Atoll 2.8.2 : Projection and Display Coordinate Systems

Projection and Display Coordinate Systems

In Atoll, you define the two coordinate systems for each Atoll document: the projection coordinate system and the display coordinate system. By default, the same coordinate system is used for both.

A projection is a method for producing all or part of a round body on a flat sheet. This projection cannot be done without distortion, thus the cartographer must choose the characteristic (distance, direction, scale, area or shape) which is to be shown appropriately at the expense of the other characteristics, or he must compromise on several characteristics¹. The projected zones are referenced using cartographic coordinates (meter, yard, etc.).

Two projection systems are widely used:

• The Lambert Conformal-Conic projection: a portion of the earth is mathematically projected on a cone conceptually secant at one or two standard parallels. This projection type is useful for representing countries or regions that lay primarily east to west.

• The Universal Transverse Mercator projection (UTM): a portion of the earth is mathematically projected on a cylinder tangent to a meridian (which is transverse or crosswise to the equator). This projection type is useful for mapping large areas that are oriented north-south.

A geographic system is not a projection, but a representation of a location on the earth's surface from geographic coordinates (degree-minute-second or grade) giving the latitude and longitude in relation to the origin meridian (Paris for NTF system and Greenwich for ED50 system). The locations in the geographic system can be converted into other projections.

Atoll has databases including more than 980 international coordinate system references, a database based on the European Petroleum Survey Group and another one regrouping only France's coordinate systems. Atoll distinguishes the cartographic coordinate systems for projection and either cartographic or geographic coordinate systems for display.

The maps displayed in the workspace are referenced with the same projection system as the imported geographic data files; thus, the projection system depends on the imported geographic file. By choosing a specific display system, you can see (using the rulers or status bars) the location of sites on the map in a coordinate system different from the projection coordinate system. You can also position on the map sites referenced in the display system: the coordinates are automatically converted from the projection system to the display system and the site is displayed on the map.

In Figure 2.5, the French Riviera geographic data file has been imported. The map shows the French Riviera projected using the cartographic NTF (Paris)/France II étendue system (coordinates in metres). On the other hand, site coordinates are stated in the geographic WGS 72 system (coordinates in degrees-minutes-seconds).

Figure 2.5: NTF (Paris)/France II étendue system used with WGS 72 system