Notes on Map Geometry and Projections

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MapWindow Users are not required to be experts in Geodesy, Mapping science or Coordinate computation. However, once in a while, information from these fields can be helpful.

Here an attempt to start wiki pages covering these topics in a simple way.

Geodesy, the Science of measuring the Earth

Map Projections

Coordinates and Coordinate Transformations

Problem Overview

When a view of Earth or part of it is required which reaches beyond the horizon, truthful reduced models of it will be needed, as for example provided by maps.

Mapping the Earth is not a trivial process.

Consider that Earth's outer surface is in rough approximation spherical, like a ball. Creating a truthful model of Earth will only be possible in creating a reduced spherical model of it. But, who wants to carry a globe in his/her hip pocket? Flat paper or computer screen maps are much easier to handle.

And here lies the problem: How to flatten out a curved surface and still be truthful?

This will not be possible at all. All we can expect is to create a flat map with geometric properties we need and with distorted properties we don't need or to keep any distortion small enough to be neglegible. (For the latter, the shear size of Earth is helpful).

In order to start this flattening process (Map Projection) we need to know two things:

1. the SIZE of Earth and

2. the SHAPE of Earth as exact as possible

Geodesy is the science which delivers these two aspects through measurement and computation.

Once the size and shape of Earth is known to acceptable accuracy, this information can be used in the flattening out process: Map Projection.

Flat maps have been in use for several thausends of years. That means Map Projections have been used with and without knowledge of Earth's shape and size for various reasons:

Today, all these map types are still in use and all are more or less based on the knowledge of Earth's size and shape empoying in their creation more or less rigid and accurate Map Projection mathematics based on positional data, e.g. Coordinates.

Since Earth does not carry an inherent natural positioning reference system (coordinates like latitude and longitude), assumptions have to fixed by common agreement as to how and from where to reference positions:

This page may contain inconsistencies and errors. Please correct or complain to --Gngdowid 04:53, 25 December 2009 (UTC)

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