Usually, such a valuation process commences with a measurement of the most and least valuable land within the taxation area. A few sites of intermediate value are then identified and used as "landmark" values. Other values are filled in between the landmark values. The data is then collated in a database and linked to a unique property reference number.

Over time with the use of sales data the map should become more accurate.

The assessment will only be close, of course, it'll never be "perfect".