Datum & Projection

Datum

The Earth is formed sort of a planar sphere. This form is named associate ellipsoid. A data point could be a model of the planet that's utilized in mapping. The data point consists of a series of numbers that outline the form and size of the ellipsoid and it's orientation in house. A data point is chosen to grant the most effective doable suited truth form of the planet.
There area unit an oversized range of datums in use. several of them area unit optimised to be used in one explicit a part of the globe. associate example is that the geodesic 1949 data point that has been utilized in New Seeland. Another example, acquainted to GPS users, is that the WGS-84 data point. WGS-84 is associate example of a data point that's used globally.
Latitude and meridian area unit unremarkably wont to talk over with a selected location on the surface of the planet. it's vital to stay in mind that latitude and meridian area unit perpetually per terms of a data point. The latitude and meridian of your current position area unit completely different for various datums. for instance, the Cathedral in central urban center is at 172.63658E, 43.53103S in WGS-84 coordinates and 172.36344E, 43.53270S in geodesic 1949 coordinates. If you're operating with latitude/longitude coordinates and you get a blunder of one or two of hundred metres, you presumably area unit victimisation the incorrect data point.
New Seeland is dynamic to a brand new frame of reference for general mapping. New Seeland crosswise Mercator's projection (NZTM) is replacement New Seeland Map Grid, for general mapping. NZTM uses a brand new official NZGD2000 data point. Unless you're a mapping or survey skilled, NZGD2000 and WGS-84 may be thought-about to be constant.

Projection

The Earth is arched and maps area unit flat. the method of flattening out the planet onto a flat piece of paper or display screen could be a calculation known as a projection. despite however you are trying, the ensuing maps perpetually have distortions. There area unit four forms of distortions that map manufacturers contemplate
• Conformality - area unit the shapes of the map options correct
• Distance - area unit the distances measured on the map correct
• Area - area unit the spaces painted on the map proportional to their true area on the planet
• Direction - area unit directions between points on the map accurately painted
There area unit several kind of projection. Some score well on one or a number of the on top of attributes et al area unit a shot at obtaining the most effective compromise on all four. Take a glance at the subsequent map of the globe, created victimisation the standard Mercator's projection.

Compare it with the subsequent map created with a Mollweide projection. investigate the relative sizes of continent and island in each maps.

Africa is fourteen times the scale of island. The Mollweide projection accurately represents areas, whereas the Mercator's projection exaggerates the areas as you catch up with to the pole. However, the Mollweide projection shears the shapes as regards to the poles.
The angle equal projection provides a compromise between conformal and equal-area projections.
There area unit several map projections in use. If you are interested, take a glance here for a way a lot of in depth and technical introduction to projections.