When deciding what coordinate system to use (or export to) after collecting GNSS data in the field with a spatial reference of Latitude, Longitude, WGS-84, HAE in meters, there’s really no “correct” one.  The decision should be based on how you want to project your data onto a map or in a computer program.  It’s especially important if you are going to send the data to an existing GeoDatabase that is already using a spatial reference.  To make sure that your field data aligns properly with existing data, it’s important to make sure it’s corrected and exported using exactly the same spatial reference as your existing database.

While it’s true that more modern versions of NAD83 are more intrinsically “accurate,” it’s more important to use the datum that matches your existing database perfectly.

The four questions to which you must know the answers regarding the spatial reference of your GeoDatabase (in the United States) are:

  1. What is the coordinate system?
    • Latitude, Longitude
    • US State Plane 1983
    • US State Plane 1927
    • UTM
    • Custom coordinate system
  2. What is the datum (and iteration of the datum)?
    • See below
  3. What is the zone (if applicable)?
    • For State Plane: North, South, East, West or Central (depending on the state)
    • For UTM: North or South with a number
  4. What are the units?
    • International Feet, US Survey Feet, Meters

If you can answer these five questions, then you are armed with all the information you need to get the field data into your GeoDatabase.

The general evolutions of datums are:

  1. United States Standard Datum (based on the Clark 1866 Ellipsoid) 1901
  2. NAD27 (Conus) 1927
  3. NAD83 (Conus) 1983
  4. WGS84 (GPS only) 1984
  5. NAD83 HARN (Conus) 2009
  6. NAD83 (2011) 2011

There are many more iterations of ITRF (International Terrestrial Reference Frame) from 2000 to 2011.  The latest iteration of the ITRF is only a few centimeters different than WGS84.

So, you can see there isn’t really a correct coordinate system or datum to use.  You just need to know where the data is going.


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