When is data created in datums other than WGS84?

Question

I'm trying to understand why we need datums other than WGS84. Since GPS coordinates are relative to the WGS84 ellipsoid what is the advantage of having other datums like for example NAD83. I understand that other datums are locally a closer approximation to the earths topological surface - but when is this advantageous in real life?

I feel the answer may be in this term from the wikipedia entry for Geodetic Datum, but still unclear how you would create location references to NAD83 without a GPS.

"A reference datum is a known and constant surface which is used to describe the location of unknown points on the Earth. Since reference datums can have different radii and different center points, a specific point on the Earth can have substantially different coordinates depending on the datum used to make the measurement. There are hundreds of locally developed reference datums around the world, usually referenced to some convenient local reference point. Contemporary datums, based on increasingly accurate measurements of the shape of the Earth, are intended to cover larger areas. The most common reference Datums in use in North America are NAD27, NAD83, and WGS 84.

The North American Datum of 1927 (NAD 27) is "the horizontal control datum for the United States that was defined by a location and azimuth on the Clarke spheroid of 1866, with origin at (the survey station) Meades Ranch (Kansas)." ... The geoidal height at Meades Ranch was assumed to be zero, as sufficient gravity data was not available, and this was needed to relate surface measurements to the datum. "Geodetic positions on the North American Datum of 1927 were derived from the (coordinates of and an azimuth at Meades Ranch) through a readjustment of the triangulation of the entire network in which Laplace azimuths were introduced, and the Bowie method was used." (http://www.ngs.noaa.gov/faq.shtml#WhatDatum ) NAD27 is a local referencing system covering North America.

The North American Datum of 1983 (NAD 83) is "The horizontal control datum for the United States, Canada, Mexico, and Central America, based on a geocentric origin and the Geodetic Reference System 1980 (GRS80). "This datum, designated as NAD 83 ...is based on the adjustment of 250,000 points including 600 satellite Doppler stations which constrain the system to a geocentric origin." NAD83 may be considered a local referencing system."

Answer 1

When you use a GPS for creating and finding the point again it does not matter as errors balance themselves.

But you need the accuracy for planning anything or plotting courses. Whenever you draw or calculate something in a GIS that you need to find in real world you stumble into this problem. WGS 84 is therefore not very accurate as it is a compromise for the whole earth. It is shperoid based on a gravitational model of the earth. The true geoid differs from this. The difference is especially high in mountain areas. It can reach deviations of several dozen meters at max (without the already discussed problems in polar areas). But the WGS 84 is what GPS uses as it is consistent for the earth and as a sidenote: being only a formula and some constants very easy to put onto small devices. Think of using a highly accurate elevation model on a smartphone...

To get a better accuracy you need the local transformations relative to the WGS 84 speroid. They are created by measuring lots of reference points in the real world and then calculating the differences to the WGS 84 for each small area close to the reference point. The more reference points you measure the more accurate it will be, but also much more expensive.

Answer 2

One important reason is that only the US military, NATO, and other allies or contractors, and other US government departments with clearance have access to true WGS84 coordinates and the control network. Everybody else is getting degraded values. It is true that the National Geospatial Intelligence Agency (NGA prior NIMA prior DMA) states that WGS84 is aligned at least to centimeters with the corresponding ITRFxx reference frame (International Terrestrial Reference Frame, xx = year).

If you can perform real-time kinematics or have other access to control points or are post-processing the data, you can recover that degradation in accuracy but to whatever coordinate reference system (CRS) that the control network is broadcasting. That will be some variant of ITRFxx or the local geographic CRS like NAD83 (2011) (USA) or NAD83 (CSRS) (Canada) or GDA2020 (upcoming in Australia).

The local systems have been derived using the ITRF network, plus the country's control network and usually fixed. This enables surveyors, GIS, etc. to not worry about the coordinates jiggling continuously. USA, in particular, keeps their control network fixed to the North American, Pacific, or Mariana plates to minimize the change in coordinates over time. When there's been enough movement (plate motion, subsidence, glacial rebound, earthquakes), the geodetic agency may do a re-adjustment or calculate a new realization and update the coordinates of the control network.

Several countries are planning to go to dynamic or semi-dynamic CRS which will more closely match what IERS does with the ITRF realizations. This is possible for a few reasons including the ability to obtain data and process it in a timely manner.