Wisconsin Geospatial News

County Coordinates go under the microscope

As we reported a year ago, questions have arisen (and continue to arise) surrounding mapping coordinate systems that are designed to serve individual county areas. A task force under the Wisconsin Land Information Association recently had its first meeting to begin to scope the issues. (A similar unchartered group met last February to gather initial information).

Several complexities are apparent. First, Jackson County has its own coordinate system, separate from the one developed by the Wis. Department of Transportation ten years ago as part of its statewide set that includes every county. Second, names being used are not always standard, and the meanings of shorthand or acronyms may not be clear. This creates more than confusion since software vendors need a single set of unambiguous terms. Third, conversion routines that are built into some software programs have yielded erroneous coordinate values up to fifty feet off, which might be traceable to misunderstandings over how the county coordinate systems are devised.

A bit of background
Coordinate systems designed for individual counties have one major advantage over older, more traditional systems. They use a mapping plane that is close to the actual ground surface rather than one that lies close to sea level. The result is that horizontal distance measurements between points on the actual ground are so similar to the values expressed in the coordinate system that the difference can be disregarded. Stated in mapping jargon, the grid-to-ground ratios are extremely small. As a consequence, surveyors do not need to “reduce” their measurements, a factor that can always be questioned regarding data referenced to other coordinate systems.

The State of Minnesota developed county coordinate systems prior to a similar project being undertaken for the Wisconsin Department of Transportation by Fairview Industries in the early 1990’s. Those Wisconsin systems are often referred to as “The Wisconsin County Coordinate System.”

Who is using what, and by what name?
It is helpful to know who has been using any of the various local coordinate systems, what names have been used, and which units of government have gone through a formal adoption process. The task force is assembling a list to document the current situation. So far it is clear that the large majority of counties are actively using the system designed by the DOT although the names they have applied are not standard. One issue regarding names is that it may not always be clear which adjustment of which datum underlies a particular coordinate system, and this lack of clarity would become an even larger issue if the National Geodetic Survey publishes a new horizontal datum (as has been suggested might happen later this decade).

Software implementation varies
Many GIS, CAD, and GPS software packages include tools to convert coordinate values. These tools rely on either straight formulas or models (that provide close approximations) depending on what is being converted. For instance, going from State Plane Coordinates to Wisconsin Transverse Mercator (both referenced to the horizontal datum NAD 83 (1991 adjustment) is a straight mathematical computation (that uses latitude/longitude as an intermediate value).

However, converting between coordinate systems that are referenced to different datums involves an additional step, one that models an approximate difference in latitude and longitude for each point.

A key point is that in the process of projecting from latitude/longitude to county coordinates (all within the same datum and adjustment), there is no transformation needed. The raising of the mapping surface from its traditional ellipsoid position (near sea level) up to a place near to the ground surface is a pure mathematical process. However, at the moment, some commercial software might be handling this process incorrectly. The task force seeks to fully inform the vendor community to ensure that coordinate conversions are being supported correctly and uniformly.

WISCON future uncertain
As a sidebar to the concerns of the task force, the future of WISCON software is in question. While there appear to be no concerns over how this package accomplishes its conversions between coordinate systems and datums, further updates to the software and its availability are not clear. The task force may be asked to provide advice on alternatives to the current WISCON arrangement under which the Wis. Dept. of Transportation pays a modest amount to the software developer for code improvements and the State Cartographer’s Office is the sales outlet for other licenses users.

WISCON, of course, is a stand-alone package. It does not handle GIS data and does not interface directly with GPS software. Neither are its functions accessible through a web interface. It also does not support the coordinate system devised by Jackson County.

The membership
To date the task force has the following membership: Ted Koch, State Cartographer (chair), Diann Danielsen (Dane County), John Ellingson (Jackson County), Pat Ford (Brown County), Bob Gurda and AJ Wortley (SCO), David Hart (UW Sea Grant), Mike Koutnik (ESRI-Minneapolis), John Laedlein (Wis DNR), Jerry Mahun (Madison Area Tech. Coll.), D. David Moyer (National Geodetic Survey), Glen Schaefer and Gene Hafermann (Wis. DOT), Peter Thum (GeoAnalytics), Alan Vonderohe (UW-Madison), Jay Yearwood (City of Appleton).

If you have an interest in participating in this WLIA task force, contact Ted Koch at the SCO.