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Tech Talk: State Plane Coordinates

July 5, 2011
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There was a time (in the 1980s and prior) when “everyone” knew about State Plane Coordinates (SPCs), including how those systems were set up and how to use them effectively. Or so we thought. The reality is that although most surveyors knew State Plane Coordinate Systems (SPCSs) existed, that’s where it stopped for the vast majority. Those who knew the operational details of proper application were pretty much only those who were required to do so--usually by a party who had the power to make the use of SPCSs mandatory. Seldom did completely voluntary use occur.

Unfortunately, myths and mistaken impressions about how to use the systems also developed, many of which are still hard to dispel. This led to misapplication of the systems, causing SPCs to be published that were not in accordance with proper use procedures and, thus, with erratic reliability that varied from pretty close to wildly incorrect.

Then came GPS in the ‘80s and ‘90s. As use of this technology steadily grew, surveyors in the ‘90s started to demand the ability to use GPS, especially RTK GPS, along with total stations, since most jobs could not be completed with only one technology. The incompatibility of the measurements resulted in various approaches to solve the data-merging problem. By then, most people understood the general knowledge about coordinates but didn’t know much about how to use them over areas of wide extent, even though the State Plane Coordinate Systems had been invented by the National Geodetic Survey (NGS) in the 1920s.

The solution converged on by manufacturers of surveying systems was to use the SPCS grid in the United States as the “common ground” to bring positions determined with the two kinds of technology. Once positions determined with both technologies were merged, it was reasoned the user could continue working on the grid or transform those positions to anything they wished.

Everyone jumped on this panacea. Unfortunately, most surveyors did not understand how the systems were set up or how to work with them appropriately. Many began to complain about known system idiosyncrasies such as distances not matching up when making measurements between points that had positions published in SPCs.

New York State Plane Coordinate zones. Source: NGS/NOAA

Surveyors dominantly perform their work assuming the earth is flat. The purpose of the SPCS is to allow working as if the earth is flat when doing surveys of sufficient extent that it is no longer a good idea to make that assumption. What the SPCS does is introduce small, systematic corrections to enable continuing use of plane surveying processes. The surveying systems we are provided today purport to automatically convert all observations to that system. However, some fundamental preconditions must be met to ensure that the so-called SPCs computed and displayed by these systems are actually so.

The first precondition is that the basis for introducing plane system coordinate value is by connection of appropriate accuracy to a point (or points) with correct SPCs or geographic coordinates in an appropriate datum that can be converted to SPCs. To do this properly, one needs to consult the NGS documentation on the use of the SPCS as well as the statutes, rules and regulations in the state pertaining to the use of the SPCS and publication of SPCs in that state. This principle is seldom known, understood or followed. For example, in one state, the statutes and regulations absolutely require connection to first- or second-order control in that state. It is not OK to connect to another survey that has SPCs published. Usually the connection is specified to be more than one-half mile or one kilometer. There are rules and processes for allowing connections that are longer than that connection distance. The internal consistency of the connecting survey and the survey of the subject project must generate (in one state) positional accuracies that are on the order of +/-0.1 feet (0.03 meters) with a 67 percent confidence level.

Most surveyors simply connect to existing points whose SPCs are published. Or, if they observe more than one (are they using total station, RTK or static GPS? And does it matter?), they simply do a least squares fit. They then decide if the quality of the fit is good and go on. But if they do connect to a nearby first- or second-order control station that is within the prescribed range, how is the connection tie done? What is the quality of the traverse? If it is static or RTK GPS, how do they evaluate the quality of the connection? Few stop to ask these important questions.

Thus, many surveyors purport to use the SPCS and publish SPCs, but very few actually comply with the procedures for doing so properly.

The SPCS is a carefully and methodically designed system. A plane system that portrays a curved surface will always require adjustments to handle the inevitable distortions that occur, especially as the surveys get wider in extent. Surveyors must understand these principles thoroughly before depending on their manufacturer’s system to make the calculations for them. Procedures for making connecting ties to first- or second-order control must be properly followed. And measurements between points with properly established SPCs have to be properly converted with elevation and scale factors to determine how they check with ground level-based measurements.


Editor’s note: NGS publications on the SPCS can be found through www.ngs.noaa.gov/PUBS_LIB/pub_horiz.shtml.

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