Selecting the Correct Datum

Selecting the Correct Datum
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Q: What datum value should I use for my site coordinates in the TAP Fixed Facility database?

A: The datum value you select depends on the source of the coordinates you enter.

Note that all values in this example are for demonstration purposes only. You should determine appropriate settings for your application.

The coordinate datum defines the reference system used for the numerical coordinate values (usually latitude and longitude in TAP).

The coordinate datum is selected with the lookup button (“…”) just below the latitude and longitude values in the TAP Fixed Facility database interface:

You should set the coordinate datum before entering the coordinate values.

When someone gives you a set of coordinates to use for a site location, you should ask for the source of the coordinates:

· If the coordinates are from a map, what was the datum of the map? Most USGS maps that I have used show the datum in the legend info. Older maps tend to use NAD27 (the North American Datum of 1927) and newer maps use NAD83 (the 1983 update). Some reprinted USGS maps show both, with adjustments for the update NAD83 datum shown in purple ink.
· If it was from a GPS, what is the configuration of the GPS? The default is probably NAD83 or WGS84 (see below), but some GPS units may allow the user to select the datum. That would be handy if you were using a NAD27 map and wanted to match the GPS readings to the map.
· If the coordinates are from an FCC license of an existing station, is the datum specified. Older licenses may show coordinates in NAD27, newer ones in NAD83. I don’t recall when the FCC made the change, maybe in the late 80’s or early 90’s.
· If the coordinates are from an FAA tower clearance the same question applies – is the datum specified. I don’t know if the FAA ever switched to NAD83. I do know (from our consulting days, before we became SoftWright) that there was an overlap of time where the FAA required everything in NAD27 and the FCC in NAD83, which, as you can imagine, caused not a little confusion.

The differences between NAD27 and NAD83 can be quite large, up to almost 100 meters in parts of the southwest US. The following image from Wikipedia shows the differences between NAD27 and NAD83 horizontal locations (not the elevations).

That means that specifying the wrong datum could shift a site horizontally to a whole new location, with an elevation quite different from the actual elevation of the desired site. So TAP lets you specify the datum of the coordinates you are using. Then the software takes care of the internal detail that some topographic elevation data files are based on NAD27, some on WGS84, etc. Any translation that is necessary takes place down deep in the software without you needing to worry about it.

The differences between NAD83 and WGS84 are on the order of one meter, negligible for the purposes of TAP. And since WGS84 is pretty much the default for most applications (like Google Earth), if in doubt and if the source of the coordinates is anything in the last couple of decades (maps, GPS, etc.), WGS84 is usually a good choice. WGS84 is the default for new sites you add in TAP as well.

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