Map Unit Coordinate System

MapLink Pro maintains an internal 32-bit coordinate system, so that each (x,y) coordinate is stored internally as a pair of 32-bit integers. This provides the best balance between resolution and performance. The units of this internal coordinate space are referred to as TMCs (MapLink Mapping Coordinates).

TMCs should not be confused with map units, which are the units of the coordinate system applied to the map as a whole. Map coordinates are always presented to the user in map units.

Normally, there is no need to worry about the internal representation of the coordinates, since MapLink Pro takes care of establishing this coordinate space and converting between it and real-world coordinates. However there may be occasions where specific mapping requirements make it useful to understand the concepts involved, and MapLink then provides the user with full control over the establishment of the internal coordinate space.

The 32-bit TMC coordinate space provides a dynamic range of 2^32 (4.295x10^9) between the smallest and largest distances that can be represented in each of x and y. This is generally more than adequate for most purposes.

Consider for example a map of the whole world. The largest distance that needs to be represented on such a map actually depends upon the projection used for the map. In the case of a typical projection (such as Mercator) where the map has a true scale at the Equator, the largest distance that has to be represented in x is the circumference of the

Earth at its Equator, namely 40074 kilometres. As this distance is equal to 4.007 x10^9 centimetres (i.e. less than 2^32 centimetres, it follows that it would be possible to produce a map of the whole world to a precision of 1 centimetre using a mapping of 1 TMC = 1 centimetre.

In practice, few map datasets require the representation of data to such a precision. The Land-Line Plus dataset produced by the British Ordnance Survey uses centimetres as the units of the coordinate space, although the absolute accuracy of the data is more typically in the range 50-100 centimetres.

By default MapLink Pro will set up an internal coordinate space that provides a reasonable compromise between precision and range, bearing in mind that the user may extend the range required by adding more data at a later time.

You may wish to override this default behaviour in the following cases:

If you are constructing a multi-layer map that allows the user to zoom in from a wide area overview down to a much more detailed view of specific areas of interest, where the dynamic range of the data approaches the 32-bit limits.

If you wish to make use of the Optimisation for Compression option for your vector data. In this case, you should define a mapping between TMCs and map units, to ensure that the optimisation does not degrade your data unacceptably.

To set up your own internal coordinate space:

1. From the Map, select Properties | Advanced.
2. Deselect the Automatic option.
3. Define the new origin for the internal coordinate system by entering appropriate values into the Map Unit Shift X and Map Unit Shift Y fields. For example, if the map units are in degrees, you may wish to relocate the origin of the internal coordinate system from its normal position (in the Gulf of Guinea where the Greenwich Meridian crosses the Equator). To move the origin to London (latitude = 51.5°, longitude = -0.2°), enter the following values:

   Map Unit Shift X = -0.2

   Map Unit Shift Y = 51.5

4. Define the new relationship between internal units (TMCs) and map units by entering the appropriate value into the TMC Per Map Unit field. If the map has been projected then the map units will be in metres, so a value of 100 would be entered to define a mapping of 1 TMC = 1 centimetre. If the map has not been projected, then the map units are the units in which the input data is coded.