This morning we found the previous days work of leveling footing forms to be incorrect. We should have checked the transit for accuracy before committing ourselves, but at least we found the problem before we continued. I’m left with a couple questions, and am interested in hearing your opinions as well.
First, the matter of the transit. How does one go about checking an instrument for accuracy? I have been told that shooting marks, moving the instrument and re-setting it, and then checking the first shots is a way of establishing consistency. If a level instrument reads an elevation difference X between points Y and Z at one location, and a different X at another location, what is going on if the tool was properly leveled?
Second, I think I learned today, correct me if I’m wrong, that a central location for a builders level or transit is the best. Minimizing the distance between you and the shots and swinging 360º is better than an outside-the-house location that increases shooting distance but minimizes swing.
Third, are water levels accurate tools? I made one today with a 5 gal bucket, 50 ft of 5/8 i.d. hose and a few feet of 1/2 i.d. clear plastic tubing. I was feeling pretty good about what I was seeing, but the crew thought I was out-to-lunch…
thanks
Nathan
Replies
The water level will always work with enough precision for any construction job. Use food dye to make the water easier to "read". Your coworkers should be ashamed.
A transit, or in your case a level, yields better results at a point central to your working points, including your reference point. Every time a level is moved and reset, the instrument height changes (difference in elevation between the reference point and crosshairs). The ahead and back readings will therefore be different each time, but the elevation difference should be the same with an accurate tool.
...that's not a mistake, it's rustic
Thanks Jeff. So, I noticed yesterday a problem on the transit, that I thought I fixed. Leveling the bubble over a set of wheels, I spun the barrel 180º and got a different reading. This happened over both sets of wheels. I was able to balance the bubble over 90º, but as soon as I did a 180º it wouldn't hold. I assumed the barrel of the transit wasn't parrallel with the base, so, I balanced the tool over 90º, then noticed the change in a 180º swing. I then took 1/2 of this difference out in tilting the barrel. I re-leveled over the wheels, and this time it held true through 360º. Should this have been set correctly? Can something else be amiss?
Going back to a water level vs transit argument. IF the transit is relatively good quality...and properly set up in the middle of the site...which one is most accurate?
An off-the point question here: When leveling footing forms, can a person level one side of the form all the way around with the instrument, and then come back and set the other side to the first with a level? Seems like a real cost-saver as 1/2 the work is then a one-man job instead of three. See any problems?
I don't trust the lasers to be as accurate as either one of the above. Am I wrong?
thanks
I only can tell you about mine. I don`t know the whole book on lasers. Mine is Spectra physics that I bought about 2 years ago. It`s not the rotating kind. It has an optical cone suspended above an LED and is self leveling within a tolerance. It splits the beam horizontally in all directions at once in a pulse. The receiver is the "inaccuarate" link in this chain, mostly due to the human factor. The LCD level mark on the receiver, at about 1/16", is about as wide as the error margin for the laser unit (3/32" @ 100') so I always try to mark on the up side of the mark. I checked the accuracy several times and it`s right on the nut. The main reason I bought this type over the rotating kind is because this one is field calibratable. Don`t have to send it to the shop. So far I love it, though I don`t get to use it as much as the average builder would.
I got the same one, how do you field calibrate it?
I haven`t done it yet because it hasn`t needed it but there`s a section in the instructions that tells how. I`ll get back to you on that.
I trust a water level, and use one to check the accuracy of the laser equipment used by my subs.
I showed my foundation crew how to use mine when I was building out in Indiana a few years ago. When we were finished placing all the TOW grade marks in the forms, I told them that the Egyptians used water levels to build the pyramids.
"So where'd they get the vinyl tubing," asked one of the clowns in the crew.
The Egyptians dug a grid of trenches and filled them with water to set the base line for grading where the pyramid would go. With the line marked on the sides of the trenches, they could cut between them with a straight edge, kinda like using a screed for concrete work. The two big ones are over 400 ft. on a side, and still nice and level. Water is still the gold standard for checking optical and laser levels. It's also ideal for jacking jobs on old houses, because you don't need line of sight.
-- J.S.
And I heard or read somewhere that as levels of stone progressed up, wooden troughs were built in a grid, up on the stone, to provide a baseline for setting and aligning pieces.
