need 4 tripods. We use a rotating optical plummet in those situations, and I feel we are over the point to within 1 mm (with a lot of care taken).
>
> Are you on Oahu? I have done a few surveys there and on Maui for the feds and the military, biggest problem was finding good vertical control. I guess with the new vertical datum in the 2020's that will no longer be a problem.

Aloha, John:
I probably borrow one...(or wait until a PLS working along the highway, then I've have third one permanently... 😛 (just kidding)

I do have the Trimble SLR which I think 0 offset.

I am on the island of Kauai. Last I heard the baseline on our island not well maintained. I have to make visit there sometime to check it out. I also wasn't sure if someone who is not a PLS can utilize the baseline...

When did you work on Maui?

Thanks for the input. My next step is to calibrate the two tribrach I have. Aloha

Of course I was kidding about the steal part...just a saying.

I was on Oahu and Maui working in 2003, and Oahu again in 2005. I really liked everything EXCEPT the traffic on Oahu. That was one of my last states on my quest for working in all 50...Oklahoma was #50. Thanks to military mapping projects to get me to all of those.

> Of course I was kidding about the steal part...just a saying.
>
> I was on Oahu and Maui working in 2003, and Oahu again in 2005. I really liked everything EXCEPT the traffic on Oahu. That was one of my last states on my quest for working in all 50...Oklahoma was #50. Thanks to military mapping projects to get me to all of those.

Aloha, John:
That's really awesome that you made your marks on all 50 states! Oahu's traffic is horrible. We love Kauai which is probably one of the most relaxed island and not too rural.

I know you were kidding...Have a great day and wonderful weekend!