Projections and coordinate systems

The Earth can be represented by various models, and all of them provide a different coordinate set for every point on the surface of the Earth. The simplest model assumes that the Earth is a perfect sphere, although we know it isn't. And as more measurements of the Earth have been accumulated, consequently, the models of Earth have become extremely sophisticated and accurate. There are models that have different data that apply to different areas of the Earth, which provides increased accuracy. NAD83 is a US measurement, and WGS84 (World Geodetic System) is for worldwide measurements.

There are clear but very minor differences between NAD83 and WGS84, besides one being worldwide and the other being US-based. WGS84 uses the WGS84 ellipsoid and has an error margin of less than 2 cm from the center of mass, whereas the NAD83 uses the Geodetic Reference System (GRS80) ellipsoid with an offset of about two meters. NAD83 uses points over the Northern American plate that do not change, and WGS84 uses points with respect to the average of stations all over the world and are not fixed. NAD83 has not changed since its inception; WGS84 has been revised to use a deviation of 105 m to +85 by the new EGM96 geoid. WGS84 is used by the US DOD (Department of Defense), and NAD83 is used by many other agencies within governments.

Although WGS84 and NAD83 have different parameters, the results have negligible effects during surveys. For example, the semi-minor axis has a difference of 0.00010482 between WGS84 and NAD83, and the inverse of flattening has a difference of 0.000001462 between the two. To better showcase this, let’s look at a comparison of the US map from the results of WGS84 and NAD83: