|
![]() |
![]() |
![]() |
The laws of spherical and Euclidean geometry show that a map projection cannot be achieved without loss
of information. There are two main types of map projection proposed by the geodetic sciences. Both of them
conserve just one characteristic; the characteristic of orientation or the characteristic of the surface.
The conformal projection respects the relative directions; all shapes are preserved by respecting the angle between
two directions. The equal-area projection represents accurately the area in all regions of the sphere. None of these
projections conserves the real distances.
Topographic mapping is established generally according to the conformal projection. The projection mode is chosen in
the
way that on the local scale the distances are preserved within an acceptable error. There are a lot of different projections.
The most common are the cylindrical projection and the conic projection. In this lesson, the basic elements are presented
for students who have no training in this field. Generally, the objects of the ellipsoid are projected orthogonally,
first on a reference sphere, then on a plain surface, the cylinder or the cone. The schemas of the different projections
consider that the reference surface is already a sphere. A number of projection variants have been proposed. In the
following, the main principles applied will be explained. The national georeferencing is generally expressed in meters,
at an origin which avoids ambiguities between longitude and latitude.
The figures above represent two possibilities of cylindrical projection. The cylinder on the left side represents
the Transverse Mercator projection. Here, the line of tangency coincides with any chosen meridian. This projection is
also called UTM (Universal Transverse Mercator) projection. For each meridian, a new cylinder
is defined. In order to hold the linear deformation in every zone acceptable, narrow zones of 6° of longitude are chosen.
This system divides the earth into 60 zones (cf. illustration below). The central meridian devides the zones 30 and 31
counted towards east. The illustration on the right shows the tangency coinciding with the great circle. This projection
is called oblique Mercator projection.
The Lambert conformal conic projection
is a conic map projection. Here, a cone is superimposed over the sphere of the earth. The cone builds the tangency line
that coincides with the parallels secant. This map projection is adopted by France. Four cones were necessary: three
on
the continent, one on the island of Corse. The meridian of origin, which is the meridian of the obervatory in Paris,
became the y-axis.
Georeferencing is the basis for mapping. The georeference defines the spatial reference systems. There are also other reference systems known. They are combined with a spatial reference system mentioned above. Among them, the postal reference system, the regional reference system and the roads reference system should me mentioned. The continental European postal reference system is based on numbers which identifies the postal office responsible for the postal mail delivery. All the numbers are assigned to a geographic region, in order to optimize the distribution. Any person resident in the same region is assigned to the corresponding number. This kind of georeferencing plays a big role in "geomarketing". In market research, a correspondence between the social strata and the postal zone, in order to elaborate specific publicity, is established. The North American postal reference system is different. The space is devided into a regular grid (Theriault 1994). The road reference systems allow the unequivocal location of any events happening on the road network. Again, there are differences between the countries.
![]() |
![]() |
![]() |
![]() |