Every once in awhile when reading posts that are so interesting you know they’d be even better with a beer. I’m sanding Sheetrock today so now I have 2 reasons to twist the top on a cold one.
I’m sanding Sheetrock today so now I have 2 reasons to twist the top on a cold one.
Well stay a while. I might post something so stupid you have an excuse for a 3rd.
After pondering and playing around in AutoCAD, I realize that having an angle (the vertical angle of the instrument) and the slope distance to a known point of known elevation is enough for the survey software to fill in the blanks so to speak; If the instrument is in fact centered (properly) over a known point everything's good... kind of...
The total station has just enough known parameters to fill in the blanks. I would need to put a little time into figuring things out but at first glance my findings match what everyone is saying: Any error would be very minor and insignificant. Even combing say a 1/8" (0.01') distance error with a 1/8" out-of-plumb error won't really affect horizontal distances; I don't think?. I guess I was thinking that providing the instrument height would give the total station one additional parameter to use to average any such errors out (similar to the 3rd, 4th, 5th... and so on... CP's when doing a resection).
@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 only thing the "distance reduction calculation" depends on is the vertical angle. The horizontal distance is the component of the distance between the instrument and the prism that is perpendicular to gravitational down. Changing the heights will just move the point up and down.
Try entering different heights. The horizontal distance won't change.
The only reason you need to put in an instrument height is if you want to know or check the height of the setup point directly under the instrument, i.e. a survey station that will be used again in the future, a change point to transfer height, a new ground control point etc.
Typically, if you're resecting as your setup method (which is the norm in construction layout), there is no need to put in an instrument height. If you are setting up over a known mark, you will generally want to measure the height of the instrument (some newer instruments have a disto that can do this for you). All this does is to serve as a check for the height calculation, because if you're reading to a backsight with a known height, the total station will calculate the height from the slope distance and vertical angle. After your setup measurement are done, the instrument should show you two different heights, one is calculated using the instrument height you put in, the other is from the vertical distance between the known points. They should be very close, if they're not, you either put in the wrong height or one of your points has shifted up or down.
(A little trick that you shouldn't abuse is that if you're a lazy slob and you've forgotten your measuring tape is set to instrument height to zero, measure a distance between two know points and the difference between expected and measured height should be your instrument height, if you're not a party leader don't let your party leader catch you doing this or else!)
In terms of measuring angles, distances and heights, the height of the instrument doesn't factor in whatsoever. I usually keep a written record of instrument heights just in case anything goes wrong.
If you're doing a traverse or putting control in, always put in the correct instrument height. For precise control, you should double up with a level run or trig heightening for good measure.
The only reason you need to put in an instrument height is if you want to know or check the height of the setup point directly under the instrument, i.e. a survey station that will be used again in the future, a change point to transfer height, a new ground control point etc.
Typically, if you're resecting as your setup method (which is the norm in construction layout), there is no need to put in an instrument height. If you are setting up over a known mark, you will generally want to measure the height of the instrument (some newer instruments have a disto that can do this for you). All this does is to serve as a check for the height calculation, because if you're reading to a backsight with a known height, the total station will calculate the height from the slope distance and vertical angle. After your setup measurement are done, the instrument should show you two different heights, one is calculated using the instrument height you put in, the other is from the vertical distance between the known points. They should be very close, if they're not, you either put in the wrong height or one of your points has shifted up or down.
(A little trick that you shouldn't abuse is that if you're a lazy slob and you've forgotten your measuring tape is set to instrument height to zero, measure a distance between two know points and the difference between expected and measured height should be your instrument height, if you're not a party leader don't let your party leader catch you doing this or else!)
In terms of measuring angles, distances and heights, the height of the instrument doesn't factor in whatsoever. I usually keep a written record of instrument heights just in case anything goes wrong.
If you're doing a traverse or putting control in, always put in the correct instrument height. For precise control, you should double up with a level run or trig heightening for good measure.
The time invested is negligible. Having the data improves error trapping and adjustment results, and prevents the head of the LS in charge from exploding (if your into that sort of thing)...
Even in the most pure planimetric survey I want my measurements to reflect the existing ground and if I do something naughty like wing a corner in on a twenty foot rod, I want that part of the story told as well.
@thebionicman I think this was what I was really trying to wrap my head around. For example, setting up on a known point and sighting a retro target, the instrument is never going to be exactly on the target. Plus with distance errors...
I did some testing in AutoCAD and figured out that there's enough 'knowns' for the instrument to do the math and figure out where it is. It would take somewhat of a perfect storm, for example, having the angles sighted a bit off, and being out of plumb just enough so that if the total station still measures the right distance and angles, it's location can easily be 0.01' off in either direction along with being who knows how many minutes out with the angle. I know for a fact that this happens in real life because I've seen bad surveys where you can visually see that the surveyor's setup was rotated.
My assumption was that having some redundancy within the 'known' values would provide a means to which the instrument can adjust the error out. Not knowing the height to a known point means it solely relies on the distance and angles to the backsight.
