GPS Manager User Manual -- Projections and coordinate grids

|  |  | Projections and coordinate grids

## Projections and coordinate grids

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.

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.

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 Portuguese Military Maps projection, used in maps published by the Portuguese Army Geographic Institute. Parameters: central latitude `39.66666666666667`, central longitude `-8.13190611111111`, scale factor `1`. The datum to be used is called "Lisboa". Military coordinates in these maps correspond to a false easting of 200km and a false northing of 300km.
- the German Grid projection (GKK: Gauss-Krueger-Koordinatensystem). Parameters: central latitude `0`, central longitude in zones of 6 degrees centred at `0`, `3`, `6`, `9`, `12`, and `15E`, scale factor `1`. Coordinates in the GKK grid have a false easting of z×1000 + 500 km, where z is the zone number.
- the British National Grid (BNG) projection. Parameters: central latitude `49`, central longitude `-2`, scale factor `0.9996012717`. The datum to be used is called "Ordnance Survey Great Britain". Coordinates in this grid correspond to a false easting of 400km and a false northing of 100km.
- the British West Indies projection. Parameters: central latitude `0`, central longitude `-62`, scale factor `0.9995`. The datum to be used should be based on the "Clarke 1880" ellipsoid. Coordinates in this grid correspond to a false easting of 400km.
- the Irish Transverse Mercator Grid (ITM) projection. Parameters: central latitude `53.5`, central longitude `-8`, scale factor `1.000035`. The datum to be used is called "Ireland 1965". Coordinates in this grid correspond to a false easting of 200km and a false northing of 250km.
- the Uniform Finnish Grid (KKJY) projection. Parameters: central latitude `0`, central longitude `27`, scale factor `1`. Coordinates in this grid correspond to a false northing of 500km. There is a single zone named `27E`.
- the Basic Finnish Grid (KKJP) projection. Parameters: central latitude `0`, central longitude in zones of 6 degrees centred at `21`, `24`, `27`, and `30E`, scale factor `1`. Coordinates in the KKJP grid have a false easting of z×1000 + 500 km, where z is the zone number.
- the Swedish Grid (SEG) projection. Parameters: central latitude `0`, central longitude `15.808277777778`, scale factor `1`. Coordinates in this grid correspond to a false easting of 1500km.

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 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.

|  |  | Projections and coordinate grids