In my area at least, the Calibration Base Lines (CBL's) that were set up by NGS years ago have fallen into disrepair, and are not being maintained. I have visited three CBL's over the last several years. On all three, at least one of the monuments were either damaged or missing, leading to inconclusive/questionable results. I could visit other CBL's but the distance to them would make the time/cost involved excessive.
I am wondering about using a calibrated steel tape for the purpose of checking an EDM. Putting aside the issue that such a procedure will be inherently inferior to the use of a properly maintained CBL, it still may produce a useful check.
My purpose in posting is to enquire what are the best physical methods for setting up a "homemade" baseline.
I have a Keson 100' nylon clad standardized steel tape. Model NR10100. I would like to set up a baseline in the field having a length of at least 450 feet (I do not have a horizontal flat surface of that length available to me to support the tape). I understand that the length of the steel tape will change as a result of tension and temperature, and I have the report of calibration from Keson stating how tension and temperature affect the length. This report indicates that the stated length of the tape is based on its being "supported on a horizontal flat surface". As I don't believe the grassy ground would be a suitable/consistent flat surface, I'm inclined to think that the proper procedure would be to make an additional correction for (catenary) sag, from and to elevated points above the ground surface.
What would be a decent method for creating temporary elevated points above the ground? I was thinking that perhaps building a lightweight wooden rigid frame (triangular, with point on top, small enough to fit under a tripod), having spikes on its feet to minimize tendency for movement, might be more stable than driving a simple wooden stake into the ground.
Measurements would be slope distances, and should properly be corrected to horizontal distances for comparison purposes?
Does anyone know of a resource which explains how this sort of thing is best performed (or, used to be performed?).
Thanks for any advice and suggestions!
Al
I've got a bookshelf full of old surveying books that are exhaustive on the subject of temperature and sag correction for taping. Elementary Surveying by Breed and Hosmer is my favorite, probably because of the number of pictures. Just find one published prior to 1965.
As for taping "bucks", try something like these:
Thanks, paden.
I've got an old copy of "Surveying" by Davis and Foote, that should do.
I like that batter board system. Just ordered a set of 8.
Understanding what is going on is very useful. The most misunderstood one here is the offset correction. There is an EDM offset, that is in the EDM, of the Total Station, or the separate EDM, as the case may be. And, there is the PRISM offset.
The quick and dirty method to check this is to set up the inst, and shoot the prism 10 ft away, and set the prism offset this way. Fact: ALL prisms have a different offset. Maybe not much different, but they all differ. Use 2 tripods, and try out all your prisms. The next method is to set up 3 tripods, 3 tribrachs, and set the instrument on the middle one, and shoot both ways. Now, move the inst to one of the end tripods, and shoot the total dist. This requires that the ground be level, and that the 3 points be nearly in a straight line.
This method is also dependent on your prism offset.
I am speaking in practical terms here.
N
What sort of variation in prism constant have you observed, Nate?
Where in Mass Al I know of a couple of good baselines.
Major
> My purpose in posting is to enquire what are the best physical methods for setting up a "homemade" baseline.
Instead of trying to tape, why not measure it with 3 or 4 different instruments? If they all agree they are likely all right.
Hey Al, come on down to the Cape. Our baseline is doing fine.
GPS Your Own Base Line
Set up 3 points in line, with 2 at a comfortable taping distance. At a minimum use 2 receivers and double occupy all pairs. Three receivers is better. Only one needs to be L1/L2, post processing in L1 only is OK and may be preferable. Mean out multiple OPUS positions, split the files and get OPUS-RS also. Use the OPUS position for SPC, elevation and scale factor. The adjusted post processed vectors are the point to point slope distances. Occupy all 3 points with your EDM and shoot all 3 points. Multiple D&R angles and distances. Over the tapeable pair shoot every prism you own (2D & 2R, distance only) to verify they are all comparable and verify their offsets.
Paul in PA
Use your own EDM ...
(or 2 or 3) to create your own base line.
Set 4 points in a straight line - call them A, 1, 2 & 3
Use a short prism pole in a rigid holder and do not change the orientation or the prism during the measurement process.
Occupy A, measure to 1, 2 & 3
Subtract distance A-1 from distance A-2 to get distance 1-2
Subtract distance A-1 from distance A-3 to get distance 1-3
Subtract distance A-2 from distance A-3 to get distance 2-3
By following this procedure all instrument or setup errors are negated while determining the distances.
Then occupy 1 and measure to 2 and 3 to check your instrument.
While measuring with one instrument is sufficient, measuring with 2 or more will increase your confidence.
SW corner of MA.
Use your own EDM ...
Good stuff. Thanks.
