I’ve done quite abit of reading in the Trimble access manual and can’t find a clear answer.
When inputting the height of instrument in a station setup, are you entering the slope height of the measure up and Access then converts it to a vertical height?
Or do I need to convert the slope height I measure manually into a vertical height?
It's been a while since I last used Access, but I recall that there were various settings where you specified what you where measuring up to- the slant distance to the bumper or the vertical distance to the ARP.
You enter the slope height measurement. The DC will then adjust the measurement to the correct number. One test to be sure each and every option is working correctly is to field test the procedure. Measure to each option at the base and rover as you locate two close together leveled points. With the R10 units I expect to match within .00'-.02' vertically. The lever arm is the best measure point we've found for vertical accuracy. Also check the DC file and you will see the adjustment Trimble makes.
You didn't state which version of Access you are using. But for several years, there's a pull down in the height field to enter "bottom notch" or "true height". Bottom notch is on S and SX series total stations at least. As a note, unless you are using the total station as a level (90 degree vertical angle) you can only physically measure a slope distance height. In my opinion, you'd need a special case not not use "bottom notch" distance measurement entry.
I use true vertical measurements a lot...there are two main cases.
1) In deformation surveys we set up a lot of times on "pedestals", which are concrete filled pipes with a metal plate on top and a 5/8" thread to attach the tribrach to. In this case we use tribrachs that are open in the middle, and we measure up from the top of the bolt (which is leveled to by differential leveling) to the tribrach plate (where the instrument or adapter sits). The bolt typically protrudes about 12 mm (1/2"). All of our total station instruments are the same height above the tribrach seat, 0.196 m. We also know the height of each GPS (with adapter) and prism type above the tribrach plate. Our adapters are 0.035 m.
2) We also do a lot of setups where there are no points on the ground, and therefore the tribrach plate is the datum point, and each instrument/prism/GPS gets a height above the plate.
As for the original question, you can measure either slope or TV (true vertical) and enter in to access, just making sure to select the correct measurement type from the drop down. If you enter slope, the data file contains both the raw measurement, the type of measurement, and the TV height. Note that all of the data in the job file is reduced to the mark (GPS vectors, slope distances, zenith distances, etc).
My two cents: if you're setting up on a tripod above a known point, bottom of notch is far and away best practice. Wrapping the tape around the instrument, or eyeballing the slant height correction is just asking for trouble. Let Access compute the slant height correction and true instrument height.
Pillars/forced centering and resections, true height is all good provided you know the offsets like John mentioned.
During post-processing it's easy to see the height correction in TBC.
Also: the traverse kits are designed so that the notch measurements are exactly the same as if an instrument was set up on the tripod. I'm not a huge fan of forced centering, but if you're leapfrogging on traverses it makes it far easier for measure-ups.
