# Horizontal Datum

Datum: with an ellipsoidal earth, two parameters define the best fitting ellipsoid, but three are used:

• a -- equatorial radius, about 6378 km (we actually know it to sub meter accuracy)
• b -- polar radius, about 6356 km (we actually know it to sub meter accuracy)
• f -- flattening, 1 - b/a, about 1/298 (or 1/300 in round terms)

Any two define the datum and third can be derived, but different datums select different combinations for their definition.

e -- eccentricity; e² = 2f - f² (used in most projection formulas)

Take this shape, and a starting point (now usually defined with coordinates so that Greenwich and equator are on the zero longitude and latitude line, and an orientation), and get lat-long grid

Developed in 19th century, local datums gave best fit for the geoid in area.  At that time they could not do a worldwide fit, and original errors in measurement mean some of these are very different from currently accepted values.

At least a half dozen datums are in wide use, and over 100 have been defined.

With satellite positioning and gravity, true geocentric coordinates and earth shape are possible.

World wide datums: WGS84 for the world and NAD83 for North America, which replaced NAD27; shifts up to several hundred meters in lat-long coordinates if using different datums

The US will soon get 4 new datums, one for the region on each tectonic plate (Caribbean, North American, Pacific, and Phillippine).  Nominally named for 2022, these are being delayed.

Other datum topics:

• How MICRODEM handles datums
• Datum shifts: the change in coordinates for a single point on the earth's surface using two different datums, or conversely the two different points on the earth's surface corresponding to the same set of coordinates referred to different datums.
• Datum caution

Last revision2/28/2022