Now, if the anchor moved a metre or 2 before it dug in, we could add another 2m of uncertainty to our swing radius. Now if we consider that the best charts are made using DGPS, we should add another 2 to 3m of uncertainty to our swing radius to allow for chart inaccuracy, making 6m or half a boat length of uncertainty to be added to the swing radius around a position that is already a boat length away from where the anchor was dropped. Trinity House says that DGPS is accurate to 5m for moving applications and even better for stationary applications, so we should add at least a 3m uncertainty to our radius of swing. In this case, it would be at about 12m from the position the anchor was dropped. With the GPS aerial at the stern, we would be a boatlength out if we set the GPS position at the time of anchoring as the position from which the allowed radius of swing were set.
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