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GPX Extensions - Position / Velocity / Time (PVT)

Overview

Position / velocity / time (PVT) elements are outputs of GPS / GNSS chipsets and location APIs.

COG and SOG are relevant to almost all activities; land, sea or air.

Name	Values	Description
<cog>	0 to 360	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. See RMC and VTG in NMEA 0183
<sog>	>= 0	Speed over ground (SOG) / ground speed (GS) in m/s is the instantaneous horizontal speed. 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. Speed may be measured by a wheel odometer, thus it can be independent of the GPS / GNSS receiver. See RMC and VTG in NMEA 0183 - units are knots or km/h so need to convert to m/s
<roc>		Rate of climb (ROC) / vertical speed in m/s 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. ROC may be measured using barometric pressure or an IMU, thus it can be independent of the GPS / GNSS receiver.
<dist>	>= 0	Cumulative distance over ground for the trip / activity in meters (m). Distance may be determined by integrating speed over time or measured by a wheel odometer, thus it can be independent of the GPS / GNSS receiver. See VLW in NMEA 0183 - units are nautical miles so need to convert to meters

Accuracy estimates for COG, SOG and ROC (typically determined by the GPS / GNSS chipset) are supported by the “acc” attribute.

Name	Values	Description
<cog acc="n">	0 to 180	Course over ground (COG) accuracy / error estimate in degrees, such that the difference between 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.
<sog acc="n">	>= 0	Speed over ground (SOG) accuracy / error estimate in m/s, such that the difference between 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.
<roc acc="n">	>= 0	Vertical speed accuracy / error estimate in m/s, such that the difference between 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.

GPX 1.2 is also expected to support position and time accuracy estimates using the “acc” attribute.

Name	Values	Description
<trkpt acc="n">	>= 0	Horizontal accuracy / error estimate in meters, such that the difference between the true horizontal position and the reported horizontal 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.
<rtept acc="n">	>= 0	Horizontal accuracy / error estimate in meters, such that the difference between the true horizontal position and the reported horizontal 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.
<wpt acc="n">	>= 0	Horizontal accuracy / error estimate in meters, such that the difference between the true horizontal position and the reported horizontal 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.
<ele acc="n">	>= 0	Vertical accuracy / error estimate in meters, such that the difference between the true vertical position and the reported vertical 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. TODO - list NMEA sentences
<time acc="n">		Time accuracy / error estimate in seconds (including milliseconds), such that the difference between the true time and the reported time 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.

Notes

• The abbreviation COG has been used to avoid potential confusion caused by the word “course”
• The abbreviation SOG has been used to avoid confusion due to the many kinds of speed; boat speed, air speed, wind speed, etc
• Rate of climb (ROC) is most applicable to aviation and has been chosen instead of the phrase “vertical speed”
• Distance is included to avoid cumulative errors which may occur due to rounding
• Horizontal accuracy estimate can be calculated from latitude / longitude accuracy estimates using the formula hAcc = √ (latAcc² + lonAcc²)

Exclusions

• Individual north / east / down (NED) and east / north / up (ENU) velocity components are available from some GPS / GNSS chipsets
  • These velocity components have been excluded because they can be determined from COG, SOG and ROC
• Glide ratio / slope is not included because it can be determined using simple formulas
  • glide ratio = horizontal distance / change in altitude
  • glide ratio = horizontal speed (SOG) / descent speed (0 - ROC)

Example Usage

Land / Fitness

• Hiking
• Running
• Cycling

Sea / Nautical

• GPS Speedsailing
• Dinghy Racing
• Yachting

Air / Aviation

• Skydiver
• Microlight
• Light Aircraft

Meteorology

• Weather Station

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