Janneke Kuysters on georeferenced satellite imagery and why it can be a real help to the cruising sailorEvery cruiser is familiar with that tingle of doubt: is my chart really up to date, and is that shallow bank still in that location? Careful \u2018eyeball\u2019 navigation in tropical waters has become a lot easier thanks to satellite overlays on digital charts, and for those sailing off the beaten track there are simple tools to make your own overlays.
\nOf course shipping routes for commercial traffic are well charted; as a necessity they have been surveyed for well over a 100 years. But those little nooks and crannies where we love to sail or anchor? In popular cruising areas like the Mediterranean or the Caribbean, chances are that your digital and\/or paper charts will be a great help in reaching those spots safely, because of the yachting and\/or commercial traffic there.
\nBut when visiting more remote spots you may notice that regular charts for chartplotters differ in accuracy per region. One supplier is often best in one area, but another might excel in another. Plotter brands are increasingly linked to a fixed supplier of charts, so in some sailing waters you may lack the accuracy you\u2019re looking for. In other areas there are no recent surveys at all.
\nThis is where satellite imagery comes in handy. If the nautical charts of an area are old or of dubious quality, a satellite image will be much more recent. If you look on, for instance, Google Earth, you can check if there have been any changes in the location of sand spits, harbour layout or channels.
\nThis is very helpful if you have a good internet connection. But offline things are different. And, of course, it is very handy if you can see your boat on screen while moving on the satellite picture, so you know exactly where that coral head is or where the shallower bits are.
\nCareful eyeball navigation on approaching an anchorage. Photo: Janneke Kuysters
\nSome chart formats like Navionics Platinum+ and C-MAP Reveal have satellite photos selectable as an optional layer; you still have the information you normally get on a chart, but you can compare it with the reality on the satellite image. This does require a multifunctional display that supports this and the applicable charting card. These may be hard to get in your location, and of course they cost money.
\nThat\u2019s where the open source (almost) free chart software comes in handy. They\u2019re very popular with cruisers because these sources of information can be used for passage planning and visiting \u2018off the beaten track\u2019 locations where a pair of eyes from the sky are essential.
\nOpen source data
\nThe two most popular are OpenCPN and Guru Maps. OpenCPN is open source chart plotting software in which you can load digital charts from various free and paid sources; it runs on Linux, MacOS and Microsoft Windows computers. There is a paid Android derivative for tablets and phones. Guru Maps is an app that can be downloaded on a tablet or phone.
\nWhen you\u2019ve downloaded the charting software, the next step is to get the geo-referenced satellite images so you can import them into your charting application. This sounds like a daunting task but, fortunately, there is lots of help available on the internet.
\nThere are various sources for satellite images: Google Earth, Bing and ArcGIS to name just a few. You\u2019ll need various sources, because the specific anchorage or island you want might have been hiding under a cloud when the Google Earth satellite passed over. Another source may have a better image of the same area. One very helpful tool is SAS.Planet: a program designed for viewing and downloading high-resolution satellite imagery. All downloaded images will remain on your computer and you\u2019ll be able to view them offline.
\nClipping tiles
\nThe keyword here is \u2018tiles\u2019. As you scroll along the satellite images of the area that you want to explore, you need to make \u2018tiles\u2019, ie clips of a satellite image. These are stored as MBTiles if you use a program like SAS.Planet. Depending on whether you have nautical charts of the area it may be useful to also prepare tiles at various zoom levels.
\nApproaching Walvis Bay, Namibia, the author saw shipping patterns on Marine Traffic that didn\u2019t match chart data
\nGoogle Earth revealed a brand new pier constructed for cargo ships
\nThe next step is to import these MBTiles into your charting application. The software generally shows the \u2018best\u2019 chart for your current zoom level. If you only prepare satellite tiles that are very detailed, when you are zoomed out you\u2019ll not see those \u2013 the system will show either the worldwide background map or, if you have those, the nautical chart that you have installed for that area. Once you zoom into the area that one or more of your tiles covers, the charting software can automatically switch to the more detailed satellite photo tile, showing the coral heads and sand spits you need to avoid.
\nThis sounds like a lot of work \u2013 and it is. If you\u2019re going well off the beaten track and need detailed information, you\u2019re in for hours of careful scrolling, selecting and downloading to make the tiles.
\nInternet access and time to prepare is paramount for a successful use of these tools. But once again, some helpful cruisers have made tiles of favoured anchorages already and have stored them in libraries that you can access online. So, before you start making tiles yourself, it pays to look around the internet to see what you can find that is downloadable.
