What is NAD83 and what does it mean for Davis County?

Among the duties of the county surveyor is to publish bearings and distances between corners of the government survey (Section, Quarter Section, and Meander corners) and to publish the spatial relationships of these corners. Though advisable, there is no statutory requirement for the data to conform to any quality standard or modern survey techniques such as GPS. However, it is reasonable to assume that given the end users of this data (property owners by way of private practice surveyors) that the data should conform to a current level of accuracy and usefulness.

Do you remember the world globe we learned geography from in grade school? Imagine that globe delaminated and laid out on a flat surface. The North American Datum of 1983 (NAD83) is an Earth centered map datum consisting of the GRS80 ellipsoid and a definition of a latitude and longitude origin. State Plane Coordinate System 1983 (SPCS83) coordinates are points projected onto a flattened plane of the GRS80 ellipsoid. A good application of this system provides users an interface to toggle between grid and ground conditions.

Why is this significant?

Turning angles and measuring distances (terrestrial methods) returns coordinate data based on the assumption that the project area is entirely upon a flat planar surface. The spatial distortion from this practice exists at a manageable level if the project area is kept small enough. The zone definitions for SPCS83 coordinates are too wide for practical survey accuracies in 2010. The distance and angular distortions are too large.

The case for Low Distortion Projections

State plane coordinates are projected onto a plane that nearly approximates sea level, but we want to know coordinates at 4,200 feet above sea level or slightly more for Davis County. We are not concerned what the distances would be on a sphere 4,200 feet closer to the center of the earth.

The state plane coordinate system allows for distortion due to the zone definitions to reach but not exceed a ration of 1:10,000. This is an introduced inaccuracy of roughly ½ foot per mile. Additionally, the distortion in angles at the extreme limits of the Utah State Plane Coordinate zones is unacceptably high.

The solution I use in my practice and which will be commonly supported in the near future by the National Geodetic Survey, is a system of low distortion projections. A low distortion projection will typically be based on the GRS80 ellipsoid, a latitude and longitude definition similar to NAD83 and a scale factor which will place the coordinate plane near the working area and plus or minus about 400 feet in elevation within the zone. The target for maximum distortion in an LDP is 1:50,000 but in many cases can be 1:300,000 or better depending on the elevation changes within the zone.

Low distortion projection zones are kept very narrow compared to UTM or SPCS zones. Consequently, the convergence between grid bearings and true north is kept reasonably small and consistent with a horizontal distortion ratio better than 1:50,000. A Low Distortion Projection must of necessity define an arbitrary origin such that it will not be confused with SPCS83 coordinates.