A mathematically ideal sheet of paper in a 3d-dimensional space can be addressed with both 2D coordinates, within the reference frame of the paper, and 3D coordinates, within the 3D space, but given that the paper has a location in space, 2D coordinates are sufficient to specify a point in 3D space.
You can do the same with geography. It's why generally only specify geographical coordinates with latitude and longitude, altitude is a given from those two, since you're as unlikely to be hovering in the air as you are to be immersed in bedrock.
You can do the same with geography. It's why generally only specify geographical coordinates with latitude and longitude, altitude is a given from those two, since you're as unlikely to be hovering in the air as you are to be immersed in bedrock.