Accuracy in Surveys

We often talk about the importance of accuracy in our surveys. It's generally understood that the quality of information is the essence of its value and usefulness. But surveys are never perfect. Measure something twice and you will always get a different answer. If you don't, then just use more decimal places - and you will see the variance!

So how can we rely on information that can never be 'perfect'?

The answer is that we have to set limits on how much variance we can accept and then strive to develop systems and controls to meet these objectives. Over time we should be able to improve accuracy and narrow the range of what we consider accurate. Although the information may not be perfect, we can set specific tolerances for what is the acceptable range of possible variation.

To understand how accurate CAD drawings and areas have to be, we have to determine the purpose that the information will serve. In commercial leasing, the rent paid for space is based on a square foot rate. Inaccurate areas will either cost the landlord in uncharged rent if the calculated area is too low or create a liability if the landlord has collected rent on too much area. How much could this amount too? If we take a modest office of 10,000 square feet at a rent of $30 per square foot for a term of five years, then an error of 5% would amount to $75,000. If a typical office building of 300,000 square feet were off by 5%, then the total lost revenue or liability would be $450,000 each year!

Another important use of surveyed office interiors is for interior design layouts. In calculating areas, columns are not subtracted from usable or rentable area and are therefore not accurately recorded. But if you are using a CAD file for construction and especially if you are using system furniture, the accuracy of every column and jog in the walls is crucial. A drawing that may have been very accurately drawn to calculate area may be unsuitable for an interior layout. It is therefore very important to determine the purpose of a drawing before doing the survey and creating the file. Because the exact location of drywall covered columns and walls may change during renovations, it is often best to create the 'as-built' survey of existing conditions just before new construction or when new furniture is required.
Once we have determined what needs to be measured and what the purpose of the information is, the next step is to determine how accurate the information has to be. What is the allowable margin of error? What is the maximum tolerance? Currently the BOMA standard accepts a variance of +/- 1% in area calculation. (If two surveys are within 2% of each other, then both are deemed to be correct!)