Dear Capt. Paul:
Recently, a skipper of a vessel put a waypoint into the GPS, and the display showed the direction and distance to that point, but it is not the same as the course to steer plotted on the chart.
Why is this? What would cause this?
Capt. Paul’s response:
Without knowing exactly the area you are speaking about, the distance between the waypoints, the brand of the GPS unit and its mapping software, I can only offer some suggestions that might be causing the deviation.
I can give you some suggestions, but you are going to have to enlist Sherlock Holmes, or at least do a little investigation yourself to determine what is actually causing the different course values.
First, check if the unit is set up for magnetic or true north, and that the settings for the position of the chart on the screen are the same. If one is different from the other, it could account for the different course readings. It certainly might be different from the magnetic boat compass, if the GPS unit is set to display true north and the boat compass is magnetic, but the chart might also be set up to display either true or magnetic north.
In addition, some units have an automatic “routing“ feature, which is activated when using certain maps or charts. This feature is much like an automobile GPS unit, where the route is automatically laid out only on established roads and highways and not a direct line course.
If so, they give an automatic routing course and distance that is in established channels etc., and around obstacles, avoiding shallow water and other obstructions shown on the existing internal mapping software.
This can cause a deviation between a “Go To” direct route to a routed course that was made by the GPS unit. This would cause a difference in the courses between the present position and the marked waypoint.
If the operator selects 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 and slightly different courses in order to clear unwanted waters and obstructions.
Next check the “Course” settings. The usual definition for “course” is a direction from the present position to a specific location.
Most units offer several different course settings. Some have a feature called “heading” and even “course ran” and “course to steer.” The course to steer setting might be taking into consideration the automatic routing that avoids what it thinks are unsafe areas.
Obviously, the meaning of these choices would naturally indicate a different value. Some units have the option of displaying all of these different choices as a value in small data window on some of their screens.
Then, if there were a time difference between noting the values, the actual position of the vessel might have changed. This would cause the unit to display a “corrected” go-to course to the waypoint. This would now be a different heading from the position where the go-to waypoint was entered in order to get to the waypoint.
If the vessel traveled any distance other than one that is a direct course to the waypoint, the go-to value would constantly be changing.
This becomes dramatic with a handheld unit when navigating to a waypoint. Most travels over land have to take into consideration terrain and topographical features, especially obstacles that would be in line with the waypoint. As the traveler moves around a tree for instance, the go-to course would naturally change.
If the distances between the present position and the newly entered waypoint are a considerable distance apart, there are other factors that come in to play. The go-to designation usually 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-plus-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.
The course could be shown as a different value.
If the vessel traveled any distance other than one that is a direct course to the waypoint, the GO TO value would constantly be changing exactly as the aforementioned over land traveler with the handheld unit.
I suggest that you do a little investigation and see if any of these suggestions help in determining the different readings you have experienced.
I would be delighted to hear from you again when you find the reason.
- Capt. Paul