The Five Key Ingredients for Mapping the Earth
There are five important steps to producing a map of the Earth's Surface. These are: sphere with north and south through the middle:
Firstly, a reference system which is used to describe the locations of points on the surface of the Earth.
For maps of large areas of the Earth this would be latitude/longitude and for simpler ‘sketches’ or maps of local areas this would be a grid. Some more complex maps may use both latitude/longitude and a grid reference system. Spheroid Diagram
Secondly, a Datum which defines the geometry/mathematics of how to transfer this coordinate system onto the surface of the Earth.
This is necessary for all maps of large areas of the Earth, but may not be needed for simpler ‘sketches’ or maps of local areas where features are able to be visually located relative to each other.
A feature of the modern electronic era is that many map users are now also using Global Positioning Systems (GPS) when they are using maps. An interesting impact of this is that more and more maps are being forced to use the same datum as the GPS system (even ‘sketches’ and maps of local areas maps) so that the reference coordinates on the map agree with the coordinates shown on the GPS receiver. Surveyor using surveying equipment.
Thirdly, a measurement system which is used calculate the coordinates of points on the surface of the Earth. World map.
Fourthly, a projection which allows the coordinates, which have been calculated using a measuring system, to be displayed on a flat piece of paper. Colourful timezone map of Australasia.
Fifthly, the Map Maker’s Art.
Question: My perception of Latitude and Longitude is that:
Since, earth is wobbly in shape, we define a datum with a particular center and particular shape. Using that datum as a reference, write down latitude and longitude into that framework.
But according to the given link and the snippet of information presented, latitude and longitude are defined first and then a datum is created to transfer the coordinate system onto the surface of the earth.
The confusion now is then, on what basis/surface/reference the latitude and longitude are defined upon?