GETDEC, Version 4.0

by Clive Ruggles

Terminology. Elevation means the height of a location above sea level. Altitude means the vertical angle between a viewed point and the horizontal plane through the observer. Elevation, then, is a distance; altitude is an angle. Cf. Astronomy in Prehistoric Britain and Ireland, p. ix.

The program calculates the apparent declination and the geocentric lunar declination of horizon points as viewed from given points on the ground. Horizon points may either be specified from survey data ('survey mode') or from map data ('map mode'). Site information may be provided either as a latitude or as a TM Grid easting and northing in all-figure form. The TM Grids currently catered for are the British and Irish National Grids, the Greek EGSA87 Grid, and UTM. Note that UTM northings in the northern hemisphere must include the initial "1" (i.e. the equator has a northing of 10000000). Map mode is only available when Grid co-ordinates are used. Contextual help is available at many points by typing '?'.

In survey mode, the program requests a plate bearing and altitude and returns

both from a specified position and (if required) from up to three offset positions. The PB-Az correction (or minus the magnetic declination) for a given site is requested before horizon point information is entered and may be altered before a further set of readings is entered. Enter '0' for the PB-Az correction if azimuths are available instead of plate/magnetic bearings.

In map mode, the program requests the elevation of the point of observation and the Grid co-ordinates and elevation of each observed point. It returns the grid azimuth, true azimuth, altitude, apparent declination and geocentric lunar declination.

Offsets are available in survey mode. They are specified in terms of an azimuth, horizontal distance and height difference from the specified position. The angle may be entered as a plate/magnetic bearing, which the program will convert to an azimuth, if desired. Up to three offsets may be specified at a time. If any offsets are specified then the program will prompt for the distance, as well as the plate/magnetic bearing and altitude, of each profile point. It will then provide a corrected azimuth, altitude, apparent declination and geocentric lunar declination for the profile point as viewed from each of the offset position(s).

Angles may be entered in a variety of formats.

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Author: Clive Ruggles