Did you know? The Importance of Accurate Measurements in Surveying

Most historians agree that the first unit of measurement was the cubit. Developed by the ancient Egyptians, the cubit was the arm length between the elbow and the tip of the middle finger. In fact, most earlier units of measure were based on the human body. An inch was based on the width of a thumb, a yard was based on the length of a man’s belt, and a foot was based on (you guessed it) the length of a man’s foot. The scientific community’s ability to measure accurately has advanced over time in ways our ancestors never thought possible. These technological advancements allow the surveying community to obtain measurements faster and more precisely.

It is also important to understand that a survey’s accuracy is dependent on the type or purpose of the survey. For example, the standards of practice for surveying in the State of Alabama (Rule No. 1.03, 16) lays out the following accuracy standards.

The relative error of closure permissible shall be no greater than the following:
  • Commercial/High Risk:
    • Linear-1 foot in 10,000 feet
    • Angular-15 seconds times the square root of the number of angles;
  • Suburban:
    • Linear-1 foot in 7,500 feet
    • Angular-20 seconds times the square root of the number of angles;
  • Rural:
    • Linear-1 foot in 5,000 feet
    • Angular-30 seconds times the square root of the number of angles.

As you can see, the permissible closure error is greater in a rural setting than in an urban area.

It is also important to understand that precision and accuracy are not the same. For example, surveyor Bob comes to a site and makes a very precise measurement between point A and point B. On the surface, it seems like surveyor Bob has made a very accurate measurement; however, after giving the data to his client, it is discovered that surveyor Bob accidentally measured between point B and point C instead of point A to B. This measurement was very precise, but it was not accurate.

Why is this important? Take a topographic survey performed for a large commercial development. The topographic area covers 40 acres, and a set of design plans are produced for the development of the site. The surveyor who performed the survey had a bad elevation that was off 0.10 of a foot. As construction begins, the contractor realizes that the 0.10 of a foot error has resulted in 6,453 extra cubic yards of dirt that must be removed from the site. That is roughly an additional 240 truckloads of dirt that must be removed.

At the end of the day, the surveyor should take care to provide the most accurate measurements possible. One mistake can prove costly for all parties involved.