State Plane Coordinates Misunderstood

The basic concept of state plane coordinates is that there are two locations within a zone that have scale factors of exactly one. Between these two points, state plane distances are less than surface distances, and beyond these points state plane distances are larger than surface distances.

In my 30-plus years since college graduation, I have been fortunate enough to live in the midwestern, eastern, and southern parts of the United States and to visit other states as well as a few countries in Europe.

As a technically trained person, I have found myself to be quite knowledgeable about surveying practices and procedures in the areas I have visited. While ice fishing in Minnesota I have seen survey crews using grain shovels to remove snow from a control point location, and I have seen photographs of vermessungpunkts in Germany and a Primo Meridiano monument in Rome, Italy.

Most recently, as a practitioner and instructor in surveying matters, it has come to my attention that there may be a significant misunderstanding of state plane coordinates.

I started a non-scientific and random review of documents related to some of my projects as a practitioner for use in my instructional endeavors. Subsequent discussion of the preliminary results of that search with students has revealed that many of the students have been introduced to those same misunderstandings by their supervisors.

The state where I practice has statutorily defined state plane coordinates to be metric with specifically defined zones, origins, and scale factors. The minimum standards for property boundary surveys in this state require that specific information be included within any subdivision plat that uses state plane coordinates and that plat distances are to be in horizontal surface feet, while state plane coordinates are to be in meters. In addition, the statutes state that no surveys are to be recorded unless they comply with the requirements of the adopted state plane coordinate system.

Several county and city planning departments within my state of practice have adopted subdivision regulations requiring placement of state plane coordinates on all new subdivision plats. Additionally, the surveyor's certification for subdivision plats must state that the plat complies with our state's minimum standards for property boundary surveys.

My preliminary research of these randomly selected plats indicates that less than one third of them comply with state statutes and administrative regulations related to state plane coordinates. A recent professional society seminar presentation by another surveyor in our state showed that roughly three out of four subdivision plats were initially deficient for coordinates in his metropolitan area.

Deficiencies noted are:

  • coordinates listed in feet instead of the prescribed meters,
  • failure to list the name and coordinates of the geographic reference station used as the basis for the survey,
  • failure to list the coordinates of that station, and
  • failure to show either a tabulation of the traverse from reference station to the subdivision or the GPS relative positional tolerance of the tie to the reference station.

At first, these deficiencies appeared to be administrative or documentary defects only; however, a further review of the data on the review plats revealed that the coordinates listed were actually surface coordinates, not state plane coordinates. The basic concept of state plane coordinates is that there are two locations within a zone that have scale factors of exactly one (remember, elevation factor must be applied to scale factor to determine grid factor, and grid factor applied to surface distances yields grid distances and coordinates). Between these two points, state plane distances are less than surface distances, and beyond these points state plane distances are larger than surface distances. This difference applies regardless of whether the state plane coordinate system is defined to use units of meters or feet.

One plausible explanation of the plat distances and the inversed distances being the same is that the preparer (of the plat-converted state plane coordinates in meters to surface coordinates in feet) conducted the survey on the surface in feet and merely picked off surface coordinates from the resultant drawing (this is, of course, in an area that has adopted metric state plane coordinates). For proper publication of surface distances and state plane coordinates, a transformation method must be used. For example, in Audodesk's Land Desktop, the projects menu contains a transformation submenu that allows definition of a point in state plane and local coordinates, and the labels menu contains a submenu for geodetic labels.

If a subdivision plat that you are reviewing (whether it be yours or someone else's) lists distances that match the distance obtained by inversing the listed state plane coordinates, you have one of two situations. Either you are on the line of scale factor exact, or the data on the plat is incorrect. A detailed reading of your local state plane coordinate statutes, regulations, or manual of practice will show you which situation you have.

Following the recent NSPS/ACSM meeting in St. Louis, one surveyor who attended the geodetic seminars was discussing readjustments to the control network and said that one of the geographic reference system stations he reviewed had moved 1.22 feet due to readjustments. I asked him to use metric dimensions when discussing state plane coordinates, as our state and all its neighbors have adopted the metric coordinate system. I then summarized the plat information that I had discovered illustrating misconceptions about our coordinate system.

A few days later I was thanked by another local surveyor for mentioning state plane coordinate misconceptions at the recent meeting. He went on to say that he had been thinking about his firm's procedures and concluded that their GPS measurements are always on our state plane system, but their survey crews use those coordinates for surface measurements by total station also. Their procedure did not account for transformation from grid to ground, and he was reminding himself to be sure to set the grid factor in the GPS controller at the beginning of the project.

There is a national debate regarding whether GIS should be under the supervision of professional surveyors. The primary use of state plane coordinates is to coordinate all surveys to a common reference system and subsequent use in mapping or GIS.

People have also made presentations at our professional society seminars regarding both state plane coordinate procedures and evaluations of whether GPS surveys meet our state's minimum standards for property boundary surveys.

If we professional surveyors are to comply with local surveying standards and to obtain jurisdiction over GIS, should we not be sure that we correctly understand state plane coordinates?

About the Author

The author is a surveyor who prefers to remain anonymous.

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