Ok, before we get too far into this, I mis spoke. I have a Spectra Precision, not Spectra Physics. Not sure if they`re related or not. It`s a model L130 and there`s a little trunnion screw for the "x"axis and the"y"axis each just below the cone window. It has holes in it and you`ll need a small pin to turn it. compare the 180 deg. readings of both axis` and split the difference with the adjustment until it comes into tolerance. Not sure how sensitive the adjustment is so move a little, check a little. You may have to do it more than once if your unit is way out. If you like I can send you the instructions.
my stabila rotating laser cost about $300. it is not self leveling, self leveling rotating lasers cost more, about $500 for one i saw in a catalogue. the advantage of a laser is that it is all a 1 man job. once you set it up you can work by yourself to set forms to grade without having a man turn the transit. mine has an receiver that can be mounted on a story pole. in my opinion water levels are slow, you must be patient. the proper way to check an instrument for level is to shoot two points in front of you, one close, one far. then shoot one point behind you, 180 degrees. you then move the instrument to the point you just shot behind you and shoot all three points again and see if you have a difference. you can also do this by shooting footings from opposite corners of a structure for a rough check, any inconsistency and it should go in for repairs. anytime you do layout with any method you always go back to the first grade you set and check it again after you have completed all your points. if it is off keep checking. with my laser after i have set all my points and checked my first point again even if it is on i move the laser to a different spot and check a few points to make sure i am exactly where i want to be, opposite corner frinstance, just as a double check to make sure it is indeed level. it is very fast, and so far has been dead accurate.
Thanks for the explanation...one bit of confusion though. When checking a level, shoot two points ahead and one behind. Then move to the rear point and shoot all three again, checking for a difference. How does one shoot the third point, if that is where he is? Do you mean close to the rear point? Why wouldn't the level be moved to a point 90º away from and in the middle of the line just shot?
the third point when shot in a straight line after moving instument is very close, say 10'. and to be exact with two pegging a transit my retired surveyor dad says to shoot on a gentle slope,about 10' away (downhill), 100 yds away, and twenty yds to rear. this can also be done on a smaller scale, but with less accuracy. the 180 degree turn at first point of taking shots checks transit for level after a turn, by cross checking in a straight line after from second point. i suppose it works as well with any angle, but this method which i am relating to you from memory i think has to do with certain distances to calibrate the instrument, i do not remember the details of exactly what to adjust on the transit for what amount of error in the instrument. this just tells you it is off or on, and distances and amount of error can be relayed to calibrating tech who would adjust.
most transits and levels are leveled in 4 positions..
wether it is a 3-point support or a 4-point support.. let's assume 3- point..
point to the first position, level the bubble.. turn 90 deg. to the 2d position, level the bubble
the 3d position is 180 deg from the 1st position, but you don NOT level the bubble, you take up 1/2 the error
the 4th position is 180 deg from the 2d position and again , you do NOT level the bubble , you take up 1/2 the error..
the instrument is now as level as it can be, assuming you were careful.. turn it and observe the bubble, if it moves more than 1 graduation on the vial, the instrument is NOT level ... start over
if it is within 1 graduation , your instrument is within the design tolerances it was made for..
if it is not, then it has to be adjusted or repaired..
also set up some bench marks in your garage or basement so you can check it from time to time..
i've used water levels.. too cumbersome, take too much time..
my builder's transit/level did me yeoman service for 25 years .. now we use a builder's laser..
every once in a while we still use the transit.. like for setting a reference line all the way around a siding job...Mike Smith Rhode Island : Design / Build / Repair / Restore
Mr Smith...you are da' man.
Thanks for the three point leveling system. The four-point method? I've always gone back and forth in 90º swings with the scope parrallel to the two wheels I'm adjusting. Is this correct? Also, what are your field tests to check your instrument, if needed?
Hey, a question I've been meaning to ask you. I lost the name that author and series you told me about...the one that you thought was better'n Patrick O'briens. Can you refresh my memory? I've stalled at book seven in the Aubrey series, as someone at the library seems to have taken a long-term liking to "The Ionian Mission".
