After looking at the need for global equal area projections and a number of different possible projections, we shall now look at the pseudo-cylindrical options. There are a lot more of these, so we cover them in an article of their own.

Pseudo-cylindrical projections are similar to cylindrical projections, but the central meridian is the only meridian that is straight. All other meridians are curved. These are all mathematical projections, with the choice of curve producing different effects. As with the equal area cylindrical projections, the spacings of the parallels (lines of constant latitude) for all of the equal area pseudo-cylindrical projections are chosen to ensure that the equal area property is maintained.

Previously in this series (1, 2), we discussed the different map projections and coordinate systems that can be used for map generation. We also looked at the reasons for the many different projections, and some of the reasons why we might wish to choose a different projection.

In this article, I shall make the case that the choice of projection for most commercial web mapping applications has been made for simply historical reasons. When used properly, the projection is adequate for many applications; it is a very bad choice for applications where spatial distribution and map statistics are important

Later this week in the second and third parts of this article, I shall explore alternative equal area map projections that could be used for these applications.

This two part article is intended as an introduction to map projections and coordinate systems. The second part of the article will be published on Wednesday. They start a new series of articles which I shall be publishing over the next few weeks. I shall look at different projections and coordinate systems, the inadequacies of the current choices, and how to do something about it

Most users of online maps ignore the issues of different coordinate systems and projections. Any concern is usually limited to whether their system of choice will read the coordinates they have. As both are typically the same, the issue rarely surfaces.  However, over the last two millenia, cartographers have produced hundreds if not thousands of different projections and coordinate systems for a wide range of different applications and locations.

Most online map services use geographic longitude,latitude coordinates on the WGS84 datum using a Mercator Projection. Although the coordinate system and datum are both logical choices for global mapping, As you shall see, Mercator is a poor choice of projection for most map applications.

So what are these different datums, coordinate systems, and projections? Why should we care?