GPS Coordinates Are Dangerous Part I

Robert J. Arnold, P.S.

I have been hearing a troubling statement from many different corners of my clientele, “Can you get me the GPS coordinate on that”?  I’ve heard it from residential clients, Architects, Professional Engineers, and yes Professional Surveyors too.  The thing is…there’s no such thing.

A coordinate system is a way to describe the relative position of discrete points.  Think back to your High School Algebra class, remember plotting points on an X Y axis?  The question from the teacher on the test might have been “Graph (-2,4) and (1,3) and write the equation that describes the line between them”.  I know some of you are having nightmares right now, just bear with me.  Fig 1 shows the graphing part of the answer, (we’ll skip the equation bit).  The dots at the ends of the line represent the coordinates from the test question.  This is an example of an “unfixed” arbitrary coordinate system.  The origin, where the X-axis and Y-axis intersect is at a random spot on a piece of paper, the Y-axis is pointed to the top of the page and the X-axis is at right angles to it.
Surveyors use this idea every day to do their work.  Our job is to properly measure discrete points and to analyze the relative position of those points to each other.

See Fig 2.  In this hypothetical case, we measured, with either a total station or GPS equipment all the points shown relative to a “fixed” arbitrary coordinate system, oftentimes called a “local coordinate system”.  Here we arbitrarily made the Section Corner the origin and the Y-axis due north, probably because that’s what the deed says, and we can start answering our questions.  Does the written record for the parcel start 100’ north of the Section corner? Do the angular and distance relationships match the written record as well?  If not, why?

Like I said, we can generate these coordinates with GPS equipment, but these are not what folks mean when they ask for, “GPS Coordinates”.  What they really mean, is usually, State Plane Coordinates.  Here’s the difference, State Plane Coordinates have a mathematically defined origin and direction. Every point measured within the State Plane Coordinate systems zone is relative to that origin and direction.  See Figure 3.

This system has been around for a long time.  It started prior to the launch of any GPS satellites and there are ways to generate State Plane Coordinates without using GPS.  However, it is very laborious and time consuming and therefore expensive and not very practical.  Conversely, determining a state plane coordinate on a position using GPS equipment is quite easy and cost effective.  For surveyors, it is a big technological advancement and has changed the way we complete our daily tasks.

On the next installment I’ll discuss the pitfalls of State Plane Coordinates and how this request for “GPS Coordinates” can be a problem.

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