Distances and bearingsUsing GPSManDatums and ellipsoidsProjections and coordinate grids

Projections and coordinate grids

Selecting and defining projections

Selecting the map projection is done by using the second (from the left) menu-buttons on the bottom of the map window if the map window is the main window, or the Projection menu-button if the lists window is the main window.

If a background image is to be loaded the projection and the datum should be set to the projection and datum used in the image (see above for the details on this). If there is no image, the map projection should be selected according to the map scale and the geometry of the region to be covered.

Projections can be either pre-defined or user-defined. There are a small set of pre-defined projections. Some of them admit particular cases, in the sense that they have parameters whose values can be fixed. The user may define such particular cases along with a coordinates grid associated to it.

Each projection has an associated coordinates grid that will be used as default position format for displaying the map cursor coordinates and when a waypoint is created from the map. This position format can be changed from the map window (menu-button near the cursor coordinates).

When defining a projection, the user may also define a new coordinates grid User-defined grids cannot have more than one zone.

User-defined projections and grids are automatically saved in a file in the GPSMan user directory, and will be loaded when GPSMan is started.

To define/change a projection there are the appropriate entries under the Definitions menu-button.

When defining a new projection, which is necessarily a particular case of a general projection, the user must select first the general projection to use, along with a name and short name. The short name is for internal use and will also serve as the coordinates grid name, if the user associates one to the new projection. The values of the projection parameters must be then given. The user may either associate to the new projection an existing grid, or create a new grid by selecting a distance unit (currently either metres or feet), by giving the values for the false easting and northing (for some projections these parameters are in fact the easting/northing of the false origin or of the projection centre), sensible bounds to the coordinates, and by choosing whether or not a fixed datum must be used with the grid. The bounds given will be used to check that the grid is not used outside its intended scope. All values of latitudes and longitudes must be given either in the datum of the grid if there is a fixed one, or in the datum being used for the map.

An user-defined grid cannot be forgot if it is currently associated to another projection or in use for displaying the map coordinates. Changing the definition of a user-defined grid may cause inconsistencies in previously projected data.

Pre-defined projections and grids

With the UTM/UPS (Universal Transverse Mercator/Universal Polar Stereographic) projection a single UTM zone is used, that of the first point displayed. Points in different zones will be projected into the same zone what may produce some deformation. There are no parameters that can be changed by the user.

The Transverse Mercator projection, also known as Gauss or Gauss-Kruegger projection, has 3 parameters: the latitude and longitude of the centre and the scale factor at the central meridian. The first two are computed as the averages of the latitudes/longitudes of the first points being mapped, while the third one has the default value of 0.9996 (used for UTM).

Particular cases of the Transverse Mercator projection are used in several maps. GPSMan pre-defines the following ones:

The Lambert Conic Conformal projection has two variants: single standard parallel (named Lambert Conic Conf 1 in GPSMan), and two standard parallels (named Lambert Conic Conf 2 in GPSMan).

The former has 3 parameters: the latitude and longitude of the centre and the scale factor at the natural origin. The first two are computed as the averages of the latitudes/longitudes of the first points being mapped, while the third one has the default value of 1 (corresponding to a tangent cone; a value of less than 1 stands for a secant cone).

The latter has 4 parameters: latitudes of the two standard parallels (along which the cone intersects the geoid) and of the false origin, and longitude of the false origin. The first two default to the extremes of latitudes of the first points being mapped, and the position of the false origin defaults to the average of the positions of these points.

The Mercator projection can be defined as a Lambert Conic Conformal projection either with the equator as its single standard parallel, or with the two standard parallels at equal North and South latitudes (i.e., symmetrical with respect to the equator).

This leads to two variants: single standard parallel (named in GPSMan Mercator 1), and two standard parallels (named in GPSMan Mercator 2).

The former has 2 parameters: the longitude of the centre and the scale factor at the natural origin. They are taken as the average of the longitudes of the first points being mapped, and as 1, respectively.

The latter has 3 parameters: the latitudes of one of the two standard parallels and of the false origin, and longitude of the false origin. The first default to the maximum of the absolute values of the latitudes of the first points being mapped, the position of the false origin defaults to the average of the positions of these points.

The Stereographic projection is an azimuthal conformal projection used both for large scale and small scale mapping. There are 3 possible aspects: polar, oblique and equatorial, which are dealt with automatically by GPSMan. A particular case of this projection is the Universal Polar Stereographic that is used in the UTM/UPS.

The Stereographic projection has three parameters: the latitude and the longitude of the centre (tangent point) and a scale factor. By default the scale factor is 1 and the coordinates of the centre are taken as the average of the latitudes of the first points to be mapped.

The Cassini-Soldner projection is a neither conformal nor equal-area projection used in the 19th century. It is still used for mapping areas with a small E-W extent. Scale is true along a central meridian and distortion increases significantly with distance from it. It has two parameters: the latitude and the longitude of the natural origin. These parameters are taken as the averages of the latitudes and longitudes of the first points being mapped.

The American Polyconic projection is also a neither conformal nor equal-area projection used before the computer era. It has a single parameter: the standard latitude, whose default value is taken as the average of the latitudes of the first points to be mapped.

Some national grids

The following information gives the parameters needed for defining some national grids. The coordinates given as parameters depend on the datum used in the definition; if there is none, GPSMan uses the datum currently selected for the map. This means that the coordinates given below must be converted if a different datum is used.

The Iceland grid is based on the Lambert Conic Conformal projection with 2 standard parallels at N64.75 and N64.25 degrees, a false origin at N65, W19 degrees, using distances in metres, a false easting and a false northing of 500km, and the "Hjorsey 1955" datum.

The Netherlands grid uses the Stereographic projection, centred at N52.15616, E5.38763333333 degrees (this corresponds to the Amersfoot OLV church, N52.15517, E5.38720 in the "WGS 84" datum), a scale factor of 0.99991, distances in metres, a false easting of 155km and a false northing of 463km, and the "Rijks Driehoeksmeting" datum. Acceptable ranges of values are: 0-290000 for x, 290000-630000 for y, 50.3-53.45 for latitude, and 3-7.45 for longitude.


GPSMan User Manual
©Miguel Filgueiras, mig_at_ncc.up.pt, DCC-FC & LIACC, Universidade do Porto, 2002

Distances and bearingsUsing GPSManDatums and ellipsoidsProjections and coordinate grids