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The boat icon's plotted position is not correct. - Printable Version

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The boat icon's plotted position is not correct. - Chuck - Raymarine - Moderator - 08-16-16 05:04 PM

The boat icon's plotted position is not correct.

The boat icon's plotted position and track displayed by a Raymarine MFD should always be plotted properly on the chart, within the accuracy of the GPS sensor and given correct georeferencing of each individual chartlet within the cartography set being used. As long as the system's GPS sensor is functioning properly and the system is configured for a WAAS position FIX, then such problems are generally the result of improper georeferencing of the chartlet contained on the chart cartridge for the area in question.

In systems featuring more than one GPS sensing device, care should be taken to ensure that the system has been configured to utilize only one of the available GPS data sources. In systems supporting Data Source Selection (ex. Raymarine's a/c/e/eS/gS-Series MFDs, etc.) the system's Data Source Selection feature should be utilized to select the source of GPS data which will be utilized by the system. The selected source should generally be installed in a location affording it an unobstructed view of the skies overhead. This may require that an external GPS sensor be installed onboard (ex. Raystar 150 GPS Sensor, Raystar 130 GPS Sensor, GA150 GPS Antenna, etc.). Failure to afford the GPS sensing device with an unobstructed view of the skies overhead may result insufficient GPS satellite signal strength being received by the GPS sensing device, resulting in an inaccurate GPS position FIX, intermittent GPS position FIX, or no GPS position FIX.

In order to resolve whether the problem is being caused by an inaccurate GPS or an improperly georeferenced cartography, the coordinates of the vessel should be compared to the coordinates of an object having a known location (i.e. bridge or some other fixed object ... do not use buoys) or those specified by another WAAS GPS (ex. handheld). The position (lat/lon coordinates) reported for the boat's position should be the same (within a few thousandths of a minute). When tracking a WAAS satellite (see MFD's GPS Status dialog) and has a low HDOP (http://en.wikipedia.org/wiki/Dilution_of_precision_(GPS)) value (ideally 1.0 or less), Raymarine's WAAS GPS sensors will typically provide accuracy of approximately 3m RMS. When not tracking a WAAS satellite (i.e. FIX Status = FIX), Raymarine's WAAS GPS sensors will typically provide accuracies of approximately 15m RMS. Should a high HDOP value be detected and the GPS sensor have an unobstructed view of the skies overhead, then it may indicated that the GPS receiver has degraded and may be in need of replacement. Note that a position error of less 0.001' is equivalent to approximately 6 feet. Should the coordinates be significantly different, then it may indicate a problem with the GPS sensor or it may indicate that the installation location of the GPS sensor is not conducive to GPS signal reception. Should the GPS sensor be embedded within the MFD, then it may be necessary to interface an external GPS sensor to the MFD and that the external GPS sensor be installed where it will be afforded an unobstructed view of the skies overhead.

If the GPS HDOP is low (see above) and the coordinate comparisons to fixed objects or a handheld WASS GPS do not indicate a significant difference, then the problem would appear to be caused by a cartography georeferencing error. In the early years of electronic charts, such georeferencing problems were not uncommon. One of the benefits of todays chartplotting products is the ability to support more than one type of cartography (ex. Raymarine's a/c/e/eS/gS-Series MFDs support cartography from LightHouse Charts, Navionics, and C-Map). Accordingly, many vessel operators will equip their systems with more than one type of chart card for the waters being navigated. In systems equipped chartplotters supporting more than on type of cartography and being equipped with more than one type of cartography, should a georeferencing error be identified, then the operator may configure the chartplotter to utilize one of the other types of cartography present within the system. Chart card contains many chartlets, each providing cartography for a specific geographic area and chart scale. Should there be a problem, then it would typically affect only the chartlet in which the georeferencing error had been detected.

In the early years of electronic charts, such georeferencing problems were not uncommon and chartplotting products often included a Chart Offset feature to permit the vessel icon to be properly positioned on the displayed chart. However, it would be necessary to turn off the Chart Offset feature when transitioning to properly georeferenced chartlets. Unless constraining the vessel's operation to a the area covered by a single chartlet, it is generally not recommended that this feature not be enabled for this and the reason specified above. Raymarine has not included this feature within the currently manufactured MFDs, as electronic cartography has matured and most such georeferencing errors eliminated.

Cartography georeferencing discrepancies as well as other cartography related discrepancies should be reported to the cartography manufacturer, Navionics, C-Map, Raymarine (for LightHouse Charts only) to seek correction of the error in a future update to the cartography. When submitting such discrepancy reports, it is recommended that a digital snapshot of the MFD’s screen displaying the chart scale and cursor or vessel coordinates be attached to the report.