As per the title. Is there any case for setting the instrument height when doing a plane survey, when setting up over a known point?
At face value in a perfect world it doesn't matter (or does it?). But as far as my understanding of total stations goes: They have built in calculations for correcting any alignment flaws. Vertical angle errors would have a very small affect on horizontal distances. But maybe any effects are insignificant. Although I might be able to figure this out in CAD with a bit of time, it's way easier just to ask someone smarter. So here we are 😉
Since the day I started using a data collector c.1995 I've simply made it a habit to enter the instrument and rod measure ups in all circumstances. There is always the chance that the data will be reused and that information will be needed. And good work habits pay off in the long run. There can be no "Oh, I didn't realize that you wanted elevations on this one" moments.
Nevertheless, I do not think that there is any fundamental reason that they must be entered if you are running local (non-projection grid). There would be a very small scaling effect if running state plane (or other grid projection system) on areas of high vertical relief.
If you keep raw data such as the vertical angle and elevation difference you should be able to verify a horizontal distance if it's questionable even though the reverse calculation may only be good to the nearest foot. A typical situation would be taking a shot and getting a reflected value off something behind the rodperson, like the head light of a parked vehicle, or the rodperson's reflective vest.
It's a guarantee that when someone says "Oh, we don't need vertical for this project", and we decide to not take the extra ten seconds to measure, input and check heights, later on someone will say "Oh, we're doing some design work out there, and we need to get elevations for all that data you gathered".
Carrying vertical is so easy with modern instrumentation that my default position is "you better have a damned good reason for not doing it".
Note that this would be for construction layout and tracking a prism. Elevations would be determined (mostly) by other means including setting up the instrument and specifying instrument height.
I guess the scenario I'm thinking of is that if the total station is on somewhat of a sharp angle would the accuracy go to sh*t given that the total station has built in corrections for things like vertical angle error, collimation, etc... If the station doesn't know exactly where it is placed, the total station wouldn't be able to make the corrections for things like the vertical angle error (which would affect horizontal distances ever so slightly... maybe?). Even if the instrument height varied by say an inch (because obviously there must be some value entered, and the actual height is always going to be around 5 feet), the measurement to the backsight would probably still be spot on (the target being relatively close to instrument height, so a very long 'flat' triangle'). But spotting a horizontal distance with that setup that is 1) closer to the machine and 2) quite far down in an excavation let's say, you'd have closer to an equal-sided triangle.
To be honest, I think that if it was really important I would have been able to find more information on it. I just see lots of people doing this in construction layout. But anything you read or watch about setting up (aside from resections) always takes the time to set HI.
@keithscadservices To put it in my own words to see if I understand the original post:
Unless data is entered into the total station or data collector to the contrary, the software will assume the height of the instrument above it's mark and the height of the prism above the prism's mark are equal. If this is not the case, there will be a small error in the horizontal distance calculation.
Right?
If I've got it right, then setting the height of the prism would be just as important (or unimportant) as setting the height of the total station.
The built-in corrections for collimation, earth curvature, etc. have nothing to do with instrument set up height.
I haven't done the calcs but off the top of my head, if the instrument height is the same as the rod height, your errors are going to almost cancel out. However if you have a large difference in instrument height and rod height, then in the situation you described looking steeply into or out of a trench you could have some meaningful error.
Automatic instrument error compensations have nothing to do with the concept you are asking about.
Unless data is entered into the total station or data collector to the contrary, the software will assume the height of the instrument above it's mark and the height of the prism above the prism's mark are equal. If this is not the case, there will be a small error in the horizontal distance calculation.
I don't know the software, but it shouldn't affect horizontal distances, only the vertical calculation. The horizontal calculation should always be slope distance times sine of the angle from vertical, regardless of the instrument and prism heights.
@bill93 it the HI & HT are not at the same height you are introducing error into the reduction equation.
Unless data is entered into the total station or data collector to the contrary, the software will assume the height of the instrument above it's mark and the height of the prism above the prism's mark are equal. If this is not the case, there will be a small error in the horizontal distance calculation.
Not quite. The typical modern total station measures HA/VA/SD (horizontal angle / vertical angle / slope distance) and/or AZ/HD/VD (azimuth / horizontal distance / vertical distance) or derives one set from the other. Then pushes all of that to the collector, so if elevations are not being carried, the software will just use the horizontal distances and completely ignore the vertical component, allowing computations to take place on a plane that may or may not have an elevation assigned to it.
Unless otherwise requested I will not give out elevation information for boundary.
Any elevation data that is accumulated during the survey is not for clients or anyone else's use. Elevation data for some jobs will be restricted to control points and of course topo/site surveys will have a full suite of elevation data.
This is my policy and it's not to be violated anytime.
I never carried elevation data during boundary surveys until GPS showed up, then it's necessary if you want any kind of accuracy, however, even now I don't give out that data, unless the job is set-up for it prior to starting.
As to the OP's question, no it won't matter (much), XY is enough to do boundary work when using a total station. The assumption is that you're surveying on a surface plane at ground. Each time you shift your total station you're on a different surface. Each total station set-up will be on a surface above or below the starting point. So you have multiple plane surfaces, presumably so close together the error is swamped by the errors inherent when using the instruments. Every 40' shift in elevation creates 1PPM, no big deal, even a 400' shift will cause only 10PPM. In 10,000' that's 0.10'.
@bill93 it the HI & HT are not at the same height you are introducing error into the reduction equation.
Can you explain in a little more detail? Is the equation something other than I gave? Does the instrument/data collector correct for the difference in plumb between points because of the size of the earth and the deflection of the vertical due to the geoid?
@bill93 employing the formula that you mention assumes that the height of the instrument and target are the same when not entering both the target height and instrument height into the data collector to include a differential in the distance reduction calculation.
@bill93 employing the formula that you mention assumes that the height of the instrument and target are the same when not entering both the target height and instrument height into the data collector to include a differential in the distance reduction calculation.
The formula doesn't assume anything, because the respective heights don't matter when computing horizontal distance. I could input zero or one thousand feet for my HI/HT on a typical total station, and come up with the exact same horizontal distance.