\nEven better, your fellow cruiser in the anchorage might have what you need \u2013 a splendid opportunity to go over and share a cup of tea.
\nMiles out
\n\u201cWe\u2019ve found satellite \u2018charts\u2019 to be invaluable,\u201d says Sherry McCampbell, who cruises their St Francis 44 catamaran Soggy Paws with her husband, Dave.
\n\u201cIn some of the areas we cruise (the Philippines and Indonesia, specifically), the available charts, even just-updated Navionics and C-Map charts, can be off by as much as a half a mile. It\u2019s pretty astonishing to see your boat navigating across an island, using your expensive chart plotter.
\n\u201cIn some places, these charts are only off 100ft, but that\u2019s enough to put you on the reef.\u201d
\nSue and John Hacking on Ocelot
\nWe\u2019ve had a similar experience. My partner Wietze van der Laan and I have recently returned from an eight-year circumnavigation on our 44ft Bruce Roberts design, Anna Caroline. Wietze recalls: \u201cWe\u2019ve always used a combination of Open CPN and paper charts. In very remote locations like Patagonia, we found that both types of charts were sometimes off by well over a mile.\u201d
\nThe help of satellite imagery (as well as good old eyeball navigation) is invaluable to navigate these waters safely. More than once we\u2019ve been anchored on land on our digital charts, while the satellite image showed us snug in a beautiful little bay near the face of a glacier.
\nEven in areas where the charts are fundamentally accurate, additional satellite data can be invaluable, says Sherry McCampbell: \u201cWe have found that the satellite charts add so much to our ability to find off-the-beaten-path anchorages. Where the Navionics chart shows a large shallow area (interpreted as \u2018you can\u2019t go here\u2019), a satellite chart may well reveal a small channel into an area that has a perfectly good anchorage for a boat with a draught under 3m.
\n\u201cIt does take some experience to be able to \u2018read\u2019 the colours in a satellite chart and interpret the colours: is it too deep, too shallow? Is there good sand, or maybe a coral head to avoid? But once you learn these skills, it totally expands your possible anchorages.
\n\u201cWe are currently in south-east Indonesia \u2013 very few cruising boats travel in this area. I was just today making new satellite charts for our next month of cruising, using Sat2Chart and SAS.Planet. It\u2019s amazing what you can see from the satellite charts: where small villages are (possibilities for small markets to buy fresh vegetables, etc), where \u2018sand fans\u2019 are that indicate a viable anchorage, where passage is possible into an inner reef area, where fishing boats are anchored. And for on shore exploration, beaches, mountains, waterfalls etc.\u201d
\nStunning Kabui Bay, Indonesia by satellite
\nEver changing tools
\nJon Hacking is cruising aboard SV Ocelot, a Wauquiez-designed 48ft catamaran. He says: \u201cThe possibilities of using MBTiles for exploring islands and anchorages are endless. It pays to invest time in getting to know the online tools and how-to-guides that are available.
\nKabui Bay, Indonesia on CM93 chart. Note the bay doesn\u2019t appear on the chart
\n\u201cThe only limit is the size of your hard drive. It pays to choose well where you store your MBTiles, because over the years of cruising, it can add up quickly. If you upload too many MBTiles in your OpenCPN, the performance of your laptop will decrease.\u201d
\nHe adds a word of caution: \u201cThese open-source systems develop all the time, so some functionalities will appear or disappear. Make sure that you check if everything is still working before you set off.\u201d
\nHis experiences are echoed by Thom D\u2019Arcy, who sailed his Vancouver 28 Fathom solo around the world. \u201cOn my circumnavigation I found the georeferenced satellite imagery extremely useful for backing up my digital and paper charts. In some poorly charted areas such as Fiji and Madagascar I even used satellite images as my primary means of navigation. These images are also very useful for researching anchorages and layouts of ports and harbours before arrival.\u201d
\nHowever, he too notes: \u201cThe applications for accessing the satellite images are constantly changing and the app I used until recently is no longer available. It therefore requires a vigilant eye on the developments in this field to be able to access the right information when needed.\u201d<\/p>\n
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Janneke Kuysters on georeferenced satellite imagery and why it can be a real help to the cruising sailorEvery cruiser is familiar with that tingle of doubt: is my chart really up to date, and is that shallow bank still in … <\/p>\n