A series in which Swellnet explains surf forecasting information, from the basics to the loftiest of concepts, using simple language and real examples. This week, real buoys vs virtual buoys.

What's the difference between virtual buoys and real buoys? No doubt some people are shaking their heads while reading that question because the answer appears to be so simple. Virtual buoys are virtual, as in they aren't real, take a boat out to the virtual buoy's GPS position and you'll find nothing but empty ocean. However, do the same with a real buoy and you'll bump into a big metal float filled with instruments such as accelerometers that's tethered to the ocean floor.

So now we've got the existential meaning out of the way we can look at each type of product, describe how they work, and assess what's good and bad about them. Buoys – both virtual and real – are critical forecasting tools, so at the end of the process you should have a better understanding of the data and you may even sharpen your own forecasting skills.

Seven wave buoys waiting to be used by Manly Hydraulic Laboratory

Aside from tangibility, the major difference between the buoys are the types of information they record. Virtual buoys predict virtual information. They predict what a spot of ocean will be doing during a designated point in time. The read out from a virtual buoy will only ever be a prediction and never a measurement. Real buoys, on the other hand, don't predict information, they measure and record what is actually happening.

Therefore, real buoys can only help with current data. Of course they can help estimate swell arrival times, yet everything they record is happening right now. But beware, just because real buoys record real information doesn't mean they're infallible. More on that later.

In contrast, virtual buoys can give both current data and they can peer into the future. They therefore have wider use to forecasters, however users must be aware of their shortcomings.

As mentioned earlier, virtual buoys are a prediction of what a designated spot of ocean will be doing as calculated by an underlying weather model. The information is derived by choosing a GPS co-ordinate somewhere on the ocean's surface.

Some websites allow you to choose a grid point, in that case you need to be aware of the nearby coastal alignment. Is it sheltering the swell? Will you get a true read out? Virtual buoys come into their own when used in the open ocean, they're very good at giving open ocean swell heights and surface conditions where a margin of tolerance is acceptable. Boat captains love them for this reason, yet surfers need to understand their limitations when predicting specific nearshore wave heights.

Coastal alignment and underlying bathymetry can significantly affect approaching swell – a two metre south swell can be 10 foot at a known swell magnet, and just 5 foot down the road - yet virtual buoys rarely take these factors into account. I say 'rarely' because some sites do calibrate their virtual buoys to better reflect local conditions. Hint: we're one of them. Rather than give raw data, we've calibrated every Australian coastline, and many overseas ones, to better reflect how they handle prevailing swell. Click the 'Surf Forecast' tab for any region.

For this reason it's best to use virtual buoys as a guide only. Don't expect exact information, look for trends in the swells – is the swell rising or is it dropping? - and keep in mind there may be a size range between what the virtual buoys say and what actually happens. And just as there may be a size range there may also be a time range – the swell could come earlier or later than predicted.

It's important to note that the further out the prediction is – i.e 12 hours, 48 hours - the less accurate the reading will be. This effect is greatly amplified on east facing coasts because they're traditionally more prone to dynamic weather patterns, and hence swells, than south or west facing coastlines.

Another thing to note is that almost all virtual buoy networks run off the same base information. The only difference between those sites is where they choose the grid points and the website's user experience - some graphs simply look more appealing than others. This is important because aesthetics can effect how you interpret data. Try to see beyond the layout and colours.

When it comes to assessing real time information – i.e what the swell is doing right now – it's always best to seek out a real buoy. As mentioned, virtual buoys run off weather models and those models can be incorrect, sometimes by a little, occasionally by a lot. So if you need the hard facts about swell then track down a real buoy. In Australia, each state government has their own network and some ports have private yet publicly available buoys. See a list below.

Perhaps the most important thing to be aware of when using virtual buoys and real buoys is their capacity to be contaminated by multiple data sets. In simple terms that means they merge information into a graph so it appears as one distinct swell. Yet that's rarely the case. More often than not there are multiple swell trains in the water. Sometimes a long period groundswell will ride underneath a large, short period windswell. When this happens the buoys struggle to resolve the mix and usually output a combination of the swells making it appear larger than reality.

This is a critical error on any coastline susceptible to wind as it skews the data. There's much more to be said on this topic and it deserves to get it's own instalment. Stay tuned next week.

Postscript: To use Swellnet's virtual buoys click the 'Surf Forecast' tab for any region. Rather than give raw data, we've calibrated every Australian coastline, and many overseas ones, to better reflect how they handle prevailing swell.

List of real buoy sites to bookmark:
NSW, Manly Hydraulic Laboratory
Kurnell, NSW Port Authority
Queensland, Queensland Government
Cape Sorell, BoM
Cape du Couedic, BoM
Western Australia, WA Dept of Transport

Previously in Know Your Product:
Wave Period