Position +data

Purpose

This document describes how position data +is represented within the position +data classes.

Values and co-ordinates

The World Geodetic System +reference datum (WGS 84) is used by LBS. This is a global co-ordinate system +in which co-ordinates are expressed as latitude and longitude in degrees and +altitude in metres above the reference datum. In WGS 84, latitude values have +the range (-90 <= Lat <= +90) and longitude values have the range (-180 +<= Long < +180).
In LBS, latitude and +longitude are held as TReal64 values and altitude is held +as a TReal32 value.
Both horizontal and +vertical accuracy are held as TReal32 values and represent +a possible error values in metres.
Bearings and headings +are held as TReal32 values and represent a direction in degrees. +Bearing and heading accuracies are also held as TReal32 values +and represent a possible error value in degrees.

Co-ordinate adjustment

The TCoordinate base +class holds the latitude, longitude and altitude of a position. If an application +attempts to assign values that are outside of the ranges specified by WGS +84 to an instance of a TCoordinate class then the values +are adjusted to lie within the correct interval.

Unassigned values

In some situations, one of the +position co-ordinates, typically altitude may not be known. This is usually +because the positioning module that was used to obtain the location fix was +unable to obtain data for that co-ordinate. Where LBS cannot assign a value +to a co-ordinate value, the value "Not a Number" (NaN) is assigned to it. +Applications must test for this situation by calling Math::IsNaN() on +each returned co-ordinate value before using it in a calculation.