The last seminar I went to on GPS accuracy was given by a fine Geodesist and Engineering Professor from the University of Wyoming. When he started making the case for centimetric accuracy for GPS elevations he seemed to me and my friends to be rushing through PowerPoint slides and giving us brief glimpses of Excel spreadsheets while waving his hands impressively. We weren't convinced, and that's why we still run differential levels.

We just completed 9 years of verification surveys at the McCarran Airport at Las Vegas, NV. The construction contracts exceeded two billion dollars. I agree with the author that old school methods must marry in with all the latest equipment. The point I want to bring forth is the calibration and adjustment of all equipment including the gun, tribracks, prisms, prism poles and yes tripod legs. Each set of layout equipment must be checked to a known off-sight base line and each of the other equipment sets. This takes patience and careful consideration in ideal weather conditions preferable at the same time in the morning. All work must start from a known and proved control point. The back sight must be from a base line point and tied to one point set in the yesterday work. The back sight check and the check to a yesterday point must check with-in 0.012'. If any check shot exceeds 0.012' a complete reset-up of instrument and back-sight and a new check procedure must be performed. If the second check procedure exceeds the 0.012' tolerance you must stop and resolve the problem. The control monuments must be of suitable size and length and encased in concrete to insure absolutely no movement. All of the above described mythology is old school and applies to all equipment.

I do not understand the terminology as described. Oh and by the way, I have done precise work on Hydro Electric Dams, as well as other projects purporting to have the necessity of precise repeatable measurements. Some of his comments are done as a matter of course for any survey, as in doubling an angle...Please explain the three congruent situations (instrument, tape, string line, sightline?????

I agree fully with the utilzation of old technology- in fact when I get down the the intimate setting out on a civils project i.e. concrete/bolt centres I tend to put away the modern gear and pull out the trusty measuring tape. I too am puzzled by the 'amigos'..Instrument and chain/tape I follow, but how do we compare a sightline? is that an angle or distance?

What about checking the diaganol? If you run 90°baselines you better pull or shoot diaganols. If you layout via radial stakeout - you better pull back into baselines at 90's.

I agree with Bruno. The diagonal will tell the tale if the layout fits within itself. Then pull perpendicular back to the baseline. If one uses direct measurements on a baseline, 90° angles turned from a base line, double centering, average direct and reverse sights and pulling diagonals is the way I was taught to do construction layout. Redundancy in measurements and properly adjusted equipment must be used as stated by Thompson. I am also confused about the " three amigos". Did I miss something?

mildeu, Ian , Joe, rwatson all of you have touched on the methods of the "Three Amigos". It is a triple check on positioning for embeds, such as anchor bolts. Using anchor bolts as an example: 1) The String Line - anchor bolt patterns run along their respective double centerlines (perpendicular to each other). Each extended centerline normally center's several bolt patterns. The centerlines are used as reference lines by the operations group (carpenters) for their formwork. Setting the end points on each line allows the opportunity to set a string line. In many cases for tighter tolerances of 2mm or 3mm, a "C" piano wire (gunite wire 300,000 PSI breaking strength) is set. The bolts are originally offset, per plan from the line. A sighted line is simply setting up a transit on one end point of the line and sighting the other end of the line to assure that the bolts are in their proper patterned location. 2) The Instrument - With the technology of total stations, robotic stations, etc.., layouts are for the most part done radially. This is fine so long as your control has been set and defined correctly, moreover; this method does improve time management and the collected information can be downloaded to check against the drawings in CAD (a fourth check). the bolts set by the instrument does give a position relationship in regards to the project as a whole (where you are in the world). A good check to assure you are in the right location. 3) The Chain - Tape (measure) - this is what all of you are describing. hard measurements of the bolts in relationship to each other - bolt to bolt, diagonals, cross references, centerline bolt pattern to centerline bolt pattern. The method is designed to reduce the three basic errors - human, instrument, and natural. When verifying these anchor bolts, they should be approved of their location against the three elements "Three Amigos". When one of the elements do not match the other two, then an error exist. The centerlines may be found to be wrong, the instrument may not be properly setup over the control, or the control is off, the instrument is out of calibration, movement of the forms may have happened, expansion and movement if you are verifying embeds on upper steel erected levels, so on and so on. So all of you have touched on these items and yes repeatability is a staple. I hope this helps clear it up for you all, and thank you for the comments.

