The problem
Many software developers have misconceptions about postal codes (usually
referred to as zipcodes in the United States) and how they relate to
geographic areas.
We hope that this guide will help cast light on some of the common
misconceptions, and thus help prevent developers from building applications
based on faulty logic.
One of
the most common problems we see
is developers struggling when the
assumptions they made about postcodes based on their home country turn out
not to be valid in other countries.
Background
The key point to understand is that postal codes are a tool used by postal services to speed up the delivery of physical mail. In some parts of the world postal codes have become a shorthand way to refer to areas, and this has lead to a mistaken, albeit understandable, perception that postcodes always correspond to locations.
Common points of confusion and misconceptions
Postcodes always represent locations
No, for most countries postcodes represent mail delivery points. Those
points may or may not be in fixed locations. For example, many countries
assign postcodes to military units. The postcodes stays the same regardless
of where the unit is physically based. In many countries large companies or
government departments have their own postal code with no correlation to
physical location.
Postcodes never move or change
No, in most countries there is on-going fluctuations in postcodes. New
postcodes are created, old codes are retired, large organization move and
take their code with them.
Occasionally countries will change their entire postcode system (fun!).
Within a country the area covered by a postcode is always roughly the same size
No, absolutely not. Generally postcodes cover roughly the same number of people. In a
dense city that might by a very small area. In a rural area it may be massive.
Postcodes work roughly the same way in all countries
No, not at all. Almost every aspect of what a postcode is and what it means
can differ across countries. The format of the code, the number of people it
represents, etc, etc.
All countries have postcodes
No, many countries do not have any form of postal code at all.
Also, some countries technically have a postcode system, but it is not
widely used.
Every geographic location belongs to a certain, unique postcode region.
While some countries have assigned postcode regions to all of their
territory, many others have not. Remote locations, for example in a large
forest, or lakes or other bodies of water, may not fall into any postal
region.
Locations have a single, unique postcode
No, often large buildings will have multiple postcodes, for example if there
are multiple, large organizations based there.
Usually it will depend on how many people are at the location (and thus how
much mail typically needs to be sent there). An example: skyscrapers in
Manhattan, New York City can have multiple "zipcodes" (as postcodes are
known in the US).
Also, when postcodes change people will often keep using the old postcode,
even after a new postcode
has been assigned to the location.
The area covered by a postcode is always within one state / province
In many countries the area served by a postcode has no correspondence with
administrative hierarchies like county, state, province, etc.
In Australia the postcode
0872
famously spans not just multiple states but also timezones.
Postal systems are unique to a specific country
No, many countries and territories of the world use the postal service of
other countries. For example Liechtenstein uses the Swiss postal system.
For logistical convenience, exclaves of one country often use the postal
system of another country. For example the German exclave
Büsingen uses both the German and Swiss postal services.
Postcodes are unique to a specific country
No, many countries use strings of numbers as their postal code.
So if you just send us a query like
12345
we have no idea which country you mean.
Please always also add a country name. For example:
12345, USA
Postcodes are always numbers
No, many countries like the UK, Canada, and the Netherlands have
alphanumeric codes.
But even in countries that use only digits in their postal codes it is best
to always treat postcodes as strings not numbers, otherwise leading zeros
can get removed by formatting software
(for example Excel). This is one of the most
common problems we see in geocoding query formatting.
Postcodes are always alphanumeric
No, some countries require dashes or spaces in their codes. For example in
Poland postal codes are five digits but with a dash between the second
and third digit, for example:
01-003.
So what should software developers do?
The first thing is you should understand and internalize that postcodes are not a great tool for communicating location. They should be avoided if possible. Nevertheless, we understand that in some countries the habit of searching by postcode or referring to locations by postcode has become entrenched. Even in that case it's important to understand the inherent limitations and challenges of using postcodes to reference locations. Know the potential problems you are building into your system. If your application will be geocoding multiple countries do NOT fall into the trap of assuming postal codes have the same characteristics in all countries (or even that all countries have postcodes) Final point: treat postcodes as string not numbers. You will thank us.Further Reading
- Wikipedia's page about postal codes is a great place to start.
- Postcode formats of the world