Coordinate Systems


Maps use a range of different coordinate systems. Most map coordinates use two numbers (ordinates) and a fixed reference point (the origin).

The most familiar map coordinates are probably longitude,latitude pairs. This is a spherical coordinate system that identifies points on the globe. The origin is a point off Africa where the Equator (Latitude=0) crosses the Greenwich Meridian (Longitude = 0).

Cartesian coordinates are often used but these are applied to projected map, and not directly to a point on the globe. For example, the British National Grid uses a Cartesian coordinate system in meters with an origin just south west of the Scilly Isles. This coordinate system works in a space projected using a Transverse Mercator projection, and not directly on the Earth’s surface.

Therefore a complete coordinate system is defined with a map projection, a datum, an origin, the orientation of the axes, and the coordinate units.

Luckily we do not usually have to specify all of these parameters, and a simple standardized name is sufficient. These names are typically defined by mapping authorities. In the US, they might be global, national, or at the State level.

The coordinate system is set by overriding the map projection on the Map Properties dialog box. This is displayed by selecting File->Properties on the main menu:

The Map Properties dialog box

The Map Properties dialog box

Clear the Auto Projections checkbox and press the Change Projection button to display the Projection and Coordinate System dialog box. The Coordinate System tab looks like this:

The Coordinate System Tab, set to use the

The Coordinate System Tab, set to use the “Caltrans California Statewide Mapping System” Coordinate System

With the large numbers of different coordinate systems, Maptitude divides them into classes (above, we have selected the class of systems that use the NAD83 datum).  Then select the ‘zone’ (i.e. actual coordinate system), and whether you wish to report the coordinates as geographic longitude,latitude or Cartesian. Here we have selected Cartesian (x,y) coordinates in meters. Here is the result:

California mapped with a California specific coordinate system

California mapped with a California specific coordinate system

Notice that California is better oriented than with the default Maptitude coordinate system. This is because the coordinate system was designed specifically for California.

By default Maptitude displays the current coordinates (e.g. in data views) using geographic (longitude,latitude) coordinates. You can switch to an XY display using the current coordinate system by selecting Tools->Geographic Utilities->Coordinate to XY on the main menu.

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