Nathan
Edited 6/19/2003 11:43:41 PM ET by WEGEMER
the 4-point follows the same sequence as the 3-point EXCEPT there are 4 leveling wheels instead of 2... you use two opposite leveling wheels simultaneously... one in the left hand and one in the right..
note that the bubble ALWAYS will move in the direction of the LEFT thumb..
when you get done you have teh same results as with 3-points
I think that would be Alexander Kent.. and his hero was Richard Bolitho ... good readMike Smith Rhode Island : Design / Build / Repair / Restore
My opinion is that a water level is more reliable. It simply can't get knocked out of whack (used properly, level is always level!) and it can be used by one person. A buddy who is pool contractor in FL uses water levels exclusively and has had problems with dumpy levels. All that said, I use both.
Any jackass can kick down a barn, but it takes a carpenter to build one.
Reliable is good, but I'm after accuracy...I'll assume the lasers and transits and levels I use to be tuned up and as accurate as one could expect, and will check them in the future. If the water level is more accurate though...I'll find a way make it an efficient use of my time...after I learn it's place in the arsenal.
Edited 6/19/2003 2:19:05 AM ET by WEGEMER
There are some issues with water hose levels too. leave part of the hose in the sun for 5 mins and it is not accurate. If you mix anything in the water, mix it in a containor and then put it in the hose. it takes at least 15 mins with the hose and bucket in the same shady spot after heating part of the hose for the temps to equalize again.
If you understand the dynamics of a waterhose level the accuracy can't be beat. I avoid them as it is a PITA to insure accuracy when doing several marks outside in the sun or in any place where there is a temp difference. that said, when used indoors for a level line that crosses two or more rooms, there is nothing better.
SamT
I have had a White LT8-300P transit for over 12 years now. I took it in for recalibration last year. It was still dead nut on. The key is in how to handle, store and transport the insturement ( and the key word is insturement, not tool).
Instructions with my transit say to level the bubble over all four post first. You should be able to swing the insturement through a 360 degree arc, stopping at any point and have the bubble level stay put with in the lines at every stop point. Self leveling insturements are a little easier to accomplish this test with, but a manual adjusting if prperly set up is just as accurate.
My insturement is transported in the truck cab with me, never in the back with the tools. It is set up, used and put up, never left sitting out on the job site. It is stored in a closet in the house, not in the shop. Maybe I baby it too much, but I can trust it.
Since mine is a transit I can also swing it 108 degrees over to check the elevation stops as well as the arc swing reading. I have only used laser levels on drop ceilings and concrete flat work and they were not mine, so I did not set them up. They both were quite accurate.
Set up is the main issue. You can't rush through it, or think it is close enough. If it is off a liitle, it is off a lot somewhere, either in distance or the swing arc.
Dave
If you wind up doing some water level / transit / laser comparisons I'm sure everyone here would be very interested if you cared to post the results!Any jackass can kick down a barn, but it takes a carpenter to build one.
Don't know what kind of gun you have, but here's the way you "peg" a level. First a "level's" scope doesn't tilt, a "transit" does.
To adjust a transit, set it up, level the plate and the scope. Turn the plate in roughly 90 degree increments and observe the plate and scope bubbles. They should agree. If the plate bubbles are out, use the adjustment screws on the bubbles until it's level all around. Then do the same thing with the scope, until it agrees. Now the bubbles are level.
Find about 200' of flat ground. Set a stake at each end. Set up in the middle. Shoot one stake)foresight) and record the reading (to the .01), turn and do the same with the second (backsight). Calculate the difference in the two elevations. Move the gun about 50' from the backsight, set up, and read it, turn and read the foresight. The difference should be the same. If not, set back in the middle, read the foresight, turn and read the backsight. If the difference is not the same as before, use your crosshair adjustment to achieve the correct reading on the second stake. Break you setups as many times as necessary to achieve accuracy.
If you're gonna use a level or transit, I'd suggest you set a couple of 2' pieces of rebar in the ground at your house for bench marks. Set them exactly on line with one another, shoot them both and get the difference in elevation between them for future reference. Check your gun at least once a month.
Turn the plate in roughly 90 degree increments... Do you mean continue the 90º increments around the 360º, or go back and forth between two sets of adjusters? (four post leveling).
"If the plate bubbles are out, use the adjustment screws on the bubbles until it's level all around. Then do the same thing with the scope, until it agrees..."
This transit didn't have a base plate bubble, only one bubble on the scope itself. This worried me yesterday, as I couldn't be sure the scope and base were parrallel (could I?)
