gpx-ideas

TrackPointExtras

Background

GPS / GNSS chipsets from a number of manufacturers provide position and speed accuracy estimates; SiRF, u-blox, Broadcom, Qualcomm, etc.

This data is typically available via binary protocols (e.g. SiRF and UBX) or proprietary NMEA sentences (e.g. $PGLOR,...,LSQ).

Estimated location and speed accuracy information is also available via the Android and Apple location APIs, summarised in the linked document.

However, GPX 1.1 provides no standard mechanism to store the accuracy estimates (or Doppler-derived course and speed), hence these GPX extensions.

Extensions

This TPX 1.0 schema defines universal extensions to be used with the GPX 1.1 schema.

The “extras” element defined by this schema is intended to be used as a child element of the “extensions” elements in the trkpt element.

For example:

<trkpt lat="50.5710623" lon="-2.4563484">
  <ele>7.90</ele>
  <time>2022-04-11T10:16:01Z</time>
  <extensions>
    <tpx:extras>
      <tpx:course>157.19</tpx:course>
      <tpx:speed>0.543</tpx:speed>
      <tpx:hacc>2.0</tpx:hacc>
      <tpx:vacc>4.0</tpx:vacc>
      <tpx:cacc>5.0</tpx:cacc>
      <tpx:sacc>0.5</tpx:sacc>
    </tpx:extras>
  </extensions>
</trkpt>

A more elaborate <src> element is also provided to give some idea of reliability and accuracy of the data:

<trk>
  <trkseg>...</trkseg>
  <extensions>
    <tpx:src>
      Apple Watch Series 8
      <tpx:device>
        <tpx:manufacturer>Apple</tpx:manufacturer>
        <tpx:product>Watch Series 8</tpx:product>
        <tpx:serial>123456789</tpx:serial>
        <tpx:name>Nathan's Apple Watch</tpx:name>
        <tpx:version>8.5.1</tpx:version>
      </tpx:device>
      <tpx:software>
        <tpx:developer>Hoolan Ltd</tpx:developer>
        <tpx:application>Hoolan</tpx:application>
        <tpx:link>
          <tpx:href>https://www.hoolan.app/</tpx:href>
        </tpx:link>
        <tpx:version>1.6.0</tpx:version>
      </tpx:software>
    </tpx:src>
  </extensions>
</trk>

Decisions + Rationale

All of the element names have been intentionally kept as short as possible to avoid too much bloat of the GPX:

The <extras> element is a complex type which ensures consistent ordering of the sub-elements.
The elements <course> and <speed> have been carried forward from GPX 1.0
- They also match the elements in version 2 of Garmin’s TrackpointExtension schema.
Estimated “accuracy” elements have been given short names - e.g. <hacc> is the “horizontal accuracy” estimate.

These short names are closely related to the names used by u-blox (e.g. “hAcc” for horizontal accuracy estimate) and Broadcom (“HErr” for horizontal error estimate) in their GNSS chipset documentation.

The accuracy estimates have been named concisely (e.g. <hacc> and <vacc>) and use entirely lower case names for consistency with the GPX schema, much like the elements <hdop>, <vdop>, and <pdop> in GPX 1.0 and 1.1.

Note: The term “accuracy” has been chosen over “error” because it is used by the Android and Apple APIs, plus notable GPS / GNSS chipset manufacturers such as u-blox. Although some chipset manufacturers use the word “error” those items essentially represent the same thing. It represents RMS, 1-sigma or 68% confidence.

Trackpoint Extensions

COG, SOG and ROC

The following elements can all be added to <wpt>, <rtept> and <trkpt> elements, under <extensions>.

They are all optional, although the values of <course> and <speed> as provided by the GPS / GNSS chipset are highly recommended in GPX 1.1.1 files.

Name	Description
`<course>`	Course over ground (COG) in degrees is the actual direction of travel relative to true north. COG is an independent measurement from position, so it is possible (and not an error) for the difference between two successive positions to be inconsistent with the COG values.
`<speed>`	Speed over ground (SOG) in m/s is the horizontal speed calculated by the GPS / GNSS chipset. SOG is an independent measurement from position, so it is possible (and not an error) for the difference between two successive positions to be inconsistent with the SOG values.
`<roc>`	Rate of climb (ROC) in m/s is the vertical speed calculated by the GPS / GNSS chipset. ROC is an independent measurement from position, so it is possible (and not an error) for the difference between two successive positions to be inconsistent with the ROC values.

Accuracy Estimates

The following accuracy elements can all be added to <wpt>, <rtept> and <trkpt> elements, under <extensions>.

These are available from a variety of NMEA sentences, binary messages and location services provided by Apple and Android.

