GPS Coordinates Are Dangerous Part II

Robert J. Arnold, P.S.

Continuing our discussion of State Plane Coordinates or what some erroneously designate as “GPS Coordinates” from last time, I should first clarify that State Plane Coordinates are not “dangerous” by themselves.  What is dangerous is what people do with the information that they think they have.

When a residential client asks for “GPS Coordinates” for his property corners, I start to worry.  First, the description of the property most likely does not use the State Plane Coordinate system as his bearing base.  This means that the coordinates he wants, which could be Latitude and Longitude as well as State Plane, will not translate directly.  In other words, North on State Plane is, most likely, not north for the parcel.  A good conversation with the client and this becomes a non-issue.

Secondly, and more importantly, is why does the client want this information?  It’s a good bet that the client wants to save some money by using his phone GPS App to stake the lines between the corners.  If he does nothing with permanence, this is not a problem.  To be sure technology has come a long way and your phone can easily get you sub meter results.  But +/- 3 feet, or even +/- 1 foot if you’ve paid for the upgrade, is not close enough to make definitive statements about the location of the line.  Don’t build a fence this way!

Another example of potentially dangerous misuse of State Plane Coordinates involves Geographic Information Systems (GIS) and their administrators.  A GIS is a wonderful tool for analyzing data that has a spatial aspect to it.  Emergency response coverage zones, crime statistics, real estate values, the list is endless.  A GIS is most effective if it truly represents or models the geographic area of interest.  A good way to tighten up the model is by using State Plane Coordinates or Latitude and Longitude to fix the map (Remember fig 3?).  With enough ground data the GIS can be made quite accurate.  Now here’s the problem.

Residents of the area don’t always know where to turn if they have problems with their parcel boundaries.  Increasingly, municipalities will put their GIS online so that constituents can access the data.  One of the basic functions of a GIS is to show parcels.  Most on-line GIS platforms will allow you to inquire on the coordinate of a pixel.  If a resident inquires on the coordinates of the corners of the parcel and then uses their phone to navigate to the points did they just figure out where their boundary line is?

The answer is no because they have absolutely no idea how good the coordinate they picked is, nor do they have the tools needed to navigate to that exact coordinate.  The real problem is that GIS administrators don’t understand this either and can very easily lead their fellow citizen down a dark and expensive path.