Keeping in mind that whatever you do as an individual will not be credible as a true calibrated baseline, I would suggest using several instruments and creating an average to "calibrate" against.
I'd doubt that using a calibrated tape will be any more accurate than that over 450 feet.
In addition, you should send out a letter/e-mail to the state association and/or the state DOT, as they are most likely the agencies who would be able to begin working on a repair or replacement for the CBL.
Last time I used a Mass CBL was a few years ago, up at the Turners Falls Airport. The first several stations were there in good shape. The farthest station was there, but they'd built up the runway and you could no longer see it from any of the other stations.
> Keeping in mind that whatever you do as an individual will not be credible as a true calibrated baseline, I would suggest using several instruments and creating an average to "calibrate" against.
>
> I'd doubt that using a calibrated tape will be any more accurate than that over 450 feet.
>
>
> In addition, you should send out a letter/e-mail to the state association and/or the state DOT, as they are most likely the agencies who would be able to begin working on a repair or replacement for the CBL.
:good:
Get a good cbl in your general area. Create your own in your parking lot or somewhere near if you want, but have a tie=in check to an official baseline.
Setting something even will a steel tape would have to have the tape checked in to the bureau-of-standards official foot and unless you are already trained, you would need to use proper taping procedures and equipment (and appropriate calculations). It would probably be cheaper to train how to establish an official baseline coupled with the support of the state association and/or the dot. I would suspect you could recruit some manpower help from the same group you get the support from.
I totally support having your own "mini"-calibration base line, where you could check measurements on a regular basis at the comfort of your own area, but you need to measure it after checking your equipment against an official cbl. Then daily measurements will always be the same, or if they differ, you can check your measurements against the official cbl and then decide if you need to get your equipment adjusted or re-measure your local baseline.
An EDM has at least three potential error sources to consider.
1. Zero offset
2. Distance scale factor
3. Cyclic error
You can easily find the zero offset of an EDM-prism pair as discussed by others. Resolving what is in the EDM and what is in the prism is trickier, and maybe not necessary.
Distance scale factor must be compared to some standard. Temperature and pressure corrections are necessary. The scale factor could be off if the electronic time/frequency standard in the EDM is off, so it is like a clock running fast or slow as it measures the time light takes to travel to the reflector and back. It has the greatest effect on larger distances where it can dominate other error sources. You won't be able to measure it against a tape. A cal baseline is best, but GPS as Paul discussed might be adequate, especially if the points are at the maximum reliable distance of the EDM and prism.
Cyclic error is often overlooked, but can be as significant as the other errors on distances up to 100's of meters, and the amount of error varies sinusoidally with the measured distance. The EDM is measuring the phase of a modulation on the returned light beam, relative to what was sent. Multiple modulation wavelengths are used to resolve ambiguity, and the shortest is usually 5 or 10 meters. Leakage between the transmitter and receiver inside the EDM will alter the apparent received phase and produce an error that varies depending on the actual distance measured. Cyclic error may be detected on a well-planned cal baseline by getting some distances right and others wrong. Getting a detailed plot of it is a lot of work, involving several setups precisely on line between two reflectors, and then increasing the distance between reflectors a certain amount and repeating.
Yes, although I haven't been there, I've heard that about the Turner's Falls CBL. There is another baseline at the Pittsfield Airport that is in disrepair - in addition to charging $100 or so for the pleasure of using it (it's on private property), at least one of the monuments there has been disturbed/removed by mowing.
I've used a CBL in Vermont (in Springfield, I believe, at an airport). This is a seldom used airport, quite a distance from my work area. The last monument out doesn't check (used more than 1 instrument to arrive at that tentative conclusion), and so may have been disturbed.
There are several CBL's in NY, the nearest to me being the Rhinebeck CBL. One of the monuments there is pretty beat up (flail mower?), and you can't quite tell where is the mark on it.
I'd love to find a good CBL in my area.
>...... I do not have a horizontal flat surface of that length available to me to support the tape.... This report indicates that the stated length of the tape is based on its being "supported on a horizontal flat surface"....
An alternative is to measure the slope distance and correct for vertical angle.
>.... I don't believe the grassy ground would be a suitable/consistent flat surface ....
I think otherwise. As long as the slope is somewhat consistent, and the grass is mowed, you should be fine.
I set up just such a supplemental baseline on a highway frontage road near an office I worked out of in Portland. My procedure was to visit a true calibration baseline first then use the newly proven instrument to measure my baseline.
Oh, maybe you misunderstood me, Norman. I would think you're right that a grassy mowed area would be just fine for establishing a homemade base line for more frequently and conveniently checking an instrument. But, it sounds like you're establishing the distance between the marks on your baseline using the newly calibrated EDM. I was referring to establishing the distance by use of a steel tape.
