STAR*NET V6 - Circe

Chapter 10 Adjustments in Grid Coordinate Systems
10.13 Custom “County” Grid Systems
Some states, Wisconsin and Minnesota to name a couple, are designing special County
Coordinate Systems. The state is broken up into small grid planes, each containing one
(or in some cases two) counties. The reason for designing these custom grid planes is to
create local reference systems that minimize the differences between actual ground
surface and calculated grid distances. These coordinate systems are mathematically
based on the National Geodetic Reference System. To minimize the ground and grid
distance differences, the ellipsoid surface is elevated to the median or most-common
ground level in the county. As a result, most “surveyed” ground distances and “record”
grid distances will be, for all practical purposes, the same. In Wisconsin, for example,
the county coordinate systems were designed to provide a maximum grid scale distortion
of 1:50,000 in urban areas.
Custom county coordinate systems are designed by the individual states or counties, and
grid parameters must be acquired from them. To create custom county grid zones in the
STAR6.CUS file, follow these two steps:
1. As described in the previous section, “Defining Custom Grid Zones,” edit into the
custom file a projection type code, your choice of a custom system name, the name
of the ellipsoid, and the supplied coordinate system grid parameters.
2. Append to each line an “Ellipse Modifier” value. This is the distance (in meters) the
ellipsoid surface is to be elevated so that it will pass through the design elevation of
the county coordinate system. This value is prefixed by an “A” or “B” character. The
meaning of this code, and the determination an actual value for this ellipse modifier
is discussed on the following page.
Below are example lines added to the STAR6.CUS file to define two Wisconsin county
grids, one a Transverse Mercator projection and the other a Lambert projection.
# T Zone Name: Ellipsoid: LatO CM FN FE Scale
# B Zone Name: Ellipsoid: LatO CM FN FE LatS LatN
#-----------------------------------------------------------------------------
T Calumet: GRS-80: 42.4310 -88.3000 0 244754.8895 0.999996 B280.44
B Burnett: GRS-80: 45.2150 -92.2728 0 64008.12802 45.4250 46.0500 B277.96
And below are the published parameters for the Burnett County coordinate system, an
NAD83 Lambert projection based on the GRS-80 ellipsoid:
Latitude of Origin: 45-21-50 South Latitude: 45-42-50
Central Meridian: 92-27-28 west North Latitude: 46-05-00
False Northing: 0 Avg Geoid Height: -26.84 meters
False Easting: 64008.12802 meters Design Elevation: 308.80 meters
The “Ellipse Modifier” attached to the end of the grid definition line is determined using
the geoid height and design elevation values as described on the next page.
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