When reading a barcode level rod (with a rod bubble) with a digital level (no data collector), is it best to:
(1) Record the lowest reading, assuming the rod cannot be held perfectly plumb.
(2) Record the most repeatable reading, assuming changes in ambient temperature at the rod are affecting the readings. We sometimes take readings across paved roads.
Both (1) and (2)?
Keeping (1) and (2) in mind, I cannot use our data collector to run levels because it only records the first reading even though multiple readings are selected in the level. If someone knows how to set MAGNET Field software for multiple rod readings, I would like to know how you do it.
I realize that our level rod is made out of fiberglass and that it has temperature corrections printed on the rod, but are temperature corrections necessary for third order level work? As stated above, our level reads a barcode rod. How does ambient temperature affect those readings? We're using a Topcon DL-502 level.
When reading by eye, when in doubt I go for the lowest value - your option 1
I don't know about you Topcon, but +/- 0.3mm is easily achievable with a DiNi
Refraction along or across your road is likely to a greater source of error than a slightly mis-plumbed rod, at least in hot weather
I'm not familiar with your specific Topcon level and level rod, but I assume it is similar to others. You can verify the level rod bubble for plumb by sighting the edges with a total station in two directions.
If you have a traditional level staff markings on the opposite side of the barcode, and your Topcon level has traditional three wire graduations in the scope, you can also check your measurements that way. Rock the rod and record the lowest value. Record all three wire readings for extra confidence.
I can't comment on the temperature correction question.
@ jimcox
When reading by eye, when in doubt I go for the lowest value – your option 1
I agree, but in my case the level is doing the reading for me.
I don’t know about you Topcon, but +/- 0.3mm is easily achievable with a DiNi
Did you mean +/- 3 mm? Do you use a data collector with your DiNi?
Refraction along or across your road is likely to a greater source of error than a slightly mis-plumbed rod, at least in hot weather
I agree.
@ kjypls
You can verify the level rod bubble for plumb by sighting the edges with a total station in two directions.
I occasionally check our level rod bubble against our prism pole rod bubble.
Rock the rod and record the lowest value.
Our leveling software has this option.
I can’t comment on the temperature correction question.
I reviewed The Standards and Specifications for Geodetic Control Networks (1984):
(1) Third order - wood or metal leveling rod construction
(2) Third order - it's not required to determine the temperature gradient
for the vertical range of the line of sight at each setup
(3) Third order - minimal observation method is center wire
If you are getting a lot of variation between successive readings it may mean that you are attempting sightings that are a bit long for the equipment. Shorten up some and maybe you won't have to make the choice.
It may also be that your turning points are unstable.
You should choose the most repeatable value. If you need a decider between an equal number of readings that vary by only the least count I'd lean towards the lower value of the two. But the difference may be fanciful. The rod is not likely to be out of plumb enough to be a significant source of error.
@ Norman_Oklahoma
If you are getting a lot of variation between successive readings it may mean that you are attempting sightings that are a bit long for the equipment. Shorten up some and maybe you won’t have to make the choice.
I believe the maximum sighting distance for our instrument is 300'. Our maximum sighting distance is 270', even though the standards mentioned here specify a maximum distance of 295'.
It may also be that your turning points are unstable.
We use a metal turning pin, although it isn't a true turning pin because it doesn't have a rounded top.
You should choose the most repeatable value.
Even if it's not the lowest value?
If you need a decider between an equal number of readings that vary by only the least count I’d lean towards the lower value of the two.
I wasn't aware that a digital level, or any level for that matter, has a least count. Please explain.
The rod is not likely to be out of plumb enough to be a significant source of error.
That is the opinion of one of our party chiefs. I reminded him that error increases with distance, and although that may be the case for shorter distances, it may be significant for longer ones.
I'm not trying to split hairs here. I want to go back to using our data collector to run levels instead of taking 3 to 5 readings by triggering our level manually and choosing the lowest value. To our current knowledge, using our data collector will only average however many measurements are set in our level. This average may or may not be the lowest value. I haven't played with any data to prove that. In addition, using our data collector will greatly ease the process of checking and adjusting a level run by allowing us to download a level run into an office program.
You are saying the Topcon DL-502 does not have a mode to return the median of a set number of readings each time you take a measurement? I would check your settings for the options. My DNA30 is set up to automatically take 3 readings and return the mean along with the spread and it is over 20 years old.
As for using the lowest reading, I'm not sure it is as applicable with a digital level as manual levels. Digital levels read a large area of the barcode and not a single point. If the rod is too far out level, the pattern will be distorted and a measurement will not be possible. You should test this out.
As Mr. OK said, shot distance could be a factor. Just because it can shot 300 feet, does not mean you should go that far. For anything that you need tight numbers on, I would stick to around 200 feet.
I wasn’t aware that a digital level, or any level for that matter, has a least count. Please explain.
If you reading to the thousandths of a foot, which is typical for digital levels, then 0.001' is your least count.
For the record, I consider a 270' sighting with a digital level to be very long. When I want maximum repeatability I'm keeping it well under 200 feet. Well under.
@ john-putnam
You are saying the Topcon DL-502 does not have a mode to return the median of a set number of readings each time you take a measurement? I would check your settings for the options. My DNA30 is set up to automatically take 3 readings and return the mean along with the spread and it is over 20 years old.
Actually, the Topcon DL-502 does have such a mode. I've been doing some research.
According to chapter 16.1, page 56 of the Topcon DL-500 series, the DL-502
works in the following manner.
Average: Displays the average value calculated from the
number of times fine measurements is repeated.
(Repeat default setting: 5 times, Repeat setting
range: 2 to 9 times.)
According to page 485 of the MAGNET Field Help (version 4.0) manual, a
level run using MAGNET Field works in the following manner.
Shots
Guides you through the sets needed to be taken for the measurement.
Click Meas to take a shot.
I'll have to investigate the shots menu.
As for using the lowest reading, I’m not sure it is as applicable with a digital level as manual levels.
I'm uncertain of the technicalities involved with how a digital level obtains the lowest reading. I was thinking about the way I used to run levels manually, and I never read (3-wire) the rod more than one time.
Digital levels read a large area of the barcode and not a single point. If the rod is too far out level, the pattern will be distorted and a measurement will not be possible. You should test this out.
What are your thoughts concerning the effect of heat waves on a measurement? We try to avoid measuring over pavement, but sometimes that's unavoidable.
As Mr. OK said, shot distance could be a factor. Just because it can shot 300 feet, does not mean you should go that far. For anything that you need tight numbers on, I would stick to around 200 feet.
Our office restricts us to distances below 270'.
@ Norman_Oklahoma
I wasn’t aware that a digital level, or any level for that matter, has a least count. Please explain.
If you reading to the thousandths of a foot, which is typical for digital levels, then 0.001′ is your least count.
For the record, I consider a 270′ sighting with a digital level to be very long. When I want maximum repeatability I’m keeping it well under 200 feet. Well under.
Thank you for the information and the advice.
The Standards and Specifications for Geodetic Control Networks (1984) have been appended (?) with
FGCS Specifications and Procedures to Incorporate Electronic Digital/Bar-Code Leveling Systems (2004).
