The construction drawings are your main basis for defining the required activities for measuring the quantities of material. Accurate estimating requires a thorough examination of the drawings. You should carefully read all notes and references and examine all details and reference drawings. The orientation of sectional views should be carefully checked. Dimensions shown on drawings or computed figures shown from those drawings should be used in preference to those obtained by scaling distances.
You should check the "Revision" section near the title section to ensure that the indicated changes were made in the drawing itself. You must ensure that the construction plan, the specifications, and the drawings are discussing the same project. When there are inconsistencies between general drawings and details, details should be followed unless they are obviously wrong. When there are inconsistencies between drawings and specifications, you should follow the specifications.
As an estimator, you must first study the specifications and then use them with the drawings when preparing quantity estimates. You should become thoroughly familiar with all the requirements stated in the specifications. Some estimators may have to read the specifications more than once to fix these requirements in their mind. You are encouraged to make notes as you read the specifications. These notes will be helpful to you later as you examine the drawings. In the notes, list any unusual or unfamiliar items of work or materials and reminders for use during examination of the drawings. A list of activities and materials that are described or mentioned in the specifications is helpful in checking quantity estimates.
The tables and diagrams in the Seabee Planner's and Estimator's Handbook, NAVFAC P-405, should save you time in preparing estimates and, when understood and used properly, provide accurate results. Whenever possible, the tables and the diagrams used were based on Seabee experience. Where suitable information was not available, construction experience was adjusted to represent production under the range of conditions encountered in Seabee construction. A thorough knowledge of the project drawings and specifications makes you alert to the various areas where errors may occur.
Accuracy as a Basis for Ordering and Scheduling
Quantity estimates are used as a basis for purchasing materials, determining equipment, and determining manpower requirements. They are also used in scheduling progress, which provides the basis for scheduling material deliveries, equipment, and manpower. Accuracy in preparing quantity estimates is extremely important; these estimates have widespread uses and errors can be multiplied many times. Say, for example, a concrete slab is to measure 100 feet by 800 feet. If you misread the dimension for the 800-foot side as 300 feet, the computed area of the slab will be 30,000 square feet, when it should actually be 80,000 square feet. Since area is the basis for ordering materials, there will be shortages. For example, concrete ingredients, lumber, reinforcing materials, and everything else involved in mixing and placing the concrete, including equipment time, manpower, and man-hours, will be seriously underestimated and ordered.
The need for accuracy is vital, and quantity estimates should be checked to eliminate as many errors as possible. One of the best ways to check your quantity estimate is to have another person make an independent estimate and then to compare the two. Any differences should be checked to determine which is right. A less effective way of checking is for another person to take your quantity estimate and check all measurements, recordings, computations, extensions, and copy work, keeping in mind the most common error sources (listed in the next section).
Failure to read all the notes on a drawing or failure to examine reference drawings results in many omissions. For example, you may overlook a note that states "symmetrical about the center line" and thus compute only half the required quantity.
Errors in scaling obviously mean erroneous quantities. Great care should be taken in scaling drawings so correct measurements are recorded. Common scaling errors include using the wrong scale, reading the wrong side of a scale, and failing to note that a detail being scaled is drawn to a scale different from that of the rest of the drawing. Remember: Some drawings are not drawn to scale. Since these cannot be scaled for dimensions, you must obtain dimensions from other sources.
Sometimes wrongly interpreting a section of the specifications causes errors in the estimate. If there is any doubt concerning the meaning of any part of the specification, you should request an explanation of that particular part.
Omissions are usually the result of careless examination of the drawings. Thoroughness in examining drawings and specifications usually eliminates errors of omission. Checklists should be used to assure that all activities or materials have been included in the estimate. If drawings are revised after material takeoff', new issues must be compared with the copy used for takeoff and appropriate revisions made in the estimate.
Construction materials are subject to waste and loss through handling, cutting to tit, theft, normal breakage, and storage loss. Failure to make proper allowance for waste and loss results in erroneous estimates.
Other error sources are inadvertent figure transpositions, copying errors, and math errors.