Need some input on this. I will be doing some precise work soon where I will need to do remote elevation and/or vertical angle to a point where accuracy to hundredths is important. I am using a Lietz SET-3 total station in very good calibration. It's the older heavy type of SET-3, not the cheap lightweight modern ones. I set up a test area in the yard to the amusement of the neighbors and passers by.
I climbed a utility pole and put a 16d nail hole in the pole and painted it to be visible. No trouble seeing the hole with the total station. I set a nail in the ground at the bottom of the pole close enough to get the prism pole against the pole and plumb. The utility pole looks vertical. I set out two tacked hubs roughly 50' from the pole and approx. 40å¡ apart coming out from the pole. Horizontal distances were measured from each hub to the nail at the base of the pole. I ran levels so I knew the difference in heights from each point. I measured to the center of the prism set up on the nail at the pole and measured the height of the instrument while set up at each hub.
Test 1: Entered the height of the center of prism into the total station, shot slope distance per instructions to prism, and selected remote elevation function. (The height of instrument not needed here). Turned cross hairs upward to bottom of nail hole and got 17.61' from first hub and 17.62' from second hub by remote elevation method.
Test 2: Measured the height from nail at base of pole to center of prism. Recorded the vertical angle at center of prism. Recorded the vertical angle to the bottom of the nail hole. Put everything into CAD considering the elevations of points, heights of instrument, and height of prism. Determined the height from nail at base of pole to bottom of hole in pole to be 17.62' from first hub and 17.62' from second hub by using vertical angle method.
Remote elevation and vertical angle measurements therefore agree.
I got the extension ladder, called Mrs. Penry outside and I climbed back up the pole. She held Zero end of tape on the top of the nail and I measured 17.66' to bottom of hole in pole. Checked several times and under full tension. I laid the tape out of flat concrete, put marks in at 0' and 25.00'. Then measured with total station to be 25.01' to check the tape.
So vertical distances with total station by remote elevation and vertical angle agree, but are 0.04' lower from what I measure direct with tape.
Instrument measures to 1-second on screen and stated accuracy of 3-seconds vertical.
The specifications in the manual under Remote Elevation says: " 1 s + every 0.5 s ". (Not sure what this translates to).
Would you apply a factor knowing you need to add 0.04' of height proportionally when at a distance of 50' from the point?
Do the hubs stick up 4 hundredths?
J. Penry, post: 389914, member: 321 wrote: I set a nail in the ground at the bottom of the pole close enough to get the prism pole against the pole and plumb. The utility pole looks vertical.
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Would you apply a factor knowing you need to add 0.04' of height proportionally when at a distance of 50' from the point?
The problem that I think your test discovered is that the 60d Nail in the ground that you measured the distance to is actually not under the points in the side of the pole that you assumed were the same horizontal distance from the instrument.
There are a number of ways of testing that, but setting up the total station on a perpendicular to the line between your original instrument station and the 60d Nail, sighting the nail and having Mrs. Penry read the tape that you ascend the ladder with in hand to hold its zero end against the pole would close the case.
I might have solved my problem. I ran a third point out to 100'. Figured the difference in elevation from it and the point at the pole by level and rod. Did remote elevation and came up with 17.65'. Figured it by vertical angle in CAD using heights of points and heights of prism and total station and got 17.65'. Now both methods within 0.01' of my direct measurement of 17.66'. It appears that the close distance has something to do with it. I can measure out 100' from the upcoming vertical point I need to get and hopefully it will match my test of 0.01'. More testing later to confirm that I needed the further distance out.
J. Penry, post: 389925, member: 321 wrote: I might have solved my problem. I ran a third point out to 100'.
As you get progressively more distant from the holes in the pole, geometry of the situation means that the errors introduced by their not being exactly the same horizontal distance as the 60d nail that was assumed to be plumbed under them will become less and less.
If you have access to a reflectorless instrument, the neatest solution would just be to measure the distance to the holes directly and reduce that to a horizontal distance to use in deriving the height differences from the Set3 zenith angles.
