Common GPS error

There are a few tricks and techniques to using a GPS. It is important that the user of a GPS understand the device and how it is used.

The first thing to understand is Chart Datum. Every chart has a chart datum. This is listed on the border of the chart. It is usually specified by a name and, possibly, year implemented. For example, North American Datum 1927 (abbrev - NAD27) refers to a standard reference datum set in 1927 for use in North America. Others are NAD83, WGS84, Ordinance Survey Great Britain, etc. If the GPS is set to a datum other than the chart datum, there is no guarantee that the position shown on the GPS will correspond with the same position on the chart. As an example, confusing NAD27 and NAD83 can result in discrepancies of about 200m.

So - Rule 1 - Set the chart datum in the GPS to the chart datum of the chart you will be using. And remember - two adjacent charts can have different chart data.

The next thing to remember is that the GPS bearing is the direction to a destination (i.e. as the crow flies). It is not the path to get there. If the bearing to a campsite points into a cove, that doesn't mean the campsite is in the cove. It may be in an adjacent cove and the path to follow is in another direction around a point to the next cove. Folks that ignore or don't understand the Chart Datum may get used to discrepancies of a few hundred meters and think that the closest cove is the right one, even if the GPS says the campsite is still another few hundred meters away!