A vehicle accelerometer system comprising:- an accelerometer for installation in a vehicle ; means for determining if the vehicle is substantially stationary; a processor configured to select accelerometer output data items for inclusion in an accelerometer output data set representative of measurements by the accelerometer at a plurality of different vehicle locations, each accelerometer output data item being representative of a respective measurement by the accelerometer when the vehicle is substantially stationary; and a storage device for storing the accelerometer output data set wherein the processor is configured to process the accelerometer output data set to determine an orientation output representative of the orientation of the accelerometer with respect to the vehicle , characterised in that : the processor is configured to process the accelerometer output data set to determine an orientation output if the number of accelerometer output data items in the accelerometer output data set is greater than a predetermined threshold; and the processor is further configured to determine a mean or median value of the accelerometer measurements represented by the accelerometer output data set across all measurements, to eliminate effects due to the inclination of the ground for any one of the measurements, and to use the determined mean or median value to determine the orientation output, representative of the orientation of the accelerometer with respect to the vehicle . an accelerometer for installation in a vehicle ; means for determining if the vehicle is substantially stationary; a processor configured to select accelerometer output data items for inclusion in an accelerometer output data set representative of measurements by the accelerometer at a plurality of different vehicle locations, each accelerometer output data item being representative of a respective measurement by the accelerometer when the vehicle is substantially stationary; and a storage device for storing the accelerometer output data set wherein the processor is configured to process the accelerometer output data set to determine an orientation output representative of the orientation of the accelerometer with respect to the vehicle , characterised in that : the processor is configured to process the accelerometer output data set to determine an orientation output if the number of accelerometer output data items in the accelerometer output data set is greater than a predetermined threshold; and the processor is further configured to determine a mean or median value of the accelerometer measurements represented by the accelerometer output data set across all measurements, to eliminate effects due to the inclination of the ground for any one of the measurements, and to use the determined mean or median value to determine the orientation output, representative of the orientation of the accelerometer with respect to the vehicle . the processor is configured to process the accelerometer output data set to determine an orientation output if the number of accelerometer output data items in the accelerometer output data set is greater than a predetermined threshold; and the processor is further configured to determine a mean or median value of the accelerometer measurements represented by the accelerometer output data set across all measurements, to eliminate effects due to the inclination of the ground for any one of the measurements, and to use the determined mean or median value to determine the orientation output, representative of the orientation of the accelerometer with respect to the vehicle . <EOS>