Name	Description
`<hacc>`	Horizontal accuracy / error estimate in meters, such that the difference from the true position and the reported position is less than the accuracy / error estimate 68% of the time. 68% corresponds to 1-sigma of a normal distribution, but this does not imply that errors are independent or normally distributed.
`<vacc>`	Vertical accuracy / error estimate in meters, such that the difference from the true position and the reported position is less than the accuracy / error estimate 68% of the time. 68% corresponds to 1-sigma of a normal distribution, but this does not imply that errors are independent or normally distributed.
`<cacc>`	Course over ground (COG) accuracy / error estimate in degrees, such that the difference from the true COG and the reported COG is less than the accuracy / error estimate 68% of the time. 68% corresponds to 1-sigma of a normal distribution, but this does not imply that errors are independent or normally distributed.
`<sacc>`	Speed over ground (SOG) accuracy / error estimate in m/s, such that the difference from the true SOG and the reported SOG is less than the accuracy / error estimate 68% of the time. 68% corresponds to 1-sigma of a normal distribution, but this does not imply that errors are independent or normally distributed.
`<racc>`	Rate of climb (ROC) accuracy / error estimate in m/s, such that the difference from the true ROC and the reported ROC is less than the accuracy / error estimate 68% of the time. 68% corresponds to 1-sigma of a normal distribution, but this does not imply that errors are independent or normally distributed.

Source Elements

GPX 1.0 and 1.1 both support <src> elements in <wpt>, <rte>, <rtept>, <trk>, and <trkpt> elements.

These are simply xsd:string types and may contain values such as “Apple Watch Series 8”.

The GPX 1.1.1 proposal introduces a more sophisticated <src> element containing <device> and <software>.

<device> contains the following optional elements:

Name	Description
`<manufacturer>`	Manufacturer of the GPS device / wearable. e.g. “Apple”
`<product>`	Product name of the GPS device / wearable, preferably without mentioning the product manufacturer. e.g. “Watch Series 8”
`<serial>`	Serial number of the GPS device / wearable. e.g. “123456789”
`<name>`	Name of the GPS device / wearable, typically chosen by the owner. e.g. “Nathan’s Apple Watch”.
`<version>`	Firmware / software / OS version of the GPS device / wearable. e.g. “8.5.1”

<software> contains the following optional elements:

Name	Description
`<developer>`	Developer of the software / application used to capture + export the GPS data. This may match the “creator” attribute but unlike “creator”, it should persist after (possible) post-processing. e.g. “Hoolan Ltd”
`<application>`	Name of the software / application used to capture + export the GPS data. This may match the “creator” attribute but unlike “creator”, it should persist after (possible) post-processing. e.g. “Hoolan”
`<link>`	Link to the software / application used to capture + export the GPS data: e.g. “`<href>https://www.hoolan.app/</href>`”
`<version>`	Version of the software / application used to capture + export the GPS data. e.g. “1.6.0”

Note: Since the <src> element is a “mixed” type you can still include a simple string to describe the device, such as “Apple Watch Series 8”.

Usage

GPX 1.1 compliant files should begin with something like the following:

<?xml version="1.0" encoding="UTF-8"?>
<gpx creator="GPS Wizard"
     version="1.1"
     xmlns="http://www.topografix.com/GPX/1/1"
     xmlns:tpx="http://logiqx.github.io/gpx-ideas/xmlschemas/logiqx/tpx/1/0"
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:schemaLocation="http://www.topografix.com/GPX/1/1
                         https://www.topografix.com/GPX/1/1/gpx.xsd
                         http://logiqx.github.io/gpx-ideas/xmlschemas/logiqx/tpx/1/0
                         https://logiqx.github.io/gpx-ideas/xmlschemas/logiqx/tpx/1/0/tpx.xsd">

It is recommended that source information be provided within the <trk> element:

<trk>
  <trkseg>...</trkseg>
  <extensions>
    <tpx:src>
      Apple Watch Series 8
      <tpx:device>
        <tpx:manufacturer>Apple</tpx:manufacturer>
        <tpx:product>Watch Series 8</tpx:product>
        <tpx:serial>123456789</tpx:serial>
        <tpx:name>Nathan's Apple Watch</tpx:name>
        <tpx:version>8.5.1</tpx:version>
      </tpx:device>
      <tpx:software>
        <tpx:developer>Hoolan Ltd</tpx:developer>
        <tpx:application>Hoolan</tpx:application>
        <tpx:link>
          <tpx:href>https://www.hoolan.app/</tpx:href>
        </tpx:link>
        <tpx:version>1.6.0</tpx:version>
      </tpx:software>
    </tpx:src>
  </extensions>
</trk>

Track points should always include course and speed when available. Ideally they should also include accuracy estimates when available:

<trkpt lat="50.5710623" lon="-2.4563484">
  <ele>7.90</ele>
  <time>2022-04-11T10:16:01Z</time>
  <sat>6</sat>
  <hdop>1.4</hdop>
  <extensions>
    <tpx:extras>
      <tpx:course>157.19</tpx:course>
      <tpx:speed>0.5429</tpx:speed>
      <tpx:hacc>2.0</tpx:hacc>
      <tpx:vacc>4.0</tpx:vacc>
      <tpx:cacc>5.0</tpx:cacc>
      <tpx:sacc>0.5</tpx:sacc>
    </tpx:extras>
  </extensions>
</trkpt>

Validation

Software developers should validate their GPX files during the development / testing process.

Validation methods are described in a technical overview of the GPX format.

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