Thermal expansion poses significant constraints on the accuracy achievable in positioning

It becomes increasingly important as the table travel is increased. Ambient temperature changes are one obvious source of thermal expansion. As the positioning table warms, its constituent parts undergo expansion, at a specific rate for any material. Accordingly, to permit any two users to agree on what constitutes an "accurate" positioning system, all critical dimensional measurements world-wide are understood to take place at 20 degrees C (68 degrees F).