Green Square commemorative geodetic disk
October 19, 2011 8:05 AM
The Green Square Complex - DENR OfficeBuilding’s dedication ceremony on Thursday, Oct. 13th, was marked with inspiring speeches by dignitaries, a team ribbon-cutting ceremony, and hundreds of people touring the building.
What was overlooked by some and even walked upon by the throng as they entered the building and gazed upward through five plus floors of the glass and steel atrium was the stainless steel commemorative geodetic disk on the floor in front of the reception desk.
The commemorative disk reads:
GREEN square 2011
Latitude (φ): 35° 46’ 56.90792” N NAD 83(NSRS2007)
Longitude (λ): 078° 38’ 31.92629” W NAD 83(NSRS2007)
Elevation: 345.0 U.S. Survey Feet NAVD 88
"GREEN square 2011" was positioned by the North Carolina Geodetic Survey (NCGS), which is a spatial data framework agency within the Division of Land Resources (DLR) of the North Carolina Department of Environment and Natural Resources (NCDENR), to:
· Commemorate the building
· Provide precise and accurate coordinates to land surveyors
· Educate the public on the importance of “geodesy”
Ironically, “geodesy” is an uncommon word, but we benefit greatly from it. Geodesy is the science of measuring and monitoring the size and shape of the Earth in order to establish a spatial reference system that everyone can use. NCGS is responsible for the development and maintenance of the spatial reference system used in North Carolina: the North Carolina State Plane Coordinate System, which is used for surveying, engineering, construction, geographic information systems (GIS), precision agriculture, navigation, communication systems and mapping.
Thus, geodesy allows modern society’s infrastructure (e.g. airport runways, bridges, buildings, freeways, etc.) to be built and function smoothly because surveyors used precise and accurate coordinates and elevation values. The Global Positioning System (GPS) device in your car that tells you how to get from your present location to your desired destination is based on geodesy. GIS also relies on geodesy in order to accurately overlay different layers of spatial information (e.g. roads, pipelines, ecological habitats, housing, pollution sources, etc.) to solve complex interrelated problems.
If you can understand society’s dependence on precise and accurate horizontal coordinates and elevation values, then you can also understand the need for metadata, which is descriptive information about the data. For example, there is a big difference between driving 65 mph on an interstate and driving 65 kph, which is equivalent to only ~40 mph. Spatial data users need to know which horizontal datum was used to reference horizontal coordinates and which vertical datum was used to reference an elevation value. In the case of the commemorative disk, the “NAD 83(NSRS2007)” tag on the latitude and longitude tells the spatial data user that the coordinates were referenced to the North American Datum of 1983 that was readjusted in 2007. The “NAVD 88” tag on the elevation value tells the spatial data user that the elevation value was referenced to the North American Vertical Datum of 1988.
Why are datums important? If you wanted to drive across a bridge that was built using NAD 83 referenced coordinates and NAVD 88 referenced elevation values, but the contractor who built the bridge assumed that the values were referenced on earlier datums [e.g. the North American Datum of 1927 (NAD 27) for horizontal coordinates and the National Geodetic Vertical Datum of 1929 (NGVD 29) for elevation values], the bridge would be off by 76 ft north-south, 79 ft east-west, and almost a foot in elevation. Needless to say, your trip across the bridge would be mighty interesting.
For more information about geodesy, please visit the following websites:
by Curt Johnson