Projections and Transformations

Projections

Projections are used internally by Modest Maps providers to convert from Locations to Coordinates.

Internally, the rawProject and rawUnproject functions are the pure mathematics of the operation (we use Wolfram Mathworld and Wikipedia to find interesting projections). These are the only ones that need overriding by subclasses. The project and unproject functions use these, and in turn are used by locationCoordinate and coordinateLocation which are then used by providers. You'll probably just want to use the versions offered by Map.

Modest Maps offers two projections out of the box: LinearProjection and MercatorProjection. The built-in providers all assume Mercator is what you want, but this is easily overridden.

Transformations

Transformations are used internally by Modest Maps projections to convert from one projection space to another, and apply any needed translations, rotations and scales to get everything lined up.

e.g. to transform from a Mercator projected Location in radians to a map Coordinate at zoom level 26, you could use the following Transformation:

var t = new MM.Transformation(1.068070779e7, 0, 3.355443185e7,
    0, -1.068070890e7, 3.355443057e7);

These magic numbers can be derived from three control points using some simple linear algebra. Modest Maps provides helpers for these. For example, to derive a similar Transformation to the above, but for a Coordinate at zoom level 0, use:

var pi = Math.PI;
var t = MM.deriveTransformation(-pi, pi, 0, 0, pi, pi, 1, 0, -pi, -pi, 0, 1);

The Transformation class then offers transform and untransform methods that operate on Points.