In combination, a portable location tracking device and a monitoring apparatus, said portable location tracking device having: a location determination apparatus configured to determine an instantaneous latitude-longitude-location of said portable location tracking device from received GPS signals or from another location system; memory (127; 129) containing a two-dimensional array corresponding to longitude and latitude; a processor configured to find a location within the two-dimensional array corresponding to the instantaneous location; a secret reference point within said two-dimensional array, the secret reference point and two-dimensional array being privately shared with said monitoring apparatus; and a data exchange apparatus configured to transmit an offset between said location within the two dimensional array and said secret reference point; said monitoring apparatus having: a data exchange apparatus configured to receive said offset between said location within the two dimensional array and said secret reference point from said portable location tracking device data exchange apparatus; and memory (137; 139) containing a two-dimensional data array corresponding to said portable location tracking device two-dimensional array and said privately shared secret reference point within said monitoring apparatus two-dimensional array corresponding to said portable location tracking device secret reference point; and said monitoring apparatus configured to apply said received offset to said privately shared reference point within said monitoring apparatus two-dimensional array and thereby determine said instantaneous latitude-longitude location of said portable location tracking device. a location determination apparatus configured to determine an instantaneous latitude-longitude-location of said portable location tracking device from received GPS signals or from another location system; memory (127; 129) containing a two-dimensional array corresponding to longitude and latitude; a processor configured to find a location within the two-dimensional array corresponding to the instantaneous location; a secret reference point within said two-dimensional array, the secret reference point and two-dimensional array being privately shared with said monitoring apparatus; and a data exchange apparatus configured to transmit an offset between said location within the two dimensional array and said secret reference point; said monitoring apparatus having: a data exchange apparatus configured to receive said offset between said location within the two dimensional array and said secret reference point from said portable location tracking device data exchange apparatus; and memory (137; 139) containing a two-dimensional data array corresponding to said portable location tracking device two-dimensional array and said privately shared secret reference point within said monitoring apparatus two-dimensional array corresponding to said portable location tracking device secret reference point; and said monitoring apparatus configured to apply said received offset to said privately shared reference point within said monitoring apparatus two-dimensional array and thereby determine said instantaneous latitude-longitude location of said portable location tracking device. a data exchange apparatus configured to receive said offset between said location within the two dimensional array and said secret reference point from said portable location tracking device data exchange apparatus; and memory (137; 139) containing a two-dimensional data array corresponding to said portable location tracking device two-dimensional array and said privately shared secret reference point within said monitoring apparatus two-dimensional array corresponding to said portable location tracking device secret reference point; and said monitoring apparatus configured to apply said received offset to said privately shared reference point within said monitoring apparatus two-dimensional array and thereby determine said instantaneous latitude-longitude location of said portable location tracking device.