Tech Talk: Why Geodesy Matters
April 27, 2011
“Geodesy” and “geodetic” are terms many plane surveyors love to ignore. As long as surveys are performed over small areas and/or the accuracy requirements are such that disregarding the curvature of the earth is an adequate simplification of the world, ignorance is feasible. However, knowingly ignoring these concepts is different from denying that geodesy is a valid science that must be understood enough to know when and why it matters.
Most of the classic textbooks used at the start of college surveying programs build a foundation in surveying ignoring the earth’s curvature. But even then, that ignorance is selective. We actually get exposed to quite a few geodetic concepts long before we get to the chapter on geodesy. True, that exposure to geodesy is extremely simplified, but it is a good template to follow and a reasonable reminder of why the existence of geodesy (or, more properly, that there is earth curvature and a science for dealing with it) cannot be denied.
If you were expecting a treatise on geodesy, forget it. I am not a geodesist. But part of my interaction with surveyors and others in the business of figuring out things are or where to put them tells me that geodesy plays more of a role in their work than they realize, and it behooves them to understand how and why.
The most obvious place that geodetic concepts enter the plane surveyor’s work is in leveling. We all know that we define the vertical as the direction of gravity. We define the horizontal as being perpendicular to gravity. That immediately should give us the signal that the horizontal, as defined at one location on the earth, is therefore different from that determined at another location. When we set up a level, we talk about making sure that the line of sight through it is horizontal. But we know that rod readings, no matter how carefully observed, are susceptible to errors due to curvature (and refraction of the line of sight by the earth’s atmosphere). We are able to ignore (not deny) the existence of that curvature by (a) keeping our sight distances short so that the more or less horizontal sights approximate the curvature of the level surface (a surface we all admit is curved) and (b) keeping our backsight and foresight distances balanced so that error from the two sources of curvature and refraction are eliminated when we take the difference of our backsight and foresight readings.<
This is about as complicated as we want things to be many times. When we do trigonometric leveling, where the curvature of the earth cannot be ignored if the sight distances are more than those we use in differential leveling, we generally abdicate our knowledge of geodesy to the manufacturer, who usually has a “curvature and refraction” correction model (sometimes more than one, and then the question is which one to use?) that lets us ignore (or is it deny?) the earth curvature. While the many errors that surveyors experience from performing trigonometric leveling cannot be discussed here, we do know that lack of understanding of how the curvature of the earth impacts those measurements is one of the reasons that surveyors give up using trigonometric leveling. It seems easier to not perform a procedure that involves understanding these concepts.
And yet, many of us routinely perform other activities that use much more in-depth application of geodetic principles and don’t think about it. The culprit, if you can call it that, is GPS. I remember in the early days of GPS, that reduction of GPS observations between control stations on the national network and the points we were trying to locate were all in geodetic terms. We ended up with geographic coordinates of our new points. Then, if we desired, we converted those coordinates to some plane system such as state plane coordinates.
During my career in the development of surveying technology, I remember clearly that the benefits of GPS seemed unacceptable to many local surveyors because results in geographic coordinates were not a desirable option. Providing facilities to generate plane coordinates of some kind was mandatory. And once surveyors wanted to blend data from GPS with data from total stations, since many projects required the total station to at least fill in areas that could not be surveyed with GPS, the practical solution for manufacturers of both types of surveying sensors was to use state plane coordinates (or some other type of plane projection system) as the “platform” to which both types of measurements could be reduced, and then combined.
So far, I haven’t talked about the ellipsoid and the geoid, ellipsoidal height and orthometric height, datums and projections, semi-major axis and flattening, or for that matter, the various types of geodesy. That is not my purpose here. Instead, it is to talk about the denial of geodesy. This denial is hugely apparent when I talk with surveyors about state plane coordinate systems. Many of them don’t understand how these coordinates come to be (or why), and pretty much only treat them as a necessary evil. Most of the time, they get by with this denial. But occasionally, disaster strikes from mishandling some aspect of state plane coordinates. My experience is that these disasters often occur because the local surveyors, despite the fact that their systems use some elementary geodetic concepts to generate state plane coordinates, make no attempt to understand how the data is manipulated.
Geodesy is here to stay. It can’t be ignored all the time. It definitely can’t be denied. So do yourselves and your organizations a favor and support whatever opportunities there are to learn about the basics of geodesy. Just because our sensors do all this for us automatically does not remove our personal responsibility to understand the concepts our “black boxes” use to get these solutions. Without that understanding, we are no better surveyors than the untrained individual who acquires today’s surveying technology in an effort to avoid hiring surveyors to get the job done.