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I'm new at this (studying for a masters of GIS) and trying to understand the WGS84 geographic coordinate system as presented in ArcGIS Pro.

WGS84 is a datum and reference ellipsoid EPSG 4326 is WGS84 using latitute and longitude as a coordinate system The data is have is being represented in ArcGIS Pro in WGS84 with lat/lon coordinates, so really EPSG 4326.

However, WGS84 is an ellipsoid, while the representation I see in ArcGIS Pro is flat. So the coordinates are being projected in some way. My lecturer assures me that there is no projection. When I use the "measure distance" tool the line between two points is curved since it's a geographic system. I get that, that part is fine. But the map is still flat, so I keep insisting there has to be some sort of projection. If there was no projection I'd be looking at an ellipsoid.

What am I missing?

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If those lat-lon coordinates that you see make a perfectly square degree grid everywhere, and if 1 degree of longitude (E-W) is the same visual size as 1 degree of latitude (N-S) at the equator and at the poles, then the projection is "Plate Caree" (aka "equirectangular" https://en.wikipedia.org/wiki/Equirectangular_projection). The EPSG:4326 coordinates are "projected" to Equirectangular by simply scaling the value in degrees up or down by a constant to fit the size of your map canvas.

If you look at Google Maps (in 2d mode, switch off the 3d sphere view) or OpenStreetMap and zoom out you'll see the lat-long boxes aren't square because the data is being shown in a Web Mercator projection. If your ArcGIS Pro map looks more like those, then ArcGIS Pro is projecting your data from its stored EPSG:4326 Coordinate System to EPSG:3857 Web Mercator on the screen. This is a non-constant stretching of the latitude - with the poles getting distorted and stretched so much that you can't show the N or S pole in Web Mercator - they're at +/-Infinity.

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