No refrigerated space is perfectly uniform. Areas near the door, the evaporator, the lights or the floor can run warmer or colder than the centre. Temperature mapping measures this by placing data loggers in a grid across the space, usually at several heights, and recording for a set period, sometimes empty and loaded, and sometimes in hot and cold seasons.
The results show where the hot and cold spots are, whether the whole space stays in range, how fast it recovers after door openings, and where to place the permanent monitoring sensor. Areas that fall outside range are marked as not for use. WHO has published guidance on mapping storage areas for time- and temperature-sensitive pharmaceutical products as a technical supplement to its Technical Report Series No. 961, Annex 9.
A pharmaceutical distributor in Ahmedabad commissioning a new cold room might map it before use and again in the peak of summer, then mark the top shelf next to the door as unusable and place its monitoring probe at the warmest acceptable point.