Effective use of the anchor alarm function on your GPS
Many GPS units for marine use provide an anchor alarm function, though little or no guidelines to help with its effective use. This article provides an overview of different anchoring situations; steady or changing wind or current, length of anchor rode, and anchor system employed (single bower or with kedge in differing configurations). All of these factors affect the extent of possible movement before the anchor actually drags, and consequently affect the setting of the GPS anchor drag alarm. A brief overview of the Global Positioning System is necessarily included, as are the implications of its operation upon the setting and triggering of the anchor alarm under the different anchoring situations and approaches. Anchor types and bottom types are not discussed. There are three diagrams and one table.
Figure 1: Anchoring approaches
Anchoring considerations and approaches
Anchoring situations can be as many and varied as those of the preceding sail; fair weather or foul, sea or river, lake or lagoon, current or tide, wind or calm. These factors will play a part in deciding which approach to employ at the time of anchoring: a single bower anchor; bower and kedge in tandem; two at 45 degrees; fore and aft, or fore and aft pivoted below the keel. Figure 1 illustrates these different anchoring approaches.
Anchoring considerations commentary
A single bower anchor can suit a range of ‘normal’ anchoring situations, however heavy weather or poor holding may dictate use of an additional kedge anchor in tandem with the bower or, where the heavy weather is expected predominantly from one direction, at 45 degrees. This latter approach is not suitable where the direction of predominant conditions are expected to change significantly as respective anchor lines could become tangled. When anchoring in a location where currents are expected to reverse, the fore and aft approach can significantly reduce swing. Alternatively, setting two anchors fore and aft and joining the lines together at a point which is let down below the keel will permit swinging to the strongest prevailing condition, yet with much reduced radius; useful in crowded or constrained anchorages.
At the chosen anchoring location the depth of water and predicted height of tide and/or swell, any existing and expected winds will play a part in how much anchor rode is deployed, as will the rode type (chain or rope). More rode of rope will be let out than for chain for the same depth of water. More rode will be let out for expected bad conditions over calm ones. The types or anchor and bottom are also important, though for the purpose of these drag alarm discussions, it is assumed that anchor type is suitably matched to bottom type.
All of these factors (steady or changing wind or current, length of anchor rode, anchoring approach) have a bearing on how much you could move or swing from your initial point of anchorage without actually dragging on the anchor. This needs to be taken in to account when taking any sightings or bearings and when setting the anchor drag alarm on your GPS.
Figure 2: Principle of the GPS anchor drag alarm function
The GPS anchor drag alarm function
Let us now look at how the GPS anchor drag alarm works in principle. When setting the alarm, you are expected to enter a distance, either in terms of meters or fractions of a nautical mile. This establishes a ‘zone of surveillance’ with a radius R (the distance entered at the time of setting the alarm) at the point where the alarm was enabled on the GPS unit within which you could move about and not trigger the anchor drag alarm. This principle is illustrated in Figure .
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GPS anchor alarm setting commentary
The point at which the alarm is set is more than likely not to be the point at which the anchor was set; it could be some way from it and possibly up to almost the whole of the length of the anchor rode let out. The distinction is important, particularly if a large amount of anchor rode has been let out; you could be, say, 40m or 0.02NM from the anchor point when the alarm is set, if any current or wind reverses you could then move up to 80m or 0.04NM from the alarm set point, and not actually drag on the anchor.
By the way, here I am using a simple rule of thumb based on the fact that one nautical mile is 1.98km, and therefore 0.01NM is close enough to 20m.
Table 1: Recommended GPS anchor drag alarm settings
Setting the zone of surveillance
Ideally the zone of surveillance must be sufficiently small so as to alert you quickly to a potential dragging anchor situation, and yet large enough not to trigger the alarm inappropriately as you swing about on a well set anchor under applicable conditions. Table 1 summarises my recommendations for the setting of the GPS anchor drag alarm given the different anchoring situations and approaches.
There is yet one further important factor to take in to account in addition to those already mentioned. This one is implicit in the functioning of the GPS system and how it determines your position.
The Global Positioning System & the accuracy of your position fix
The Global Positioning System is based on a constellation of 24 satellites operated by the United States’ Air Force. Each satellite orbits around the Earth every 12 hours, therefore the constellation appears constantly in motion with individual satellites rising from one horizon and setting in another. The satellites broadcast specific information which your GPS receives and decodes. It must be able to ‘see’ three or more satellites in order to derive your position in terms of latitude, longitude and (with four or more satellites) altitude. Your GPS unit constantly tracks satellites in the constellation and always aims to switch to those providing the best signal at any given moment. Interference or a weak or blocked signal can affect the position determination. Therefore if you are surrounded by cliffs or headlands, or if your GPS unit is not well installed, or if it is a handheld unit down below deck, then the accuracy of your position determination may be adversely affected. Moreover, if the satellites being received at a particular moment are all grouped together in a small portion of the sky, your position determination will be less accurate than if they were spread across the sky. Your GPS unit will display an indication of the accuracy of your position determination either as GDOP (Geometric Dilution of Position, a small number as close as possible to 1) or alternatively as EPE (Estimated Position Error, a measure in +/- metres). It can also display which satellites are currently in view and actively being used to determine your position. I find it useful to make a record of the GDOP or EME when setting my GPS anchor drag alarm.
Figure 3: Effect of changing accuracy of position determination
Effect of changing accuracy of position fix
From this discussion you will appreciate that the accuracy of your position fix can and does change from one moment to the next. Consequently, even if the GPS anchor drag alarm does sound, you may not actually have dragged on the anchor at all. Instead the accuracy of position determination may have changed, e.g. it may be that suddenly your GPS unit can only see three satellites instead of four, and your EPE has gone from +/- 7m to +/- 30m. Your GPS unit calculates that you have strayed outside the zone of surveillance and sounds the alarm. This situation is illustrated in Figure 3.
In the end you will need to apply a certain amount of ‘latitude’ (sic) to the sounding of the GPS anchor drag alarm. Do check the GDOP or EPE as well as making the usual good visual checks before rushing to reset your anchor or change anchorage.