"Move the gun about 50' from the backsight, set up, and read it, turn and read the foresight. The difference should be the same..."
Did you mean that the second shot should be in the same line as the first set of numbers? What are your acceptable tolerances for a test like this?
"If not, set back in the middle, read the foresight, turn and read the backsight. If the difference is not the same as before, use your crosshair adjustment to achieve the correct reading on the second stake. Break you setups as many times as necessary to achieve accuracy."
Hmmm...looked for crosshair adjustments on this tool and couldn't find any. Could you explain why I'd want to make a second reading from the same spot read the same as the calc's of the first reading, by using an adjustment of the crosshairs? It would seem to me that one could assume any of these tools to be relatively precise. If a second reading from the same position indicated a different difference between the two marks, I'd assume I didn't level the instrument to the same "balance" each time...
Thanks very much for your comments.
Nathan
Edited 6/20/2003 12:13:18 AM ET by WEGEMER
Edited 6/20/2003 12:17:49 AM ET by WEGEMER
Sounds like you must have a "builders" level or transit, not a true surveying instrument like I'm familiar with.
As far as the plate and scope bubble, check the level in 90 degree increments thru 360 degrees, and adjust the bubble accordingly. If you can't get it to level, it means your plate may be warped from tightening the leveling screws too much.
When you set up a level and read a rod (foresight), turn and read a backsight, you get the difference in elevation between the two points. No matter how many times you move the gun and shoot the same two points, you should get the same difference. A short foresight and a long backsight was a proceedure we used to adjust the crosshairs on surveying instruments for the varying distances we had to shoot. There should be no difference in the readings.
The cross hair adjustment is necessary to take out errors in the instrument.
To check the ultimate accuracy of your gun, do a "closed loop."
Set a stake (bench mark), set up and read it (+), set another stake at roughly the same distance(temporary bench mark), turn and read and record that (-), break setup, read the TBM (+), turn back and read the BM (-).
Example:
BM 100.00
FS +5.00
HI 105.00 (Height of Instrument)
BS -4.50
TBM 100.50
Break and move, shoot TBM
TBM 100.50
FS +5.25
HI 105.75
BS -5.75 @ BM
BM 100.00
Hope this helps you out.
I dont use transits any more , but I have more experience using them than any thing else. They must be checked on every job and I checked mine with a water level. I used to build poultry houses that were four hundred feet long and saw quickly I couldnt live with the tolerence , but 100 feet is fine on most that are calibrated. One that is off is junk to me.
Water levels still have their place in the construction arsenal. The best use for them is shooting all the closets in a home , the cabinet lines in kitchen , railing areas , etc. This can all be done in one sit up with the level sitting in one room . They are wonderful at going around corners to get shots in one set up. With a pulley, rope , and a step ladder the height of shelves can be found and shot with the water level making it very fast for closets. Thats one of my speed tricks for trimming which I need because Im getting slower my self . I still use the water level for small things as its so easy and fast to set up such as porches , walks , decks and such. There again it they will shoot all the walks , front and back porches in one set up. They are painfully too slow for footings on a house with all the dirt piled up. Water levels are very accurate as long as you understand them . I believe if they are in the sun and stabilize their , they are accurate in the sun . I agree about shady-sunny areas. Ive shot 400 foot buildings with them in dead sun and they are dead on . Most problems people have with them are getting bubbles in the line . I use a clear line all the way and put my thumb over the line when moving to a new set up as thats when the bubbles accur from jar of moving and changing levels rapidly. I guess the obvious nono would be to lose water out the end by lowering to to low .
Lazers are for footings and ground elevation . They are here to stay, as they will pay out in short order .
Tim Mooney
Tim Mooney
A water level is more accurite then a builders level. When you make your own water level buy clear tubing so you can make sure all of the air bubbles are out of the line. If you have air bubbles in your line it will give a in-accurite reading. Useing garden hoses with the clear plastic ends won't let you see if all bubbles have come to the top, thats why clear tubing is best.
