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the skipper vessel has put a waypoint into the gps and the display shows the direction and distance to the waypoint, but it is not the same as the course to steer plotted on the chart.
why is this?

GO TO Course Different from ROUTING course
~ Captain Paul's response to: ~ MENDES ~

Without knowing exactly that area you are speaking about, the distance between the waypoints and or the brand of the GPS unit its mapping software, I can only offer some suggestions that may be causing the deviation.

First, check if the unit is set up for Magnetic or True North and the settings for the position of the chart on the screen. If one is different from the other, it could account for the different course readings.

Secondly, the GO TO designation indicates a DIRECT “as the crow flies” distance and direction and does not take into consideration the curvature of the earth. Most marine charts are depicted in a Mercator projection to allow the navigator the ability to see the curved surface of the earth on a chart’s flat surface. To accomplish this view, certain features must be depicted in a manner that coincides to the chart. The farther away from each other the greater the differences in this GO TO course would be. The effect would become more distinct at over a straight 15 mile distance. The greater the distance between the two positions, the greater the difference will be as it would be laid out on a GREAT CIRCLE course and not a line of sight course as would be shown on a flat chart or piece of paper. One line would be a curved arch on the surface of the earth the other would be a straight line going through part of an arch linking the two positions of the surface of the earth.

In addition, some units have an automatic “Routing“ feature activated when using certain maps or charts. If so, they give an automatic routing course and distance that is in established channels etc., and around obstacles, shallow water and other obstructions between the present position and the marked waypoint. This Routing feature is much as an automobile GPS unit has where the route is automatically laid out on established roads and highway and not a direct line course.

If the operator were to select a non auto routing setting, the unit would then provide a direct, line of sight heading to reach the desired waypoint.
The routing feature will guide the vessel in a sometimes longer distance to clear unwanted waters.

~ Captain Paul ~