National Surveyors Week

25 March 2021

It is hard to overstate the larger-than-life presence NGS has in the surveying profession. Early in my career, I learned the many benefits to be gained by tying my surveys to the NGS National Spatial Reference System (NSRS). Here I want to focus on two related aspects of the NSRS that impacted my work as a surveyor and are undergoing changes today: the State Plane Coordinate System (SPCS) and the foot (not the body part, but the unit of measure).

First, SPCS. Originally created by NGS in the 1930s, SPCS is a system of flat mapping grids that cover entire states or large parts of states. Over the years, it has grown in popularity and is widely used by surveyors. But there is a complication: the distance between a pair of SPCS coordinates is usually shorter than the actual ground distance, often by more than a foot per mile (which is a lot for a surveyor). This "short distance” problem happens because flat models are used to represent the curved surface of the Earth, just like how a paper map gives us a slightly distorted representation of a round globe. The distances are usually too short because the flat models used when SPCS was originally designed could not account for actual ground topography, since detailed elevation models were not available.

Second, the foot. Sometimes, SPCS coordinates disagree because of mix-ups between using the old U.S. survey foot and the new international foot. Through essentially an accident of history, these two types of feet are both used by surveyors. They differ by a miniscule amount (about 1/8 inch per mile), but SPCS coordinates are so large that the cumulative difference can reach several feet. It may seem absurd, but it is a real problem with real costs.

Today, as part of our NSRS modernization efforts, we are updating SPCS, with the (enthusiastic!) cooperation of our customers. The new SPCS is being designed to greatly reduce this “short distance” problem, by using accurate elevation models to represent the actual ground surface. That makes surveyors happy.

NSRS modernization also provides a unique opportunity to fix the onerous dual foot problem. NGS worked collaboratively with the National Institute of Standards and Technology on addressing the issue. At long last, the U.S. survey foot will be retired, eliminating a problem that has persisted for more than six decades.

It is deeply satisfying (and humbling) to be an active part of these two changes, changes that will greatly benefit not only surveyors but the entire geospatial community. For that, I am grateful. Happy National Surveyors Week to all!

Michael Dennis
National Geodetic Survey
Michael Dennis Portrait

Michael Dennis
Geodesist, National Geodetic Survey

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