The earth isn't flat, but our charts are. When we create a flat chart to represent the curved surface of our oceans we have to choose a 'projection' that converts one to the other. The downside of the way we do this (and the way everyone has to do this to make maps that you'll recognise) is that a straight line on the chart isn't the same as a direct line on the earth's surface. This means a swell that appears to be a straight shot to your local break might actually not make it and also means the swell window for some regions can be larger than you realise (You can fly in a straight line from Brazil to Indo, whatever your map appears to tell you, which means ocean swell can travel that way too).
The solution for surfers are great circle lines, these show you exactly what a direct line looks like on the chart, so exactly the directions swell can travel freely to a particular location. We're adding great circle lines on our charts directly to your local break based on it's specific swell window so you'll have an 'at a glance' idea of whether a storm is relevant to your break. Want a due west swell? Look for storms on the charts sitting on the 270 degree line with swell heading along it. It's a pretty neat addition if you like your forecasts long range and with our in-house 16 day modelling and this tool we can start to analyse the charts for swells that might not arrive on the beach for almost three weeks.
The large circles are also a useful addition. In this instance they're drawn at 500 mile intervals from our home break. It's kind of interesting to know how far away a storm is, but more importantly 14 second period swell (pretty typical at peak for a good surf here) travels almost exactly 500 miles a day in open ocean. So you can think of the circles as day lines, each line a swell has to cross is a day before it brings surf to your local beach, the swell in this example is four days away from our local beach at this stage.