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May 31 2011

Rescue Your Floating Data

We’ve previously discussed some of the basics of geodesy, ellipsoids and the geoid,  as well as datums and projections. Now, it’s time to pull it all together into what a coordinate system is. Knowing about coordinate systems matters because this is what you interact with inside of a GIS or even CADD currently. Your project will likely have a defined coordinate system that everyone will be working off of and that data will be collected in. If it doesn’t, you should remedy that immediately or face needless time delays and headaches later! Data means almost nothing if you cannot define the coordinate system that it was built in. It’s just points or lines in space apart from its system.

Simply put, a coordinate system is just a reference system with specified datum and projection. Standard coordinate systems use particular projections over zones of the earth’s surface. Each zone then has a specified origin.

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There are 3 distinct types of coordinate systems:

  1. Global Cartesian: Provides coordinates for the whole earth and has to do with how satellites see the earth. Global positioning satellites (GPS) utilize this system.
  2. Geographic: Latitude and Longitude coordinates which allow for a wider reference – nationwide, worldwide. Whenever you’re working in Google Earth, you’re seeing it in the geographic coordinates system, WGS84.
  3. Projected: The most localized coordinate system that utilizes zones which are tuned for the highest accuracy on the ground. The 2 most prominent projected coordinate systems are Universal Transverse Mercator and the State Plane Coordinate System.

Universal Transverse Mercator (UTM)

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UTM is a global system developed by the U.S. military based on the  Transverse Mercator projection and WGS84 for the datum. There 60 zones, each 6 arc degrees wide spanning the earth. Typically, you see this system in meters.

State Plane Coordinate System

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The State Plane Coordinate System is a civilian system developed for defining legal boundaries. Each zone has been defined to give the best representation and are deliberately along county boundaries within each state. East-West states (e.g. Colorado) use the Lambert Conformal Conic projection. North-South states (e.g. Wyoming or Nevada) use Transverse Mercator. Colorado, where I live, has three zones (North, Central, and South) to give accurate representation. This system provides the greatest accuracy for local measurements.

References

State Plane Coordinate System Zones (NAD83) for Google Earth (KMZ)

Coordinate Systems Overview by Peter H. Dana

“The State Plane Coordinate System” Part 1, Part 2, and Part 3 by Dan Doyle

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