Gravity's Importance

As the country moves toward the adoption a new vertical datum based entirely on the determination of Earth’s gravity field, surveyors will need to better understand the foundational aspects of how these data have been collected and managed as well as the relationship of gravity observations to the traditional and classical efforts of leveling.

In this month’s issue, Kelly Bellis, a land surveyor from Maine, has done an outstanding job of piecing together an overview of the development of gravity observations, especially as they have been performed and continue to develop in the United States by the National Geodetic Survey and its predecessor agencies.

The study of Earth’s gravity field is one of the most important aspects of physical geodesy.  While seldom mentioned in land surveying text books or discussed at local surveying, mapping, and GIS conferences, it is nonetheless one of the most important issues that surveyors deal with on a daily basis. Two fundamental issues come to mind. 

One of the first things any surveyor does when he or she goes to the field is set up an instrument.  It doesn’t matter if it’s a total station, level, or GPS receiver; all have some form of a level vial or bulls-eye bubble.  From their earliest days in the field surveyors are taught how important it is that their instruments be level, but what does that mean? To most surveyors it’s just about bringing the bubble to rest between scribed marks on a vial or into the center of the bulls-eye. They don’t give much, if any, consideration that what they are really doing is bringing the instrument into alignment with the local attraction of Earth’s gravity field. 

The second most important aspect of understanding gravity is related to running levels.  The basic premise of levels is to determine the difference in height from one point to another, but what’s really being performed is estimating the difference in gravity potential from one point to another—in other words, which way water and other fluids will flow.

The feature articles on gravity in this issue and the next provide a background on efforts to determine Earth’s gravity field and observations on the future importance of gravity to the profession of surveying.  I recommend you pay close attention, because the way we measure will soon fundamentally change.
~Dave Doyle

Dave Doyle is the chief geodetic surveyor at National Geodetic Survey and a member of our editorial board.

» Back to our December 2012 Issue

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