Using locality information from external sources, distance comparison
can be calculated when postcodes or similar fields are available in
data sets. This field comparator uses a geocode look-up table (where
for each entry longitude and latitude are stored) to calculate the
distance in kilometers between two values. The argument
`geocode_table`

must be set to a reference to a geocode look-up
table as defined and loaded using methods from the
`lookup.py` module (see Chapter 14 for
more details).

An additional argument is `max_distance`

(a positive number in
kilometers) which gives the maximum distance that is tolerated. If the
calculated distance between two fields is equal to or less than the
value in `max_distance`

, a partial agreement weight is
calculated using the following formula.

If the distance is larger than `max_distance`

, the disagreement
weight is returned.

In its current version no frequency dependent weight calculation is
possible, thus the arguments `frequency_table`

,
`freq_table_max_weight`

and `freq_table_min_weight`

can not be used with this field comparator function.