Free stationing, which can be used for most layout surveys (not just building survey), has several advantages (1) the theodolite (or total station) can be set up to avoid obsta- cles, (2) setup centering errors are eliminated, (3) fewer control stations are required, (4) the setup station can be located close to the current layout work, and (5) the lay- out work is greatly accelerated if total stations are employed. Figure 18.15 illustrates resection techniques being used to provide the coordinates of the theodolite station.
In this case, the theodolite or total station is set up at any convenient location (called a free station). Angles are then taken to a minimum of three coordinated control sta- tions; a fourth control station could be taken to provide a second computation of the instrument station as an accuracy check. If both angles and distances are to be taken to the control stations, only two, or preferably three, stations are required. By using
Target on Permanent Mark Target on Permanent MarkTheodoliteTheodolite
Target Target Figure 18.13Establishing control by “wiggling-in.” See also Figure 6.15.
Steel Tape Theodolite
Figure 18.14 Optical plumbing, combines with reference azimuth points (RAP) for polar layouts.
trigonometric relationships (identities) to solve the resulting triangles, the coordinates of the instrument station are computed—most total stations are now programmed to solve resection problems. Once the coordinates of the free station are solved, and the total station is back-sighted to a known reference point, the total station programs can then compute the polar ties to any layout point.
Figure 18.15 Location and alignment of the instrument by resection (free-stationing; see also Section 3.3.3).
Theodolite
Primary Point
Primary Point Primary Net
review Questions
18.1. What are the steps required to provide the horizontal layout of a building? List the steps.
18.2. What are the steps required to provide a vertical location layout for (a) a one-story building or (b) a multistory building? List the steps in point form.
18.3. What are the advantages of using a rotating laser rather than a rod and level on a single-story building site?
18.4. What is the advantage of using a visible-beam laser instrument rather than a conventional level in a construction site?
18.5. Why is it advisable to utilize all available benchmarks (BMs) at a construction site?
Figure 18.16 Use of BIM model in layout. (Courtesy of McCarthy Building Companies, St. Louis, Missouri)
Emerging technology for construction layout is combining 3D building informa- tion models (BIM), robotic total stations, laser guidance, and wireless communications to allow field layout or verification of key building components to further increase the accuracy and productivity of construction layout. Figure 18.16 shows a construction surveyor setting out points. The BIM model for the building is loaded into the total station's computer, and is visible on the tablet PC mounted on the surveyor's rover.
The surveyor taps the PC screen to indicate the building component to be located, and the total station directs the rover to the point. The technology allows the surveyor to set and save new points in relation to the design BIM model. The rapid advances in construction surveying technology make it important for surveyors to be committed to continuing education and active involvement with surveying professional societies.
Quantity and Final SurveyS
19.1 ConStruCtion Quantity MeaSureMentS:
General BaCkGround
After the line and grade for a project have been established, the construction surveyor’s next concern is to supply the project supervisor with survey measurements—and resul- tant quantities—that reflect the progress achieved by the contractor. Progress payments, which are based on quantities supplied by the surveyor and the construction inspector, are processed either at the end of a regular time period (e.g., monthly) or at the completion of previously agreed-upon project stages. The contractor usually also employs a surveyor to provide similar data; this ensures that questionable quantities are quickly discovered and remeasured.
Some construction projects are bid on a lump-sum basis, where the contrac- tor’s one-price bid covers all the work required to complete a project. The demolition of a structure is a good example of a situation where a lump-sum bid is appropriate.
Here, the owner (e.g., private company, municipality, state, or province) simply wants the structure removed; the demolition technique is of little importance to the owner (blasting would be an exception). The owner simply wants the job done as cheaply and expeditiously as possible.
Most municipal and state/province construction projects, however, are bid on a unit-price basis, where all the facets of the job are defined in detail. The owner (e.g., municipality, highway department, developer, railroad) specifies the line and grade (and cross section) of the facility; also specified are the types of construction materials to be used; the qualities of the finished product (e.g., compressive strength of concrete);
the compaction level of earth and granular fills; and the appearance of the finished product.
The owner will list all categories (units) of materials and operations and will prepare an estimate as to the total quantities for each item—that is, total cut and fill, total length of fence, total volume of concrete placed, total area of sod or seeding, and the like. See Table 19.1 for other typical construction categories, with tendering units. The contrac- tor bids a price against each unit item; all items are then added to compute the total unit bid. Finally, these individual total bids are summed to produce the grand total for the contract bid. The contract is usually awarded to the qualified contractor who submits the lowest bid.
n i n e t e e n
The contractor is paid for the work as it is completed (progress payments) and when all the work is completed (final payments); usually the owner will hold back a certain percentage of the final payment until after the guarantee period (two years is typical) has elapsed.
Table 19.2 shows typical measuring precision, in both feet and meters, for layout and quantity surveys. Surveyors, working for both the owner and the contractor, record the progress payment measurements in their field notes; these notes must be accurate, complete, and unambiguous. Figures 19.1–19.4 illustrate typical survey notes for quantity surveys.
Lineal units, as shown in Table 19.1, are added to obtain final quantities. Simply add- ing the scale tickets for the various items totals weight units. Both the area and the volume units require additional work from the surveyor, however, before the unit totals can be de- termined. The following sections illustrate the more commonly used techniques for both area and volume computations.