I have a project that I need to use a vertical angle offset to measure the height of a cell phone tower. I have read the manual, and it looks very straight forward, however, the only question I have is after you take your first shot with a good rod height, it appears that the software knows that you have a zero rod height for the second shot. Is this A correct assumption? Thanks in advance

Just guessing, but I assume it uses the prism height only to determine the elevation of the base. I would think that it just uses the vertical (zenith as appropriate) angle to the top and the horizontal distance to the prism to calculate the difference in elev between the instrument and the top. Right triangle, 2 angles and included (horizontal) distance. Shouldn't be any "prism height" in this part of the measurement at all. It won't work if the prism isn't directly below the top. If you have any doubt, try it with a height you can measure directly beforehand, perhaps against a wall.

I used this routine a lot for cell tower heights on a Huskey running TDS. Unless they've made major changes, it takes a normal shot on the prism and then uses only the vertical angle from the second "shot" to calc the vertical offset. Rod height shouldn't come into play for the second shot. Depending on the tolerances for you project, be careful using the vertical offset routine. If you can't be certain that the offset aiming point is directly above the first shot, you can see significant errors. Is prismless available to you? That would remove the horizontal uncertainty and avoid the potential vertical errors.

For towers I found that I couldn't get reliable data using vertical angle offset due to the combination of horizontal errors and steep vertical angles. I resorted to setting at least two and preferably three control points around the tower as far away from the tower base as site conditions allowed. I recorded HI, BS, etc for each set-up in a fieldbook and recorded horizontal and vertical angles to each point of interest on the tower from each of the set-ups. This allowed me to intersect three lines to determine the horizontal position of each point and inverse the true horizontal distance from each set-up. I set the DC for manual data entry, set my occupy, backsight, HI, Hr=0; then I manually keyed the horizontal angle, horizontal distance, and vertical angle into the vertical angle offset routine and stored the point. Doing this from three control points gives you three answers for each point of interest. It's a lot of work & time, but my tolerances went from feet to tenths.

Thanks for the replies. I do have a two reflectorless instruments, and a 90 degree eyepiece. I plan in using redundant observations. I'll see how things check, and let you know. Thanks again.

For the vertical angle offset in TDS you must enter a Rod Height of zero, otherwise it will subtract the HR from the computed value.