2. Sight the rod at the instrument height, and record the zenith angle. You would typically have a piece of flagging tied around the rod at the height of instrument to facilitate this.?ÿ This cancels the instrument height / rod height from the height difference calculation. But if you can only read the rod to ?ñ0.05' at 500' your elevation error will be that much due to sighting alone. Add other sources of error.?ÿ?ÿ
You can also use the theodolite like a level, not set up over the elevation reference point.?ÿ Check the vertical collimation as described above.?ÿ If it is close, then you can set the zenith angle to 90 (or make a slight correction) and not measure the height of the instrument (another source of error).?ÿ Just take readings seen on the rod at the elevation bench mark and the unknown points.
An architect I know showed me how to topo a small area with a card table, scale, and tape.?ÿ Sort of like an alidade and plane table.?ÿ He would measure the distance to building corner or other point of interest then sight down the scale and tick off the distance.?ÿ It worked reasonably well.?ÿ Then you use a level and rod to get elevations.
I think the British have a program where they teach ordinary people to "survey" like that, the purpose is they have so many historic sites that it is not possible for professionals to get to them all but the more minor ones can be documented by volunteers.?ÿ Better than nothing.
I would say if you can't read the rod then you are going to far.?ÿ Set some "fly points" around the area to be topo'd, a small spike or wooden stake (the little ones are called "ginees").?ÿ Then locate those with the T1A then get topo from each one using the stadia method Mark/Norman outlined above.
You could mount the most basic and compact Leica Disto or similar laser on the telescope and get the slope distance directly to a simple bullseye target on a pole adjustable to match instrument height. You'd still have calcs but only slope dist x cos and sine of the vertical angle to obtain horizontal distance and difference in height. With the laser you can also shoot to building corners, roof points etc.
In the 1970s, I probably put in over a thousand hours shooting stadia topo with a T1 and a T1A. The procedure was much like what Norman Oklahoma describes, except that we didn't shoot a flag on the rod to cancel out the height of the instrument. We made that adjustment in the office.
We set additional points, as mentioned by Dave Karoly, to keep the maximum shot distance down to about 300 feet. It's possible to read a rod to 0.01 ft. at 300 feet, but that's about the maximum range. At those distances it was only necessary to read angles to the nearest minute, which saved time. We ran a level loop through all the setup points rather than carrying elevations by stadia. These were 3-person crews, with the third person being the notekeeper.
Yaroshio, your vertical tacheometry is likely to be more accurate than stadia if the rod is held plumb and steady, and bearing in mind what Bill93 said about the vertical angle collimation. There will probably be a foot or two of tolerance in the horizontal distances and a tenth or two in the elevations. You can find out on the base line.
