Do I dare post this?
From Active captain this morning
Interesting article as we anchored a lot on our trip and only used the Raymarine Anchor alarm only twice. I also have the drag queen app, but I would forget about it and by the time the anchor was set it was too late to use it.
>>> Anchor Circles >>>
Last week's newsletter brought another large set of responses from the
ActiveCaptain community. We continue to be amazed at the quality and
abilities of boaters. It doesn't matter what type of boat you have, the
experience out there is much better than any other source we can find.
Especially when the topic is anchoring.
So the math major in me wants to bring up another topic. It's a detailed
one that nearly everyone ignores. You see, so many of the responses
involved explanations of how the navigation product or app used for
anchor alarms draws a circle. Other people watch the track their boat
creates and the circle drawn as they swing with wind and tide.
Circles, circles, circles.
And yet, none of them should look like circles.
So I checked a couple of anchor alarm apps to see what they do. And sure
enough, some draw perfect circles based on your alarm distance. In all
cases, the circles are drawn over Google Maps, satellite imagery, or
nautical charts. And all of them are drawing the swing area incorrectly.
By now, some of you are saying that I've lost my mind. Of course it's
a circle with the anchor in the middle and the radius equal to the alarm
distance. The thing being forgotten is that the Earth is not flat. It's
a sphere. And nearly all the map data, satellite imagery, and nautical
charts are drawn with a Mercator projection.
The Mercator projection of the Earth produces some exceptional features
that are perfect for navigation. The main one is that bearing lines
appear as straight lines. In nearly all other projections, maintaining a
constant bearing will create a curved line. Not so with Mercator.
The disadvantage of the Mercator projection is the distortion that
happens as you move away from the equator. This is what makes Greenland
So the problem with many existing anchor alarms is that the alarm circle
itself needs to be projected through the Mercator projection too. But
doing that involves a lot of trigonometry and many of the developers
writing these apps don't have a good understanding of it. The reality
is, a proper anchor alarm circle projected through the Mercator
projection should look squashed and not perfectly round as long as
you're not near the equator. As soon as you start to get 30 degrees of
latitude away from the equator, the circle will not look like a perfect
So go check your anchor circles drawn by alarms. Are they squashed?
If not, perhaps you should be using a different anchor alarm.
A second fun exercise is to plot your track through tide cycles as you
swing around your anchor. That won't ever be a circle either - if you
look carefully, it will also be a little squashed.