# GPX - GPS Exchange Format

GPX (the GPS Exchange Format) is a light-weight XML data format for the interchange of GPS data (waypoints, routes, and tracks) between applications and Web services on the Internet.

OGR has support for GPX reading (if GDAL is build with expat library support) and writing.

Version supported are GPX 1.0 and 1.1 for reading, GPX 1.1 for writing.

The OGR driver supports reading and writing of all the GPX feature types :

- waypoints : layer of features of OGR type wkbPoint
- routes : layer of features of OGR type wkbLineString
- tracks : layer of features of OGR type wkbMultiLineString

It also supports reading of route points and track points in standalone layers (route_points and track_points), so that their own attributes can be used by OGR.

In addition to its GPX attributes, each route point of a route has a route_fid (foreign key to the FID of its belonging route) and a route_point_id which is its sequence number in the route.
The same applies for track points with track_fid, track_seg_id and track_seg_point_id. All coordinates are relative to the WGS84 datum (EPSG:4326).

If the environment variable GPX_ELE_AS_25D is set to YES, the elevation element will be used to set the Z coordinates of waypoints, route points and track points.

The OGR/GPX reads and writes the GPX attributes for the waypoints, routes and tracks.

By default, up to 2 <link> elements can be taken into account by feature. This default number can be changed with the GPX_N_MAX_LINKS environment variable.

## Encoding issues

Expat library supports the following built-in encodings :

- US-ASCII
- UTF-8
- UTF-16
- ISO-8859-1

If your GPX file is not encoded in one of the previous encodings, it will not be parsed by the GPX driver. You may convert it into one of the supported encoding with the iconv utility for example and change accordingly the encoding parameter value in the XML header.
Note : Windows-1252 differs only on a small range of characters with ISO-8859-1. Patching the encoding parameter value to ISO-8859-1 may be enough to make it work...

## Extensions element reading

If the <extensions> element is detected in a GPX file, OGR will expose the content of its sub elements as fields. Complex content of sub elements will be exposed as an XML blob.

The following sequence GPX content :

        <extensions>
            <navaid:name>TOTAL RF</navaid:name>
            <navaid:address>BENSALEM</navaid:address>
            <navaid:state>PA</navaid:state>
            <navaid:country>US</navaid:country>
            <navaid:frequencies>
            <navaid:frequency type="CTAF" frequency="122.900" name="CTAF"/>
            </navaid:frequencies>
            <navaid:runways>
            <navaid:runway designation="H1" length="80" width="80" surface="ASPH-G">
            </navaid:runway>
            </navaid:runways>
            <navaid:magvar>12</navaid:magvar>
        </extensions>

will be interpreted in the OGR SF model as :

      navaid_name (String) = TOTAL RF
      navaid_address (String) = BENSALEM
      navaid_state (String) = PA
      navaid_country (String) = US
      navaid_frequencies (String) = <navaid:frequency type="CTAF" frequency="122.900" name="CTAF" ></navaid:frequency>
      navaid_runways (String) = <navaid:runway designation="H1" length="80" width="80" surface="ASPH-G" ></navaid:runway>
      navaid_magvar (Integer) = 12

Note : the GPX driver will output content of the extensions element only if it is found in the first records of the GPX file. If extensions appear later, you can force an explicit parsing of the whole file with the GPX_USE_EXTENSIONS environment variable.

## Creation Issues

On export all layers are written to a single GPX file. Update of existing files is not currently supported.

If the output file already exits, the writing will not occur. You have to delete the existing file first.

Supported geometries :

- Features of type wkbPoint/wkbPoint25D are written in the wpt element.
- Features of type wkbLineString/wkbLineString25D are written in the rte element.
- Features of type wkbMultiLineString/wkbMultiLineString25D are written in the trk element.
- Other type of geometries are not supported.

For route points and tracks points, if there is a Z coordinate, it is used to fill the elevation element of the corresponding points.

The GPX writer supports the following layer creation options:

- FORCE_GPX_TRACK: By default when writting a layer whose features are of type wkbLineString, the GPX driver chooses to write them as routes.
  If FORCE_GPX_TRACK=YES is specified, they will be written as tracks.

- FORCE_GPX_ROUTE: By default when writting a layer whose features are of type wkbMultiLineString, the GPX driver chooses to write them as tracks.
  If FORCE_GPX_ROUTE=YES is specified, they will be written as routes, provided that the multilines are composed of only one single line.

The GPX writer supports the following dataset creation options:

- GPX_USE_EXTENSIONS: By default, the GPX driver will discard attribute fields that do not match the GPX XML definition (name, cmt, etc...).
  If GPX_USE_EXTENSIONS=YES is specified, extra fields will be written inside the<extensions> tag.

- GPX_EXTENSIONS_NS: Only used if GPX_USE_EXTENSIONS=YES and GPX_EXTENSIONS_NS_URL is set.
  The namespace value used for extension tags. By default, "ogr".

- GPX_EXTENSIONS_NS_URL: Only used if GPX_USE_EXTENSIONS=YES and GPX_EXTENSIONS_NS is set.
  The namespace URI. By default, "http://osgeo.org/gdal".

Waypoints, routes and tracks must be written into that order to be valid against the XML Schema.

When translating from a source dataset, it may be necessary to rename the field names from the source dataset to the expected GPX attribute names, such as <name>, <desc>, etc... This can be done with a OGR VRT dataset, or by using the "-sql" option of the ogr2ogr utility.

## Example

- The ogrinfo utility can be used to dump the content of a GPX datafile :

      ogrinfo -ro -al input.gpx

- The ogr2ogr utility can be used to do GPX to GPX translation :

      ogr2ogr -f GPX output.gpx input.gpx waypoints routes tracks

  Note : in the case of GPX to GPX translation, you need to specify the layer names, in order to discard the route_points and track_points layers.

- Use of the <extensions> tag for output :

      ogr2ogr -f GPX  -dsco GPX_USE_EXTENSIONS=YES output.gpx input

  which will give an output like the following one :

          <?xml version="1.0"?>
          <gpx version="1.1" creator="GDAL 1.5dev"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xmlns:ogr="http://osgeo.org/gdal"
          xmlns="http://www.topografix.com/GPX/1/1"
          xsi:schemaLocation="http://www.topografix.com/GPX/1/1 http://www.topografix.com/GPX/1/1/gpx.xsd">
          <wpt lat="1" lon="2">
          <extensions>
              <ogr:Primary_ID>PID5</ogr:Primary_ID>
              <ogr:Secondary_ID>SID5</ogr:Secondary_ID>
          </extensions>
          </wpt>
          <wpt lat="3" lon="4">
          <extensions>
              <ogr:Primary_ID>PID4</ogr:Primary_ID>
              <ogr:Secondary_ID>SID4</ogr:Secondary_ID>
          </extensions>
          </wpt>
          </gpx>

- Use of -sql option to remap field names to the ones allowed by the GPX schema !

      ogr2ogr -f GPX output.gpx input.shp -sql "SELECT field1 AS name, field2 AS desc FROM source_layer"

## See Also

- Home page for GPX format

- GPX 1.1 format documentation