Great Circles

Important notes

  • This feature is only available to our Premium and Forecast members.

Why do I need this tool?

Have you ever noticed the path of a flight is curved on a 2D map of the Earth?

In order to make a 3D object (globe) into a 2D picture (map), you need to warp reality and make some countries appear bigger while shrinking others to fit everything in.

This works fine for most purposes!

But when traveling in a straight line, your path on a 2D projection (map) will appear curved. If you path was drawn on a 3D object (globe), the line would appear straight.

The essential theory you need to wrap your head around: If you're looking at a 2D map of the Earth, and you can draw a straight line from a distant swell to your surf spot - you can be almost certain it isn't traveling in your direction.

So, forecasters draw curved lines on a 2D map to represent straight lines. This helps them identify a swell that is traveling directly towards a surf spot from thousands of miles away.

 

How do I use the Great Circles?

You can't make everything make sense at the same time, unfortunately. We have to determine 1 "destination" (i.e. surf spot) and draw lines from that single point that reach across the entire map.

1. Determine your destination.

If you click through to Charts from a spot page, the Great Circles will already be overlaid on your map.

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If you click through to Charts from the homepage, or use the direct URL surfline.com/surf-charts, Great Circles won't automatically appear. Click the button highlighted below and choose from the following options

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  • Draw circles from a custom location (wherever the centre of your screen is over)
  • Draw circles from a spot in your saved favorites
  • Draw circles from a Custom Forecast you set up on your account (Custom Forecasts)

Screenshot 2024-02-20 at 15.20.20.png

Once you've selected your destination, hit the close (X) button to hide the menu and show the Great Circles.

 

2. Identify your basins of interest

We know that the black lines track a true straight path across a 3D globe.

Some of the guidelines will have a degree readout (highlighted in red below).

Screenshot 2024-02-20 at 14.39.46.png

This means if I were to look at a compass from my current position, point it towards 280º, and walk in a straight line, my path would follow that specific guideline when projected on a 2D map.

Next time you're checking the surf at a favorite spot, pull out your compass app and see the directions you're exposed too (not blocked by islands or headlands). Then when back at home, see which corners of the ocean you should be keeping your eyes on based on the swell window you identified.

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Or, zoom right in to your surf spot and enable the Great Circles. Use the guidelines to determine your swell window.

Screenshot 2024-03-19 at 11.41.23 AM.png

Following a line from the surf spot to a blocking coastal feature (above) allows me to identify my basin of interest (below). I'm interested in the activity above (and moving in the direction of) the red line.

Screenshot 2024-03-19 at 11.31.15 AM.png

 

 

Questions or issues?

Write to support@surfline.com and attach screenshots of anything you refer to so a dedicated Support agent is able to take a look.

 

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