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Frequently Asked Questions - Great Circles
  • What is a Great Circle? See the Wavebasics tutorial, about 2/3 rds of the way down the page.
  • How are these charts produced? Stormsurf used the excellent GNU map drawing tool call Generic Mapping Tool (GMT).
  • What are these charts used for? In combination with the output from the QuikSCAT satellite, one can determine if the winds inside a storm (fetch) is aimed at your beach. If sufficient wind is blowing parallel to a great circle path that terminates at your beach, then a swell could arrive. Conversely, if the wind is blowing at a significant angle to the great circle path to your beach (greater than 45 degrees), then the odds of a swell making it to your beach with it's maximum potential size starts decreasing. The great circle really becomes important if the fetch is greater than 1000 nmiles away from your beach. Look at the summer great circle paths for Northern CA, and you'll begin to get a sense of how strange (and important) this tool is. Secondarily, these Great Circle paths are constructed to identify the swell window for many breaks. If the fetch is outside the bounds of the great circles depicted, then it's likely land will block the swell from arriving at your beach.
  • Why are there not charts for my home break? Each chart takes about 3-4 hours to produce. Every data point is manually adjusted and positioned on the map. Needless to say it took an extraordinary amount of time to produce the images contained currently in this site. We tried to provide a well rounded selection of charts for the areas with the highest concentration of viewers serviced by Stormsurf. But we just ran out of time and energy. There are still broad areas we want to work on, notably South America, and we plan to expand as time permits. Conversely, the charts provided are fairly serviceable as-is if you live near a break already documented. For the high level charts, there is little change in swell direction from the points on the chart to another break 100 miles up the coast (maybe only a degree or two at most). So you can visually transfer the great circles (within reason) to get a rough sense of such paths for nearby breaks.
  • How do the 'Close-up' charts correspond to the high level charts? What we did was first produce a high level chart for one location, say Southern Australia - Victoria. Then we used the exact same starting points (the ones over Antarctica) and zoomed the end-point perspective in, constructing individual charts for Port Cambell, Bells, and Ocean Beach etc. Notice that the swell angles change just slightly from one break to the next, even though the starting points are mostly the same. As we moved through the breaks the swell window changed slightly, with some great circles starting to cross over land. So we either added, deleted or moved a path or two to fit within the swell window. In some instances we left paths over land if it seemed reasonable that the swell would wrap into the nearby beach. Granted, we don't have enough site specific knowledge about every break depicted to know how much wrapping actually can occur. It's a best theoretical guess.
  • Why are the swell angles/great circles not depicted in even units of say 5 or 10 degrees? In an attempt to build an aesthetically pleasing image, and to ensure the paths don't drive over land so that the real swell window could be defined, we chose to build the images as you see them. In this way, one can see the true swell window, accurate to 0.5 of a degree. Note: Swell angles were calculated using the WGS84/NAD83/GRS80 earth model, which accounts for the fact that the earth is not a perfect sphere.
  • Why are distances not depicted, say in units of 600 nmiles? Because we don't forecast that way and don't advocate that any one else should either. The most accurate way to determine distance is to capture the starting and end coordinates and plug them into a great circle distance calculator like ours here.
  • Why are some locations swell windows separated into subcategories? Notice that in many instances the charts are broken down by seasonal swell windows. For example, for Southern California there are charts for Summer and Winter, and within the Summer category there are Southern Hemisphere and Hurricane charts. This was done mainly as a way to drill down/zoom in and get to the peculiarities of each. If we chose to produce only one chart for each location, then we couldn't get enough detail to depict all the special conditions associated with each seasons swell windows. In the end, we chose to go for detail and accuracy rather than a broader approach.

 

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