Thanks everyone, very much. As I was still unclear on how to field test a level or transit this morning, I made my own test on a David White transit, a Dewalt laser, a water level, a David White builders level, and another nice builders level that we ended up renting. Man, we counted five leveling tools on the job at one time...we rationalized it by telling ourselves the boss needs to make sure our tools are calibrated. In the back of my mind, though, I'm knowing there is a good chance this has all been operator error. Since I've spent some of the budget on leveling fears, I'll make sure I come out of it with some understanding of WTHID' ng.
We started with a White transit. I didn't note the model number. It was set up midway between two points about 150 ft apart, the longest level shot I had. We shot a forsight and a backsight and calc'd the difference between the two points. We then moved the instrument to within 20 feet of the backsight and in-line with the first shot. We re-shot the original two points and noted the difference. The transit showed 7/16 difference. The same test with a Dewalt laser showed 13/16 difference. My concern with the Dewalt was two-fold. First, the two leveling vials have a small bubble in a vial of glass with a sharp curve to it. An adjustment to the leveling post doesn't easily show a change in bubble position. Second, the reciever had about a 1/4" slop range for the level signal, and the point of level indication would vary to 1/8" at 75'. The problem I had with the David White transit was the single bubble vial at the top of the scope, and none on the base. I think the scope on this machine isn't parallel with the base, as a swing of 180º will show the scope out of level with a previous level showing. With effort, I was able to coax the scope into reading level each time after 180º swings by taking out 1/2 the error shown after the swing.
I didn't feel like I had enough information to make a decision about what to do, so I made another test. I found center of two corners on the longest axis of the house, 112 ft apart. Shot the two corner ftng forms. Then we moved to within 10 ft of each corner, and re-shot each corner again. The three differences were noted on a story pole. This was done for the DW transit, the DW laser, the water level, the DW builders level and the other rental builders level. I have the info on a story pole, and will bring it to the table here if the test I did is worth something.
Well, good evening everybody. I'm going to take this thread, copy it out and put it into the foundation notes folder that is going to be started soon. First foundation for me here...
Nathan
Great discussion.
The Egyptian method, water in a tube is a fine method, but I think I can refine the problems there. The problems with the water in the tube are:
1 It leaks out if someone drops their end.
2 Bubbles mess it up. Small bubbles from the sun or burps from the accidental drain tend to make the response sluggish. In the instructions it was suggested to use deionized water. I never tried it. Soda is a no no.
3 Final thought is with the Sounding type (photocel with an audio alarm) on the water tube) those are accurate like the terms fastening and breaking torque relate to bolt fastening. When you approach the level from the below grade side the alarm goes on at one point. When you have the alarm on and aproach from above grade the level is a different place. Compare the two and you are a Meniscus off. You can be within 1/8" as a ballpark measurement if you are consistent. Boredome and mosquito slapping could confuse measurements.
If I had the scratch a laser would be the cat's patutti. I used a tripod leveling rotating laser with a measuring stick that sounded off when you hit the mark. Fast accurate and really long range. Laser would have to be the best. 'Til then, I'll level like an Egyptian
Water levels
There are two types of water levels out there: One has a bucket or other reservoir at one end. The other is just a tube open at both ends.
The second form is severly temperature dependent. If you use a 100 foot garden hose as the tube, then a 5 degree C (9 F) will make a 1/10 foot change in the length of the water column. This change is split over the two ends, so can make a 1/20 out of level mark.
The hose itself will change volume with changing temperatures. Rubber is an odd material that expands with lowering temps.
This issue bit me today. I made a simple water hose level. Went back to check the first measurement. 1.5" different. Since these were to be the trim cuts on posts for a pole barn, I really want them closer to level than this.
A reservoir at one end acts to fill in the difference. It's level will drop in proportion to the area of the surface to the area of the pipe. E.g. Using a 5 gallon bucket (diameter 12 inches) and 1/2" garden hose, the error will be roughly 1/600 as much.
This does not mean that the reservoir system is error free. Consider what happens if one end of the hose is in the shade, while the remainder is in the sun. It is fairly easy to get 30-40 degree C water in a black hose. The reservoir water changes temp much more slowly than the free end, and due to relative motion water moves in and out of the reservoir keeping the end closer to reservoir temp.
Answer: Check your work.
Go back to the start and see if your 'level' is still level.
If you have a clear end at each end of the hose, then you can verify AT EACH MEASUREMENT that the level is still on the mark at the source and at the destination. This makes it difficult to do solo.