gpx-ideas

GPX Extensions - Sea / Nautical

Overview

Sea / nautical elements include speed through the water (STW), wind measurements, tidal measurements and water depth.

Name	Values	Description
<stw>		Speed through the water (STW) / boat speed in meters per second (m/s) See VHW in NMEA 0183 - units are knots or km/h, but recommend converting from knots to m/s n.b. different to <sog> in gpx_pvt which is speed over ground
<dist>	>= 0	Cumulative distance for the trip / distance through the water since last reset in meters (m) See VLW (water distance since reset) in NMEA 0183 - units are nautical miles so need to convert to meters n.b. different to <dist> in gpx_pvt which is trip distance over ground
<awa>	-180 to 180	Apparent wind angle (AWA), relative to the bow in degrees (°) See MWV (reference = R) and VWR in NMEA 0183
<aws>	>= 0	Apparent wind speed (AWS) in metres per second (m/s) See MWV (reference = R) and VWR in NMEA 0183
<twa>	-180 to 180	True wind angle (TWA) representing wind-over-water, relative to the bow in degrees (°) See MWV (reference = T) in NMEA 0183
<tws>	>= 0	True wind speed (TWS) representing wind-over-water in metres per second (m/s) See MWV (reference = T) in NMEA 0183
<set>	0 to 360	Direction of the water / current, relative to true north in degrees (°) See VDR in NMEA 0183
<drift>	>= 0	Speed of the water / current in meters per second (m/s) See VDR in NMEA 0183 - units are knots so need to convert to m/s
<depth>	>= 0	Depth of the water below the surface in meters (m) See DBS (not DBK or DBT) in NMEA 0183 - often determined by chartplotter or fishfinder

Background

• Non-sailors can read a page written by Raymarine to understand the difference between apparent wind, true wind, and ground wind.

Notes

• Boat speed / speed through the water (STW) is typically measured using a paddlewheel or ultrasonic transducer
• Distance through water is included to avoid possible accumulated errors when deriving from speed every second
• True wind is typically wind-over-water (using heading and water speed sensor) but can be wind-over-ground (using heading, COG + SOG)
  • Strictly speaking you do not need to record TWA and TWS.
    • However, recording the true wind (TWA and TWS) will help to maintain precision
  • There is no need to record true wind direction (TWD) which is relative to true north in degrees (°)
    • It can easily be calculated from TWA and the heading
      • TWD = (TWA + heading) % 360
• Tidal elements “drift” and “set” can be calculated from other elements but have been included to maintain precision
  • See page about set and drift which explains how they are determined using a vector triangle
• Rate of turn (ROT) is supported by the gpx_imu extension
  • ROT represents the rate of change of heading, typically measured by external rate of turn turn indicator (ROTI)
  • ROT should not be derived from COG information - link
• “Ground distance since reset” (VLW sentence) should go in <pvt:dist> - see gpx_pvt

Exclusions

• IMU data such as heading, pitch and roll are handled by the gpx_imu extension
• Meteorological data is handled by the gpx_met extension
  • <atemp> and <wtemp> should be used for air temperature and water temperature
  • <gwd> and <gws> should be used for ground wind direction and speed, not to be confused with TWD and TWS
• Engine data is handled by the gpx_eng extension
  • e.g. tachometers, temperatures, pressures, fuel, batteries, etc
• VMG relative to the wind is not included at this time because it can be calculated from SOG + COG and TWD

Example Usage

Sea / Nautical

• Yachting

Meteorology

• Weather Station

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