Inspired by OfflineReverseGeocode found at this Reddit post. You may also be interested in GeoSharp. Uses KdTree. Built and tested on .Net 4.5.

This library provides classes for downloading, reading and parsing, writing and composing files from GeoNames.org and provides (reverse) geocoding methods like NearestNeighbourSearch() and RadialSearch() on the downloaded dataset(s).

This library is available as NuGet package.

Code Quality Rank: L4
Programming language: C#
License: MIT License

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Logo NGeoNames

Inspired by OfflineReverseGeocode found at this Reddit post. You may also be interested in GeoSharp. Uses KdTree. Built and tested on .Net 4.5.

This library provides classes for downloading, reading and parsing, writing and composing files from GeoNames.org and provides (reverse) geocoding methods like NearestNeighbourSearch() and RadialSearch() on the downloaded dataset(s).

This library is available as NuGet package.

Basic usage / example / "quick start"

var datadir = @"D:\test\geo\";

// Download file (optional; you can point a GeoFileReader to existing files ofcourse)
var downloader = GeoFileDownloader.CreateGeoFileDownloader();
downloader.DownloadFile("NL.zip", datadir);    // Zipfile will be automatically extracted

// Read NL.txt file to memory (NL = ISO3166-2:The Netherlands)
var nldata = GeoFileReader.ReadExtendedGeoNames(Path.Combine(datadir, "NL.txt")).ToArray();   
// Note: we "Materialize" the file to memory by calling ToArray()

// We're going to use Amsterdam as "search-center"
var amsterdam = nldata.Where(n => 
        n.Name.Equals("Amsterdam", StringComparison.OrdinalIgnoreCase) 
        && n.FeatureCode.Equals("PPLC")

// Initialize a reversegeocoder with our geo-items from The Netherlands
var reversegeocoder = new ReverseGeoCode<ExtendedGeoName>(nldata);
// Locate 250 geo-items near the center of Amsterdam
var results = reversegeocoder.RadialSearch(amsterdam, 250);  
// Print the results
foreach (var r in results) {
            CultureInfo.InvariantCulture, "{0}, {1} {2} ({3:F4}Km)", 
            r.Latitude, r.Longitude, r.Name, r.DistanceTo(amsterdam)


The library provides for the following main operations:

  1. Downloading / retrieving data from geonames.org (Optional)
  2. Reading / parsing geonames.org data
  3. Utilizing geonames.org data
  4. Writing / composing geonames.org data

The library consists mainly of parsers, composers and entities (in their respective namespaces) and a GeoFileReader and GeoFileWriter to read/parse and write/compose geonames.org compatible files, a GeoFileDownloader to retrieve files from geonames.org and a ReverseGeoCode<T> class to do the heavy lifting of the reverse geocoding itself.

Because some "geoname files" can be very large (like allcountries.txt) we have a GeoName entity which is a simplified version (and baseclass) of an ExtendedGeoName. The GeoName class contains a unique id which can be used to resolve the ExtendedGeoName easily for more information when required. It is, however, recommended to use <countrycode>.txt (e.g. GB.txt) cities15000.txt or cities1000.txt for example to reduce the dataset to a smaller size, You can also compose your own custom datasets using the GeoFileWriter and composers.

Also worth noting is that the readers return an IEnumerable<SomeEntity>; make sure that you materialize these enumerables to a list, array or other datastructure (using .ToList(), .ToArray(), .ToDictionary() etc.) if you access it more than once to avoid file I/O to the underlying file each time you access the data.

Downloading / retrieving data from geonames.org (Optional)

To download files from geonames.org you can use the GeoFileDownloader class which is, in essence, a wrapper for a basic WebClient. The simplest form is:

// Downloads (and extracts) geoname data in NL.zip from geonames.org
    .DownloadFile("NL.zip", @"D:\my\geodata\geo");

// Downloads (and extracts) postalcode data in NL.zip from geonames.org
    .DownloadFile("NL.zip", @"D:\my\geodata\postalcode");

