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George Bryant Gives Insight into Space-Based Positioning

Where ARE You?


I was fortunate to recently attend a seminar titled “Modernization of the National Spatial Reference System” sponsored by the Virginia Association of Surveyors. I went to this seminar for two reasons. First, I needed to acquire professional development hours, which are required by the State to keep my land surveyor’s license active. The second reason was to gain knowledge about improvements that are being made to our State Grid coordinate system and how GPS (global satellite positioning) and increasingly other space-based positioning systems are being applied to land surveying.

Now, I am sure that if you are not a land surveyor you might wonder how this seminar could have anything to do with you and your daily existence. The information that I came away with was entirely different than what I expected to hear. The speaker was Mr. David Doyle, the Chief Geodetic Surveyor for the National Geodetic Survey (NGS). Rather than hearing a lecture about improvements to the Virginia State Grid System, I learned that NGS is considering scrapping the current way that we calculate our location – using coordinates - and is creating a new, more accurate system.

Let me give a layman’s description of how our State Grid Coordinate system works. I will leave out the historic evolution and tedious formulas that are currently being used. First the world was flat, then it was round; today we know that it is neither. Picture a person plotting coordinates on a flat piece of paper or on a perfectly round ball. The task seems pretty easy. Now, take that same piece of paper and ball it up and call it Earth. The shape appears to be round, but it truly is not. The crevices in the ball represent our rivers and the lumps on the ball represent our mountains. Now try to plot a coordinate on that balled up piece of paper.

NGS has developed an approach to help us do this. Currently we use a horizontal coordinate datum referred to as the North American Datum of 1983 (NAD 83) and a vertical datum known as the North American Vertical Datum of 1988 (NAVD 88). To picture how these work, imagine a round ball with a piece of paper balled up around it. The paper represents the Earth’s surface, but the ball inside the paper is the surface on which we will calculate our coordinates.

Due to their elevation differences, some States could rest on a bigger ball and some States could rest on a smaller ball. This fact is why each State adopts its own State Grid system, and that sea level is not the same elevation for each State. The coordinate relationship between States can be calculated, but the process is no fun.

So why is this such a big deal? Well, do you use programs like Google Earth, MapQuest, package tracking services, emergency 911 or a navigation program in your car? Thousands of programs tie into our coordinate systems via satellites and generate information that we use daily. We have all noticed that while using a navigation system our location appears close, but maybe not exactly where we are.

What NGS is proposing to put in place by 2022 is a new national coordinate system that will replace both NAD 83 and NAVD 88 with more contemporary systems. These new systems will adhere to internationally adopted standards that will enhance the use of the improvements in space-based positioning for a vast array of applications. This technology not only creates a more accurate surface to determine coordinates, but also takes into account changes to the Earth’s surface and movements of the Earth’s tectonic plates. Scientists believe that we may be able to use a simple cell phone to acquire or find points (coordinates) within a few centimeters accuracy in real time anywhere in the world. The application of this technology appears limitless as our society continues to progress.

For a surveyor, what are the implications of these changes? GPS is already being used by contractors to define limits of disturbance, to grade sites, and to install pipe and curbs. Their machinery is only going to get more accurate. Construction staking has always been a source of employment for my profession, but as time goes on this will change. Today, my plats reference a State Grid Coordinate and use bearings and distances to define the shape of a piece of property. In the future, will I have to give coordinates on each property corner so that a teenager with a cell phone can go find his parents’ property corners for them? In our current legal system, a coordinate is considered one of the weakest ways to define a property corner. Could the rules of boundary law be rewritten with a new hierarchy, where coordinates have a much higher importance? I find this new wave of technology to be exciting most of the time, but once in a while I have to ask myself, “Where am I?”