WhereAmI/OSM Binary Format

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OSM Binary Format

This is an alternative to the xml format commonly used for distribution of OSM map data. There are also a set of related additions for use in a mobile context OSM Mobile Binary Protocol.

Note that this format is rarely used as a general purpose binary format, for which PBF Format is preferred.

It is not related to OSMbin.

The Format

The formats base structure is a series of binary blocks, which specify their length followed by a type followed by encoded data.

  • Length - 32bit unsigned integer length (length does not include the 4 bytes used to specify the length)
  • Type - 8bit unsigned integer type (as defined in Types)
  • Data - binary data any format

To save space long/lat values are encoded as 32bit signed integers from the floating point values to six decimal places (eg lat/lon * 1000000). This gives ~11cm accuracy which is plenty for any mapping project. Time/Date fields are the number of seconds since the Unix Epoch (January 1 1970 00:00:00 GMT). Strings are truncated to 255 and are given as a length followed by characters utf8 encoded.

OSM nodes are not transmitted in their entirety, only interesting nodes are included, (nodes with some additional meta data - poi's).

Ways reduce the data needed to be transmitted by only sending the positions of nodes as long/lat pairs, the first long/lat pair using 32bits then subsequent ones are 16bit diffs from the previous node position, if the diff is too big then fake nodes filling the gap are given.


[block length=0x02 0x00 0x00 0x00] 0x05 0x3B

This is an optional 6 byte header at the beginning of an OSM Binary format file. (decided because: 0=O, 5=S, 3=M, B=B)

This just indicates that the content is obf for use in file type recognisers and such.


  • Length (32bit)
  • Type - 'i' (8bit)
  • Node Id (64bit)
  • Longitude (32bit)
  • Latitude (32bit)
  • Primary property (8bit) see Node Property
  • Feature Tags (optional) see Feature Tags


  • Length (32bit)
  • Type - 'w' (8bit)
  • Way Id (32bit)
  • Node Count (32bit)
  • Longitude (32bit)
  • Latitude (32bit)
  • Followed by:
    • Longitude Diff (16bit)
    • Latitude Diff (16bit)
  • Feature Tags see Feature Tags


  • Length (32bit)
  • Type - 'r' (8bit)
  • Relation Id (32bit)
  • Relation Type (8bit) see Relations Types
  • Members Count (32bit)
  • Followed by:
    • Member Type/Role (8bit) see Relations Roles
    • Member Id (32bit way id/64bit poi id) (repeated as necessary)
  • Feature Tags see Feature Tags

Usage and intended use

The OSM binary format is intended for the following types of clients:

  • mapping programs

It is not intended for:

It is optimized for:

  • general usage

This format is supported by the following clients: