#Date Directive

Although vectors defined in data files can be assigned a segment attribute, a component of which can be a text string like "15 Jan 97", an explicit date attribute can be assigned as well. The program will, if you enable this feature, automatically calculate a magnetic declination whenever a #DATE directive is encountered.  Another benefit of using date directives is the ability to color surveys by date using a color sequence, or gradient defined for this purpose. Assigned dates will also be among the attributes of exported shapefiles allowing a GIS program like ArcView, for example, to color surveys by date range.

The directive's format is

#DATE         yyyy-mm-dd

where yyyy is the year in the usual Gregorian calendar, mm is the month of the year between 01 (January) and 12 (December), and dd is the day of the month between 01 and 31.  Example:

#Date        1997-01-30

While some date formats common in the U.S. (mm/dd/yy, mm-dd-yyyy, etc.) are accepted by the program, it is recommended that you use the above form.  It is an international standard (ISO 8601) and is less likely to be misinterpreted by anyone viewing the data.

Declination Calculation

To enable automatic calculation of magnetic declination, you also need to define a geographical reference that will apply to the surveys you intend to compile. (Establishing a reference also makes it possible to generate UTM coordinates.)  After you do this, calculating declinations from dates is still an option that can be selectively turned on or off via the Geographical Reference page of a project item's Properties dialog.  When the option is turned on, a #DATE directive is effectively equivalent to

#Units DECL=n

where n is the declination computed from a mathematical model of the Earth's magnetic field. Its value, of course, depends on both the date that measurements were taken and your position on the Earth's surface. You can easily determine what the computed declination will be by checking the Geographical Calculator that's built into Walls. (The calculator can be launched via a toolbar icon, an M-labeled north arrow.)