Geodetic Control Monument Coordinates Now Published in Feet
By Richard Morey, Minnesota Department of Transportation
At the request of several Minnesota surveying and GIS organizations, the National Geodetic Survey (NGS) will now publish state plane coordinates for Minnesota’s geodetic control monuments in feet as well as in meters; the conversion will be based on the U.S. Survey Foot. These monuments are used by the surveying and GIS communities, and most users would like the coordinates published in English (feet) units.
For the current datum, NAD83, NGS publishes the coordinates only in metric units until a state informs them which conversion factor the state would like them to use to convert meters to feet. There are two different conversions:
- The international (SI) based upon 1 inch = 2.54 centimeter
- The U.S. Survey Foot based upon 1 meter = 39.37 inches.
While the difference between the two factors is not large, it makes a difference over large distances such as those represented by the state plane coordinate system.
States can specify the conversion factor either through legislation or through a letter from the main surveying and GIS organizations within the state agreeing upon which conversion factor to use. Minnesota had not enacted such legislation, although the preferred conversion factor may be inferred by the fact that M.S. 505.20 specifies that NAD27 coordinates are expressed in “U.S. Survey Feet” and NAD83 coordinates are expressed in “meters.”
In the absence of legislation, the following groups requested that NGS provide Minnesota’s geodetic control monument locations in English coordinates using the U.S. Survey Foot as the conversion factor:
· Minnesota Department of Transportation
· Minnesota Society of Professional Surveyors
· Minnesota Association of County Surveyors
· Minnesota GIS/LIS Consortium
· Minnesota Governor’s Council on Geographic Information
The letter is posted at: http://www.gis.state.mn.us/Policies/NGS_coordinates.pdf
For more information, contact Richard Morey at: email@example.com or 651-366-3504.