You can specify the BaseUri in the GeoFileDownloader constructor or just pass an absolute url to the DownloadFile() method if you want to use another location than the default http://download.geonames.org/export/dump/. The static 'factory methods' CreateGeoFileDownloader() and CreatePostalcodeDownloader are the easiest way to create a GeoFileDownloader; these use the built-in values for the BaseUri. The GeoFileDownloader has properties to set a (HTTP) CachePolicy, Proxy and Credentials to use when downloading the file. The filedownloader, by default, downloads a file only if the destination file doesn't exist or when the destination file has "expired" (by default 24 hours). It uses the file's CreationDate to determine when the file was downloaded and if a newer version should be downloaded. The "TTL", how long a file will be 'valid', can be set using the DefaultTTL property of the GeoFileDownloader class. You can also use the DownloadFileWhenOlderThan() method which allows you to explicitly set a TTL. When a filename is specified (e.g. d:\folder\foo.txt) the file will be named accordingly.

ZIP files are automatically extracted in the destinationfolder; the original zipfile is preserved because the GeoFileDownloader needs to know which files are supposed to be in the zipfile and thus in the destinationdirectory in their extracted form.

Reading / parsing geonames.org data

Once files are downloaded using the GeoFileDownloader, or by using your own custom/specific implementation, the files can be accessed using the GeoFileReader class. This class contains a number of static "convenience methods" like ReadGeoNames() and it's "sibling" ReadExtendedGeoNames(). but also ReadCountryInfo(), ReadAlternateNames() etc. There is a "convenience method" for each entity.

// Open file "cities1000.txt" and retrieve only cities in the US
var cities_in_us = GeoFileReader.ReadExtendedGeoNames(@"D:\my\geodata\cities1000.txt")
        .Where(p => p.CountryCode.Equals("US", StringComparison.OrdinalIgnoreCase))
        .OrderBy(p => p.Name);

Again, please note that Read<Something> methods return an IEnumerable<T>. Whenever you want to access the data more than once you will probably want to call .ToArray() or similar to materialize the data into memory. The GeoFileReader class has two static method (ReadBuiltInContinents() and ReadBuiltInFeatureClasses()) that can be used to use built-in values for continents and feature codes which are not provided by geonames.org as downloadable files. You can, however, craft your own files for this purpose and use the ReadContinents() and ReadFeatureClasses() if you want to specify your own values / update built-in values (should NGeoNames's values be outdated for example).

You can also add your own entities and, as long as you provide a parser for it, use the GeoFileReader class to read/parse files for these entities as well:

var data = new GeoFileReader().ReadRecords<MyEntity>("d:\foo\bar.txt", new MyEntityParser());

As long as your parser implements IParser<MyEntity> you're good to go. A parser can skip a fixed number of lines in a file (for example a 'header' record), skip comments (for example lines starting with #) and you can even specify the encoding to use etc. Examples and more information can be found in the unittests.

Another thing to note is that the GeoFileReader will try to "autodetect" if the file is a plain text file (.txt extension) or a GZipped file (.gz extension). Support for GZip was added to keep the footprint of the files lower when desired. This will, however, trade-off I/O speed and CPU load for space. The ReadRecords<T>() method has an overload where you can explicitly specify the type of the file (should you want to use your own file-extensions like .dat for example).

Support for compressing downloaded files using the GeoFileDownloader on the fly is planned for a later version; for now you will have to GZip the files manually.

The GeoFileReader also supports the use of Streams so you can provide data from a MemoryStream for example or any other source that can be wrapped in a stream.

As you'll probably realize by now, the GeoFileReader class combined with LINQ allows for very powerful querying, filtering and sorting of the data. Combine it with the GeoFileWriter to persist custom datasets (custom "materialized views") and the sky is the limit.

Utilizing geonames.org data

The 'heart' of the library is the ReverseGeoCode<T> class. When you supply it with either IEnumerable<GeoNames> or IEnumerable<ExtendedGeoNames> it can be used to do a RadialSearch() or NearestNeighbourSearch(). Supplying the class with data can be done by either passing it to the class constructor or by using the Add() or AddRange() methods. You may want to call the Balance() method to balance the internal KD-tree, however; this is done automatically when the data is supplied via the constructor. Even if you choose to store your data in a database or custom (binary?) fileformat or anything else; as long as you provide an IEnumerable to this class you'll be able to use it.

