WGS84

The WGS84 is an ellipsoid, datum and coordinate system (Archive) that is widely used in cartography, geodesy and navigation fields, including use with Google Maps. WGS84 represents the world with a spherical coordinate system. Such discussions on this topic often revolve around the claim that, since WGS84 provides accurate information and represents the earth as a sphere, it must be evidence that the earth is a sphere.

When assessing these claims it is found that the process is a complex system which pulls information from a large collection of smaller flat maps to provide accurate information to users. The spherical coordinates of the WGS84 are projected onto a map with planar coordinates when performing the transformations and providing geospacial data.

Geographic Coordinate System
A Geographic Coordinate System is a spherical coordinate system which uses longitude and latitude.

From What are geographic coordinate systems? (Archive) by ArcGIS:

Projected Coordinate System
A Projected Coordinate System is a planar coordinate system.

From What are Projected Coordinate Systems? (Archive) by ArcGIS:

Datums
A Datum is an anchor point which ties two systems (geographic and projected) coordinate systems together.

From Datums by ArcGIS (Archive) we read:

State Plane Coordinate Systems and NAD83
State Plane Coordinate Systems are widely used for geographic data, and are based on a flat earth 'plane surveying' methods with simple XY coordinates:

https://en.wikipedia.org/wiki/State_Plane_Coordinate_System (Archive)

Lower down in that article we read that the State Plane Coordinate Systems are associated with the North American Datum of 1983:

The United States Government echoes the same association:

https://catalog.data.gov/dataset/united-states-stateplane-zones-nad83 (Archive)

The Earth is Not Round!
[https://gis.utah.gov/nad83-and-webmercator-projections/ '''The Earth is Not Round! Utah, NAD83 and WebMercator Projections'''] (Archive) By the Utah Automated Geographic Reference Center

Utah's Automated Geographic Reference Center provides an article with a descriptive title of "The Earth is Not Round!" which explains the coordinate systems in use and how the flat earth maps tie into the spherical coordinate systems for data retrieval. NAD83 is the mentioned as the best coordinate system for geospacial data:

NAD83, as we recall above, is based on a planar coordinate system.

Anchor Point Datum
From the 'The Earth is Not Round!' article it is mentioned "The UTM NAD83 projection uses the GRS80 ellipsoid and a center-of-the-earth anchor point as its datum." It has been asked what the ellipsoid and center-of-earth anchor point that is associated with the NAD83 is referring to. If the NAD83 is composed of state plane maps with a flat XY coordinate system, why is there a reference to involvement of a globe model?

The answer to this is that the ellipsoid and anchor point datum associated with NASD84 is necessary to interconnect with other spherical coordinate systems such as WSG84, in order to connect correctly at the correct location and interchange data.

The article explains

Note "real world."

Then it talks about the flat model:

The passage is speaking of an anchor point to connect the two types of systems together.

The accompanying image is the flat map with anchor point:



WGS84 Requires Flat Maps
WGS84 relies on flat projections to give out accurate data.

https://www.gpsworld.com/data-collection-of-wgs-84-information-or-is-it/ (Archive)

As we will recall from the definitions section, a geographic coordinate system is spherical and a projected coordinate system is flat. NAD83 provides a flat coordinate system.

WGS84 itself does not define a projection, and requires a flat coordinate system for operation:

https://www.gpsworld.com/data-collection-of-wgs-84-information-or-is-it/ (Archive)

Take a look at the list of projections for the ARCGIS software:

ArcGIS 10.7.0 and ArcGIS Pro 2.3 Geographic and Vertical Transformation Tables (Archive)

These are the world-wide transformation tables used with ARCGIS, of which NAD83 is one of them. These are all flat coordinate systems and their associated transformations. WGS 84 is taking data from flat systems, because as the "Earth Not Round!" article says, these flat systems that are more accurate and give out more accurate figures.

Spherical Coordinates Projected To a Plane
We find further references that the process actually involves projecting a curved surface of geodetic positions onto a plane:

Caltrans Surveys Manual (Archive)

Geographic (spherical) coordinates are projected to the plane coordinates. Not the other way around. Further, they "must" be projected.

The West Virginia GIS Technical Center (Archive) mentions the same projection of spherical coordinates onto a plane:

We read that the geographic (round earth) coordinates can be projected onto a planar (flat earth) coordinate system to accurately display distances.

The reader should ponder why spherical coordinates are projected onto a plane, and how spherical coordinates projected onto a planar coordinate system could display data and distances accurately if the earth is a globe. If the earth is a globe, the opposite should be true. Projecting spherical coordinates onto a plane should make data and distances more inaccurate. Not accurate.

Classified Coordinate Transformations
Of interest, some coordinate transformations are not publicly known.

http://desktop.arcgis.com/en/arcmap/latest/map/projections/choosing-an-appropriate-transformation.htm (Archive)

Flat Maps as Standard
We find several sources which describe that flat maps are the standard in GIS work.

From https://www.e-education.psu.edu/geog862/book/export/html/1644 we read:

Another comment: https://www.pobonline.com/articles/86585-geodetic-surveying-made-plain