Measuring the World from Different Perspectives: Geodetic and Boundary Monuments Serve Different but Complementary Purposes
by Angus W. Stocking, L.S.
In the late ’80s, I was working as a land surveyor for the California Department of Transportation and had the good fortune to be assigned to my district’s first GPS crew. In those days, GPS work was “static,” which meant that after setting up the receiver, the attending surveyor had little to do but sit around for several hours while satellite data was gathered.
On one gorgeous fall day, I set up over a brass monument placed on a hill with commanding views of the Pacific Ocean and rolling agricultural fields. While I waited on the receiver, I was entertained by a biplane pilot practicing aerial acrobatics. That day remains one of the most pleasant memories of my entire surveying career—but ironically, in one sense I wasn’t “surveying” at all.
Licensed professional surveyors have the right to establish and reset boundaries, the legally defined but ultimately intangible lines that enclose parcels of land. These invisible lines are marked out on the ground by boundary monuments, which are permanent markers (usually made of stone or metal) placed at property corners, or occasionally at a known bearing and distance from a corner. Land survey monuments can be as humble as a pile of rocks or a length of rebar, but more often they are engraved aluminum or brass caps designed for the purpose.
But the “brassy” I set my GPS receiver over that day was no boundary marker. Rather, it was a geodetic monument, and it had nothing to do with boundaries. Instead, geodetic monuments mark, as permanently as possible, a precise location on Earth. So the work I was doing was more useful to geodetic surveyors (geodesists) than to land surveyors.
Both disciplines are complex. Land surveyors apply physical evidence, case law, and historical research to the problem of finding, setting, and restoring property corners. To them, monuments are usually the most important piece of evidence used when determining boundary. Interestingly, in the United States, Canada, Australia, and other countries where boundary law derives from principles established in Great Britain, a found monument nearly always establishes a corner location, provided it has not been moved—and even if it was originally set incorrectly! The idea is that the original monument (or its preserved location) is at the spot accepted as the corner at the time of setting. So for land surveyors, recovering monuments or evidence of monument position is a top priority.
By contrast, geodesists are concerned with accurately depicting the shape of the Earth, and tracking changes to its surface shape over time. Their work is vital to navigators, astronomers, cartographers, seismologists, and people in several other disciplines. Geodesists are well aware that our planet’s crust is unstable, that it expands, contracts, and shifts. And this means that they expect monuments to move about, and to move in relation to each other. By continually using GPS and other techniques (such as Very Long Baseline Interferometry, which relies on multiple synchronized observations of astronomical phenomena), geodesists refine their knowledge of the world’s shape and how that shape changes over time.
Most of the NGS monuments found by geocachers are geodetic monuments rather than land survey monuments, but there is some overlap between the two types of markers. Monuments set as part of the United States’ Public Lands Survey System (PLSS)—which defines primary boundary lines in much of the country—are often extremely stable, so geodesists will sometimes make use of them to “densify” control networks. That is, the geodesists will determine the coordinates of available monuments to increase the amount of known points in a region. Similarly, land surveyors will sometimes go out of their way to reference a well set geodetic monument in a legal description or on a map.
Until recently, boundary markers were only rarely tied to the global coordinate systems used in geodesy. But with the advent of inexpensive GPS receivers, it’s becoming common for land survey markers—especially PLSS monuments—to be tied to state plane systems that can be converted to latitude and longitude. Many of these boundary monuments are on private property, of course, and are not particularly exciting to anyone except the landowners. But it can be thrilling to track down and find a PLSS corner, many of which were set well over a hundred years ago. You might be the first to see it in decades! Next time you plan to go geocaching or benchmark hunting, consider inquiring with your county assessor or surveyor, or looking at local topo maps, to find out where there might be boundary markers along your route.
Angus W. Stocking, L.S., is a licensed surveyor with 15 years experience. He now lives in Paonia, Colorado, and is a full-time freelance writer specializing in infrastructure and technology. You can see some of his work at www.ColoradoWriting.com
Originally published on March 17, 2008.