// Create our ReverseGeoCode class and supply it with data
var r = new ReverseGeoCode<ExtendedGeoName>(

// Create a point from a lat/long pair from which we want to conduct our search(es) (center)
var new_york = r.CreateFromLatLong(40.7056308, -73.9780035);

// Find 10 nearest
r.NearestNeighbourSearch(new_york, 10);

Ofcourse there's no need to dabble with lat/long at all:

// Read data into memory
var data = GeoFileReader.ReadExtendedGeoNames(@"D:\foo\cities1000.txt")
        .ToDictionary(p => p.Id);

// Find New York by it's geoname ID (O(1) lookup)
var new_york = data[5128581];

// Find 10 nearest
var r = new ReverseGeoCode<ExtendedGeoName>(data.Values);
r.NearestNeighbourSearch(new_york, 10);

Or simply find by name:

// Read data into memory
var data = GeoFileReader.ReadExtendedGeoNames(@"D:\foo\cities1000.txt")

// Find New York by it's name (linear search, O(n))
var new_york = data.Where(p => p.Name.Equals("New York City")).First();

// Find 10 nearest
var r = new ReverseGeoCode<ExtendedGeoName>(data);
r.NearestNeighbourSearch(new_york, 10);

Depending on how you want to search/use the underlying data you may want to use other, more optimal, datastructures than demonstrated above. It's up to you!

Note that the library is based on the International System of Units (SI); units of distance are specified in meters. If you want to use the imperial system (e.g. miles, nautical miles, yards, foot and whathaveyou's) you need to convert to/from meters. The GeoUtil class provides helper-methods for converting miles/yards to meters and vice versa.

The GeoName class (and, by extension, the ExtendedGeoName class) has a DistanceTo() method which can be used to determine the exact distance betweem two points.

Both the NearestNeighbourSearch() and RadialSearch() methods have some overloads that accept lat/long pairs as doubles as well.

Writing / composing geonames.org data

The NGeoNames.Composers namespace holds composers (the opposite of parsers) to enable you to write geoname.org datafiles. For this you can use the GeoNameFileWriter class which, like the GeoNameFileReader class, has generic methods for writing records (WriteRecords<T>) and static "convenience methods" to write specific entities to a file. If you wanted to 'transform' a file like allcountries.txt to a file with data from, say, the Benelux) you could supply the GeoNameFileWriter with data from BE.txt, NL.txt and LU.txt or data from allcountries.txt or cities1000.txt filtered with a LINQ query to only data from these countries.

Below is an example of what this would look like (with an extra filter added to filter out records with population < 1000):

// Filter 'allcountries.txt' to only BE, NL, LU entries with a population of >= 1000
      .Where(e => new[] { "BE", "NL", "LU" }.Contains(e.CountryCode) && e.Population >= 1000)
      .OrderBy(e => e.CountryCode).ThenBy(e => e.Name)

// ...or...

// Join BE, NL en LU datasets, filter records with a population of >= 1000
        .Where(e => e.Population >= 1000)
        .OrderBy(e => e.CountryCode).ThenBy(e => e.Name)

A word about "extended format"

The GeoNamesReader and GeoNamesWriter and the (Extended)GeoName parsers/composers always assume the ExtendedGeoName format (e.g. 19 fields of data) unless explicitly specified. The parameter extendedfileformat may pop-up on some method overloads; whenever this parameter is passed false the class will assume a 'simple' (or non-extended) format with only 4 fields of data: Id, Name, Latitude and Longitude. This format is more compact; especially when writing GeoName entities instead of ExtendedGeoName entities to a file. However, to remain compatible with the original files you probably don't want to use this 'simple' format. Make sure you understand the consequences before you do!


The NuGet package comes with a Windows Help File (NGeonames.chm) with lots more information. You can also build this help file, or other formats, yourself using Sandcastle Help File Builder. And finally you can use the richly commented code if you don't want to build or use help files.

Project status

The project will be updated from time-to-time when required. I am happy to accept pull-requests; if you're interested in contributing to this library please contact me. If you have any issues please open an issue.

Build status


Licensed under MIT license. See [LICENSE](LICENSE) for details.

Logo / icon sourced from iconninja.com (Archived page)

*Note that all licence references and agreements mentioned in the NGeoNames README section above are relevant to that project's source code only.