Geocoding converts physical addresses into geographic coordinates while reverse geocoding does the opposite. Geocoders are software or web services used for various purposes. The three common geocoding methods are by address, postcode/zip code, and border. Each method has its difficulties, and there are various geocoding systems available, some free and others requiring a fee. Examples include Google Maps, Yahoo PlaceFinder, and USC Geocoder.
Geocoding refers to converting a physical address, such as a street address, into geographic coordinates, commonly longitude and latitude, to display your location on a map. Reverse geocoding does the opposite calculation and finds a street address when a set of geographic coordinates is provided. A geocoder, or geocoding system, is software or a web service that performs geocoding. A geocoder can be used for a variety of purposes, such as getting directions, cartography, and georeferencing. Geotagging involves adding a geotag, i.e. geographic data such as latitude and longitude, to various types of media such as photos, videos, RSS feeds, and websites.
The three most common methods of geocoding are: by address; by postcode, zip code or local equivalent; and by border. When geocoding by address, a geocoder commonly uses a reference file with data such as addresses and street names that have already been mapped. It then compares the data in the reference file to the address list it’s geocoding. A process called address interpolation is often used to estimate the location of a specific address, which means that the system estimates where a specific address is based on the general information in the reference file. For example, a geocoder can figure out which end of a block or street an address is based on the range of building numbers, or which side of the street a house is on based on the odd or even building number.
Using postcodes can be problematic, especially in rural areas where each postcode covers a very large area. In urban areas, this method is more accurate because each postcode covers a much smaller geographic area. Geocoding by boundary is the least reliable method, because it will only show a general area where a specific address is located.
There are difficulties with all geocoding methods. Even the most reliable method, using physical addresses, has its problems. For example, new addresses may not have been added to the geocoder database yet, or streets may have the same name but are in different geographic areas. There are a variety of geocoding systems available, and some are free, while others require a license fee. Some examples of geocoders are the relatively well-known Internet-based services such as Google Maps, Yahoo PlaceFinder and USC Geocoder and systems such as Geohash, C-squares and ISO 6709. There are also text-based alternatives that do not require the user to be online.
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