Leica makes a great system for measuring the true vertical height of any TPS system. They system consist of a small arm that attaches to the tri-brach and a tape that is slope corrected (possible because the horizontal offset is constant). Since all of the TPS instruments and posts have the same vertical offset it can be used with everything.
The Trimble measuring stick is not calibrated for slope. Since you specify that your measurements are to the 'bottom of notch', the software/data collector is able to calculate the true vertical measurement based on your slope input.
The REALLY correct way to measure is to set up the tripod with the head level and over the point. Then measure down through the hole in the top of the tripod to the point and add the distance of the trunnion axis above the tripod head.
It depends
Bridges industrial bolts never measure HI
Terrain model measure HI over point all the the time +-10mm to be rough
It depends on the end use requirements for works bridges bolts or terrain shapes
Some days on terrain models i guess at the rod height on rock face type work
Other days for bridges industrail bolts can spend 1/2 hr taking out 1mm
to force robot onto Site Accepted Bench mark
Peter
I hit the "measure HI" button on the optical plumb.
But it reads out in meters, so I can't use it.
"But it reads out in meters, so I can't use it."
LOL...good one!
I just do boundary work so all mine are 5.00'.
As someone mentioned below, the Trimble S6 has a notch on the side. Once you enter that into the data collector, the TRUE HI is computed and stored. I feel it is accurate to 1 or 2 mm when done properly.
I always use forced centering. The Trimble S6 and a Zeiss S10 I occasionaly use have a trunnion axis height of 0.196 m (which is the same as the Wild T2 I used to use). This is the height above the base, not above the bottom of the tribrach (which is variable). So, Dave's idea about measuring through the tripod center to the base isn't really valid.
Anyway, I use 0.033 m trirach adapters, plus prisms that are 0.05 m above the adapter.
So, once I move ahead I subtract 0.196 m from the TRUE HI (looked up in the data collector), then add 0.083 m to get the prism HI.
I usually use the S6 to set the backsight anyway, so I always have access to an accurate HT.
Like Loyal said, maybe small corrections, but they add up.
The slope distance, same operator for the whole job,( close to the same HI ) should be very precise. Would you not carry that same difference on all of your shots and it would be self compensating to the initial back sight? Kind of like sawing a half a foot from the bottom of your level rod and running a level loop, the numbers come out fine.
jud
I measure the distance using a folding wooden ruler, cut a hundreth and then round down to the nearest hundredth. Basically I'm cutting 0.015'. I use the wooden rule because I believe it is more accurate than a tape, especially on a windy day.
