What Is The WGS84 Datum?
From Coloradohikingtrails
To better understand what the WGS84 datum means, it helps to think about how we define a location (i.e. a latitude and a longitude) on the Earth's surface:
The Earth is not flat, and the Earth is not a sphere. So scientists and geographers use an ellipsoid, which is a simplified model of Earth’s shape. A datum is a set of reference locations, including elevations, describing more precisely the surface of the Earth.
There are several common datums still in use:
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NAD27 (North American Datum of 1927): A system derived from land-based surveys, using the Clarke (1886) ellipsoid.
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NAD83: A satellite-based system using the center of the earth as a reference point using measurements taken in 1980, and adopted internationally as GRS80 (geodetic reference system 1980).
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WGS84 (World Geodetic System 1984): A refinement of GRS80 used by the US military (and therefore by all GPS systems).
Each of these reference systems uses a different ellipsoid. The differences between these models, considering the size of the earth, are very small (no more than about 70 meters). But if you are trying to find a specific location, and you are using two different datums, or don't know what datum you're using, it will impact your success.
A little history on how these datums came about helps to understand them:
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NAD 27 - The System Prior to 1986
The North American Datum 1927 was established long before we had any ability to locate the center of mass of the earth by tracking satellite orbits.
The ellipsoid selected as a best-fitting surface for North America was the Clarke 1866 ellipsoid, and astronomic observations were used to position and orient it at a point in the central U.S. Technically it's at Meades Ranch in Kansas <a target="_blank" href="http://maps.google.com/maps?ll=39.224079,-98.541807&spn=0.3,0.3&t=k&q=39.224079,-98.541807" title="Meades Ranch, Kansas" class="external text">39°13′26.686″N,</a> <a target="_blank" href="http://maps.google.com/maps?ll=39.224079,-98.541807&spn=0.3,0.3&t=k&q=39.224079,-98.541807" title="Meades Ranch, Kansas" class="external text">98°32′30.506″W</a> (39.224079, -98.541807). This is at or very near the geographic center of the forty-eight contiguous U.S. states).
Survey control networks extended out from the Meades Ranch datum point all across North America to provide access to what became known as the NAD27 reference frame.
Considering the available technology, this was a very good system, but with the advent of new technologies it became readily apparent that a more accurate system was needed.
NAD 83 - The System from 1986
NAD83 is the spatial reference system currently adopted for most applications in the United States. The NAD83 reference frame was originally produced by a major mathematical readjustment of all the control networks in North America.
NAD83 vs. WGS84
The U.S. Military operates the Global Positioning System (GPS), a constellation of satellites that is the prevalent navigation tool, both now and for the future. The Global Positioning System (GPS) has its own reference frame called the World Geodetic System 1984 or WGS84.
When the NAD83 reference frame was originally computed, every possible effort was made to keep it exactly the same as the WGS84 reference frame used by the GPS. The two reference frames were essentially equivalent at that time. Since that time, the center of the Earth's mass has been more precisely determined and as a result, the point of origin of the WGS84 datum has been moved about 2 meters from its original location. NAD83 did not move along with it. The bottom line is that the NAD83 datum and WGS84 datum are no longer co-incident as they once were. But for most purposes, they are close enough (the accuracy of your GPS Receiver (GPSr) will probably introduce more error than the difference between WGS84 and NAD83.
So Why Do I Care?
In your day-to-day life, you don't (unless you are a cartographer). But if you are using coordinates (latitudes and longitudes) from a GPSr or from a web site, and the coordinates are based on a different datum than the one your map or GPSr is using, enough error can be introduced into your measurements to make the difference between finding something or not.
For most purposes, NAD83, GRS80, and WGS84 are equivalent, so any differences between these datums can be disregarded. But the difference between coordinates using the NAD27 datum and coordinates using WGS84 (or NAD83) can be tens or even hundreds of feet. (The difference between the two datums depends upon your distance from the central datum, so you can't just add or subtract a number to adjust for the difference.)
The problems start to arise when you are trying to locate your position on a map, or someone gives you the latitude and longitude of a point you want to go to, but you are using different datums. Here's some examples of how they can cause you problems:
- You're out on a hike, and are not sure that you are on the correct trail. You take a GPS reading to get your coordinates, and then plot the latitude and longitude of your coordinates on the map you are carrying to see where you are. But the point plotted on the map is on one side of a nearby river, but you're sure that you are on the other side of the river. So now you are even more confused about where you really are. The problem is, the map uses the NAD27 datum. But your GPS is set to WGS84.
- You decide to try your hand at geocaching. You find a nearby geocache at <a target="_blank" href="http://www.geocaching.com">www.geocaching.com</a>, enter the coordinates in your GPS, tell your kids to find trinkets to add to the cache, load up the wife and kids in the SUV, and head out. Three hours later, you return home frustrated and tired, because you couldn't find the cache. What happened? The coordinates from geocaching.com used the WGS84 datum, but your GPS was set to NAD27 when you entered them as a waypoint.
- A friend gives you the latitude and longitude of an old ruin that you would like to visit. You enter these as a waypoint in your GPS, and navigate to these coordinates - and no ruins are to be found. Your friend criticizes your navigating skills, but in fact his GPS was set to NAD27, and yours was set to WGS84.
How do you keep these mistakes from happening?
- Make sure you know what datum your map uses. Many maps, such as those by Trails Illustrated, still use the NAD27 datum. If your GPS is set to a different datum, any coordinates you measure on your GPS and attempt to plot on the map may be wrong (the amount of the error is dependent upon where you are located). With most all GPS Receivers (GPSrs), you can change the datum used for displaying coordinates in the unit's setup menu. All GPSrs use WGS84 internally, but by changing the display datum, it converts the coordinates to the datum you choose before displaying them. So if your map is based upon NAD27, change the datum on your GPSr to NAD27.
- If you get latitude and longitude coordinates from a friend (or from a geocaching web site, hiking web site, etc.), make sure you know what datum the coordinates are based on. Set your GPSr to that datum before entering them as a waypoint. Once you enter the waypoint, you can change the datum on the GPSr; it will internally adjust all stored waypoints to the new datum. For example, you are given coordinates for a geocache. You confirm that they use NAD83. So you set your GPSr to NAD83 (or WGS84, they are effectively the same), and enter the waypoint. But you want to use a map to help navigate to the geocache, and the map uses NAD27. So after entering the geocache waypoint, change the datum on your GPSr to NAD27. It will internally change the waypoint coordinates to NAD27, and all coordinates you measure with your GPSr can be accurately plotted on your map.
Here at ColoradoHikingtrails, we use WGS84. So as long as you use WGS84 or NAD83, you should be able to accurately navigate to the coordinates listed on this site. If you use a map that's based on NAD27, just make sure to change the datum on your GPS after you enter the coordinates.
