Prism Backsite Help

Couple of things come immediately to mind. First, I don't use Leica prisms, but I'm pretty sure that their circular prisms are not 0 offset in actuality. You can read Leica literature that will explain the offset.

Secondly, I would question whether side shots taken to monuments were actually taken with the prism reported on your raw data file.

thanks for the reply, checking into leica looks like circle prism is 0.0mm

For the sideshots which we take a sideshot of our backsight as a check, i was always under the impression that everything in our .dat file from the instrument that follows the :
# Prism type: Leica Circ Prism 0.0
meant that its using circle prism and then when the instrument is changed to 360 it shows :
# Prism type: Leica 360° Prism 23.1

Still stumped and maybe there are multiple or hard to read punches on the monuments that they centered over. Its just ironic how its a .08' difference which is very common as a prism change error.

It all depends on what instrument you're using.

Typically Leica TS have a +34.4mm offset built into them, making the Leica 0 offset prisms actually have a -34.4mm offset.

What kind of total station are you using?

* just noticed the 1203 noted in the file.

MAKE SURE YOU'RE NOT ON REFLECTORLESS.

from long ago experience the difference between 0 offset and -30 offset is 0.08'

30mm = 0.098 feet

Note that Leica prisms default zero to equate to -34.4 mm. So a prism offset of 34.4 equals a zero “net” offset. Whenever a prism constant is changed, a note is written to the raw (RW5) file. When you select one of the prisms from the list (Circle, Mini, etc), the value that is shown in parenthesis is the actual value sent to the instrument. i.e. 0.0 for Circular, 17.5 for Mini. If you select "30mm" or "40mm", we send - 4.4 and 5.6 respectively (30-34.4 and 40- 34.4). The user may also type in any value they choose. In this case, we will send that value exactly, unmodified, to the instrument.

(360) (23.1): Prism offset of 23.1 (Leica model).
Circle (0.0): Standard Leica round prism.
Mini (17.5): Leica mini prism.
ReflTape (34.4): Equates to zero offset (wall, surface).
30mm: Other manufactures (Sokkia, Seco).
40mm: Other manufacturers.

They make it dummy proof so that in the office you can confirm which prism was used. I just can't see why we have 2 shots that say circle prism is set but two distances that are about .08' off. Field Crew double checked the points and said there were only one punch on the mons and also double checked prisms.

Hey mate,
If you're using Leica prisms with a Leica instrument the readings should be sweet. How long was the traverse? Could heat shimmer have been a contributing factor, or was there a scale factor setting from the previous job left in the setup?

Well traverse was about 2500' off two existing control points from only one end.
the opposite end has nothing to hold or tie to. Everything is ground distances that instrument has shot. All other setups seems to fit really well (less than 0.02') when occupied and so this one setup is the only one that has me curious.

I meant to say grid distances not ground, im not using any scale factor and its an open leg traverse.

> I meant to say grid distances not ground, im not using any scale factor and its an open leg traverse.

Got me bamboozled, although it's something I've seen in the past with my 1200, it's usually something I put down to heat shimmer on hot days. Maybe time for a baseline check and calibration?

> For many years have had the field crew (and myself) forget to change prism from the topo rod (360 prism) to the circle prism when checking backsight. the difference is .077' anyways usually not an issue but have come across one i cant explain. First we had ran control and then did our topo using that control and in the data files it confirms which prism they have selected. but on a recent trip out to use the control we are about .08' different on the two monuments horizontally. Ill paste in the differences below any ideas?

Okay, maybe I'm missing something but your data is not making a lot of sense to me. If I understand you correctly you are seeing a difference of 0.077 feet when you the prism you are physically using and what you tell the TPS1200 it is. The Leica circle prism has a relative offset of 0 mm while the 360 prisms have a relative offset of 23.1 mm which converts to 0.076 feet. The instrument simply takes the shot and corrects for the prism offset you entered. If you keyed it in wrong prism then the recorded distance will be off by the difference in offsets between the two prisms. Other than getting it correct in the field is to change the prism type in LGO or just add (or is it subtract, I always have to check) the difference in offset to your slope distance. This is a real pain when you take a couple of hundred topo shots with the wrong prism entered. Been there, done that.

The TPS1200 has a great built in routine that will check the back sight for your. Hit the 'User' key and choose 'Check Recorded pt/Backsight Pt'. It defaults to the backsight with prism and height settings from the set up. All you do is point at the backsight (or tell it to turn to it) and hit the distance key. It will report the deltas, record the shot and then return to your last topo point with your previous settings. I try to do it at least once per half hour.

These are short shots so I would doubt it is a problem with scale factor or atmospheric corrections unless they were way way way off.

As for the comments on heat simmer causing the problem, heat wave usually affect sighting and not EDM distances.

Good Luck

It looks like operator error to me.

In the original traverse, was the same instrument used? The raw data listed for the original traverse does not indicate what type of prism was used. Were only circle prisms for the original traverse?