J. Penry, post: 389925, member: 321 wrote: I might have solved my problem. I ran a third point out to 100'. Figured the difference in elevation from it and the point at the pole by level and rod. Did remote elevation and came up with 17.65'. Figured it by vertical angle in CAD using heights of points and heights of prism and total station and got 17.65'. Now both methods within 0.01' of my direct measurement of 17.66'. It appears that the close distance has something to do with it. I can measure out 100' from the upcoming vertical point I need to get and hopefully it will match my test of 0.01'. More testing later to confirm that I needed the further distance out.
If plumbness is suspect, wouldn't you set up at 90 degrees from where you are, and see if both the top and bottom points are precisely above one another? You say "the pole looks vertical", but I'm not sure what that means, numerically.
rfc, post: 389947, member: 8882 wrote: If plumbness is suspect, wouldn't you set up at 90 degrees from where you are, and see if both the top and bottom points are precisely above one another? You say "the pole looks vertical", but I'm not sure what that means, numerically.
"The utility pole looks vertical.". Meaning the utility pole was not visibly leaning causing the direct measurement to be off.
I would not use a "direct measurement" for this. I would use an angle-angle intersection to determine the horizontal position of the high nail. Then I would use the horizontal distances based on the intersection from the two setup points along with the vertical angle to solve the vertical difference from the instrument, then add the height of instrument above the setup points, then add the elevation of the set up points to the vertical heights. You have two solutions.
If you are very far away, it's important to add curvature and refraction. You can get extremely good results with this procedure. The danger in your technique is that with a steep vertical angle, any error in the horizontal distance (assuming the prism shot is the same horizontal distance as the high nail) will affect the vertical difference quite a bit.
Does it really need to be to within a hundred? I know several times we get asked for a bid and the specs they need, seems everyone wants everything to the hundred. We say sure we can do it to a hundred, but it's going to take us longer and going to have perform the procedure a few times to make sure we close within the hundred, which will add time and cost to the job.
They say hmmm let me talk to the engineer, most of the time they could back and say within a foot or a couple of tenths is all we need.
Just to add to what Kent has said.
You say "I set a nail in the ground at the bottom of the pole close enough to get the prism pole against the pole and plumb." So f the utility pole is vertical that nail will not be below the upper hole, but offset by about a prism pole width away from it.
At 50' your vertical angle will be around 20 degrees, so your calculated height will be low by about 0.03' (17/50=0.36 x offset prism to utility pole) With hubs 20 degrees either side of the nail line then both would have the same error. It is always better to do this sort of activity with the sightlines at right angles.
At 100' the error would have reduced to alsmost nothing (0.18 x offset).
Curvature won't come into it at short distances. With a sightline up in the air refraction won't be a problem on a short distance and in this situation. (BUT... beware of refraction when the line is close to the ground OR there is a large temperature gradient. When surveying through brick kilns - turned off but still at 45C in the middle whilst the entrances were at 0C - I've found refraction of up to 0.4 metres over less than 100 metres)
I suggest a different test. Tie your level rod to the pole (or hang your 25' tape from a nail high up the pole) and shoot the face of it at different levels. Compare elevation results with the rod readings.
Are you meaning your Direct and Reverse reading to determine your Zenith angle?
Put some reflective tape on the utility pole and see if the EDM will measure to it.
At 50 feet there is potential prism centering error, also the chance that the EDM is reading the reflection from the face of the prism rather than the return from the center.
Though it's not what you're measuring, can you get a closure on the slope distances between your control points? Trig levels need precise distances.
Some prisms have an anti-reflective coating and some don't. I learned that the hard way after thinking I had properly determined the prism offset of an oddball prism -- shot it close up and where's the extra tenth coming from?
I would test the offset of that particular prism with the 3 tripod or 4 tripod test and use only that prism for all the points, even if it takes more walking.
Sounds like backing up to 100 feet helped, not as steep a zenith.
I know when we would use those old Lietz SET-? total stations in the day, we would turn off the automatic compensator to get good verticals. You would have to index the instrument prior to use when the compensator was turned off but we could get much tighter verticals than with it off??? my 2 cents, Jp
How high up is the point, can someone get to the point? It may just be something as simple as dropping a string line with a plump bob, then measure the string line to get the elevation.