# Defining Latitude and Longitude before defining Datum?

Reference material:

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?

• I don't think the ingredients were meant to be in any order of precedence. You need all five in order to begin mapping. Listing them using ordinals may be somewhat misleading. – Octopus Sep 19 '18 at 18:46

Latitude and Longitude are defined upon an ellipsoid.

Essentially you can have different lon/lat. NAD 27 lon/lat and WGS84 lon/lat will have subtle differences. As a rule, when people nowadays refer to lon/lat the mean WGS84 datum and WGS84 ellipsoid.

(The example is extracted from the eye opening book of PostGIS In Action from Regina O. Obe and Leo S. Hsu.) So back to your question and numbering of thigs:

1. You start by modeling the earth using some variant of a reference ellipsoid, which should be the ellipsoid that deviates least from the geoid for the regions on earth you care about. The most common ellipsoids are GRS80 (specially used in North America) and WGS84 (used by GPS systems).

2. You use a datum to pin the ellipsoid to an actual place on earth and you assign a coordinate reference system to the ellipsoid so you can identify every point on the surface.

• So, the first point is basically defining an ellipsoid? – Tarun Maganti Sep 19 '18 at 12:48
• If so, isn't this: `Firstly, a reference system which is used to describe the locations of points on the surface of the Earth.` also datum? Because you are locating or pointing the points on the surface of the earth. – Tarun Maganti Sep 19 '18 at 12:49
• Indeed. I like to use the analogy of an egg and a stone. First you select the egg (ellipsoid) that covers your irregular stone(geoid). Then you select how you place this egg by defining the center of its axes (datum) and then you assign the coordinate system. – Roberto Zeeland Sep 19 '18 at 12:59

In the context above, datum should be understood as the local adjustment of an ellispsoid. The ellipsoid are approximations of the global geoid (geoid = the "true" shape of the Earth) and you can define Latitude and longitude on each Ellispoid. However, locally, the ellipsoid can be far away from the actual surface of the ellispoids, which may results in "less good" map projection. You will then define a datum (rotation, shift and rescaling of the ellipsoid) where you redifin the Latitude and Longitude according to this transformation and selected lat/long of origin. For global coordinate system, the datum can be the same as the ellipsoid (e.g. WGS 84 (datum) nealy fits to GRS80 (ellipsoid))

• Why isn't it enough to develop an ellipsoid which fits the local space instead of adjusting the original one to fit the "local" area? – Tarun Maganti Sep 19 '18 at 12:46
• because it is easier to transform the coordinate systems from one datum to another datum if they share the same "globally optimal" ellipsoid. Converting from one datum to another is always tricky, so if you don't know what is the ellipsoid being used, you will first need to compute a system of equation to transform to one of the reference ellipsoid (and you will end up defining this datum based on the reference ellipsoid that you selected for this computation). – radouxju Sep 19 '18 at 13:05