What is a Datum?

So what exactly is a datum? If you look in the textbooks, the official definition is:

"A geodetic datum is any numerical or geometrical quantity or set of such quantities that serves as a reference or base for other quantities." (James R. Smith, 1997, Introduction to Geodesy, page 83)

This definition basically says that datums are references. References are simply the "starting points" from which measurements are taken. A reference can be as simple as giving a friend directions to your house by saying, "I live across the street from Bob's house." These directions use "Bob's house" as their reference or datum point. Note that these directions will only make sense to someone who understands your reference: someone who didn't know where "Bob's house" was would still have no idea where you lived.

In surveying a datum is very similar to the above example: simply a point of reference. Oh, if only it were that simple....

The introduction of GPS and GIS has made this one of the most complicated issues dealt with on a regular basis by surveyors. Local jurisdictions including cities, counties, states, utility districts, etc. all require surveys to be referenced to a given datum. The problem arises in that they don't all have the same datum requirements and there are just so many to choose from. Throw on top of that different datum requirements from the multitude of federal agencies we provide surveys for and you begin to understand the confusion that can arise when the word datum is spoken.

Here is just a partial list of datums we work with on a regular basis:

First is the Horizontal datums.

1. North American Datum (NAD) of 1927
2. North American Datum (NAD) of 1983; This one is even more complicated because of continuing adjustments over the years so you get NAD83/86, NAD83/91, NAD83/97, and so on.
3. High Accuracy Reference Network (HARN) and High Precision Geodetic Network (HPGN)
4. And, of course, the always fun "Assumed"

We also have the added complication of having 2 zones within our State Plane Coordinate System. A North and South Zone that cuts through the State from west to east at about Tacoma.


Now we move on to the Vertical datums which are even more complicated and convoluted.

1. National Geodetic Vertical Datum (NGVD) of 1929
2. North American Vertical Datum (NAVD) of 1988
3. National Geodetic Survey Geoid-96
4. USACE Ballard Locks Datum
5. Tidal Datums (varies across the state along tidal shorelines)
6. And, again, "Assumed"

With GPS it gets even more complicated as you begin dealing with geoids, ellipsoids, and earth surfaces. I think another picture is the best way to get this idea across in as few words as possible:

Hopefully you get at least a concept of what we are dealing with whenever we are asked to provide our mapping on datum and why this just triggers us asking the inevitable question: What datum?

Until next time,

Cheers!