Declinations and Grid Corrections

The following #Units parameters specify positive or negative angular quantities, the default units being decimal degrees. Note that they specify how data should be interpreted (what reference direction to assume), not necessary how you'll want data converted or displayed on a map.

DECL = <true north minus magnetic north>

This specifies the magnetic declination that would need to be added to the azimuth measurements of compass and tape (CT) data lines to obtain a vector's true north relative direction. If you choose to use a geographical reference with your project, you can enable the option to specify declinations indirectly via the #DATE directive, in which case the #DATE directive becomes equivalent to a #Units DECL=x directive, where x is the declination computed from a model of the Earth's magnetic field.  For more information on this option see Geographical Reference Page.

#Fix and RECT data lines are not affected by the declination setting. They have similar but separate settings as described below.

GRID = <true north minus grid north>

The implied vectors corresponding to #FIX coordinates would need to be rotated clockwise by this amount to obtain true north relative vectors. The default GRID correction is the UTM grid convergence angle when a geographical reference has been assigned to the respective branch of the project tree. (See Geographical Reference Page.) In this case, the default setting for GRID assumes that all #FIXed points are defined in the data with UTM (or UTM-relative) coordinates.  Also, the UTM zone and geodetic datum are assumed to be the same as that of the reference.

Therefore, when working with GPS position fixes and UTM coordinates, you'll not normally have any GRID= arguments on your #Units directives. That's because the assigned geographical reference will already be providing a suitable default.

Important: Without either a GRID correction explicitly specified or a geographical reference assigned, #FIX coordinates are assumed to be true north relative.

RECT = <true north minus RECT north>

The displacement vectors corresponding to RECT data lines would need to be rotated clockwise by this amount to obtain true north relative vectors. The default value is zero, meaning that RECT vectors are assumed to be true north relative by default. For example, the GPS download feature produces track data in the form of RECT vectors. Since those vectors are UTM grid-aligned, the #units option RECT=<grid convergence angle> is specified for you in the output file.

Note: This option is not to be confused with "RECT" (without the equals sign) which specifies that RECT data lines follow the #Units directive.