mildeu, Ian , Joe, rwatson all of you have touched on the methods of the "Three Amigos". It is a triple check on positioning for embeds, such as anchor bolts. Using anchor bolts as an example: 1) The String Line - anchor bolt patterns run along their respective double centerlines (perpendicular to each other). Each extended centerline normally center's several bolt patterns. The centerlines are used as reference lines by the operations group (carpenters) for their formwork. Setting the end points on each line allows the opportunity to set a string line. In many cases for tighter tolerances of 2mm or 3mm, a "C" piano wire (gunite wire 300,000 PSI breaking strength) is set. The bolts are originally offset, per plan from the line. A sighted line is simply setting up a transit on one end point of the line and sighting the other end of the line to assure that the bolts are in their proper patterned location. 2) The Instrument - With the technology of total stations, robotic stations, etc.., layouts are for the most part done radially. This is fine so long as your control has been set and defined correctly, moreover; this method does improve time management and the collected information can be downloaded to check against the drawings in CAD (a fourth check). the bolts set by the instrument does give a position relationship in regards to the project as a whole (where you are in the world). A good check to assure you are in the right location. 3) The Chain - Tape (measure) - this is what all of you are describing. hard measurements of the bolts in relationship to each other - bolt to bolt, diagonals, cross references, centerline bolt pattern to centerline bolt pattern. The method is designed to reduce the three basic errors - human, instrument, and natural. When verifying these anchor bolts, they should be approved of their location against the three elements "Three Amigos". When one of the elements do not match the other two, then an error exist. The centerlines may be found to be wrong, the instrument may not be properly setup over the control, or the control is off, the instrument is out of calibration, movement of the forms may have happened, expansion and movement if you are verifying embeds on upper steel erected levels, so on and so on. So all of you have touched on these items and yes repeatability is a staple. I hope this helps clear it up for you all, and thank you for the comments.

mildeu, Ian , Joe, rwatson all of you have touched on the methods of the "Three Amigos". It is a triple check on positioning for embeds, such as anchor bolts. Using anchor bolts as an example: 1) The String Line - anchor bolt patterns run along their respective double centerlines (perpendicular to each other). Each extended centerline normally center's several bolt patterns. The centerlines are used as reference lines by the operations group (carpenters) for their formwork. Setting the end points on each line allows the opportunity to set a string line. In many cases for tighter tolerances of 2mm or 3mm, a "C" piano wire (gunite wire 300,000 PSI breaking strength) is set. The bolts are originally offset, per plan from the line. A sighted line is simply setting up a transit on one end point of the line and sighting the other end of the line to assure that the bolts are in their proper patterned location. 2) The Instrument - With the technology of total stations, robotic stations, etc.., layouts are for the most part done radially. This is fine so long as your control has been set and defined correctly, moreover; this method does improve time management and the collected information can be downloaded to check against the drawings in CAD (a fourth check). the bolts set by the instrument does give a position relationship in regards to the project as a whole (where you are in the world). A good check to assure you are in the right location. 3) The Chain - Tape (measure) - this is what all of you are describing. hard measurements of the bolts in relationship to each other - bolt to bolt, diagonals, cross references, centerline bolt pattern to centerline bolt pattern. The method is designed to reduce the three basic errors - human, instrument, and natural. When verifying these anchor bolts, they should be approved of their location against the three elements "Three Amigos". When one of the elements do not match the other two, then an error exist. The centerlines may be found to be wrong, the instrument may not be properly setup over the control, or the control is off, the instrument is out of calibration, movement of the forms may have happened, expansion and movement if you are verifying embeds on upper steel erected levels, so on and so on. So all of you have touched on these items and yes repeatability is a staple. I hope this helps clear it up for you all, and thank you for the comments.

I agree with all of this. I stress this to the students I am teaching. Check, Check and Confirm. It is what we do. With modern equipment surveyors get lazy. And finally when working with a chain(tape) never ever forget the adjustments and corrections!

It amazes me how may young surveyors think that they "party chiefs" just because they can layout the coordinate points that someone else calculated and don't understand why the points along a line they just set don't line up when checked against a "string line". Nothing beats the experience gained by making an error.

I agree with all of the above comments about the methods and checks that need to be performed. I started my construction surveying career in the late "˜90s, so while I am not that "old-school", we did not have the benefit of robotic total stations and data collectors that are now commonplace on many construction projects. I continue to see many examples where simply pulling out a $10 pocket tape would have revealed incorrect points that were setout with a $60,000 instrument. I stress pulling diagonals and I try to get our younger surveyors to realize that control points close to on-going work can be disturbed and sometimes do move due to the heavy machines moving around them. As a precautionary practice I tell them that it is prudent to check into two or three previously setout points before setting out any additional points, especially if the work is tying in these previous points. I am currently working on a multistory steel frame building and I would like to hear some thoughts about checking the verticality of steel beams using the refelectorless technology. In the past, I have sighted along the flange of the column and noted any deviation from the vertical. I have my doubts about whether I can achieve the same accuracy taking reflectorless measurements along the flange of the column.

As a 19-yr-old greenhorn chainman (June 1951) on a MT Hwy Dept constr. survey crew, the 1st two things they taught me: (1) How to "throw a chain"; (2) How to mark a "bluetop". Ah, good ol' days!

Many long years ago on a steel tank project I did penetration and fitting locations just after sunrise (for visability). This was to have the tank in a thermally neutral or static condition ie, no leaning due to thermal expansion of one side caused by sun heating southerly while the northerly side was in the shade of the south side. I applied tape length corrections sparingly and seldom because the tank was steel similar to the tape. Elevation taping was from a level datum, 18" dumpy level, for measuring the elevation of the penetrations and fittings. Horizontal location was radial from within the tank with a 30" transit. Sights were not long. A leaning tank would have made either measurement land at the wrong point. This was summer in Alabama with large swings in temperature. The south side of the tank would become almost to hot to touch in the summer sun. The finished tank would be enclosed in a building with hopefully minimal temperature change after project completion. How this works with a measuring device with minimal correction for temperature while the project material expands and contracts needs consideration.