Navigation and… satellite imagery?

Classic maps and satellite imagery

Every yachtsman knows this subtle feeling of doubt. Is my card valid? But that shallow shoal is still in this place? Careful “by eye” navigation in tropical waters just got a whole lot easier thanks to satellite imagery. Especially they will help those who like to explore places far from the beaten path. 

Of course, shipping routes for commercial transport have been mapped for a long time and in detail. Many of them have been used for over 100 years! Unfortunately or fortunately, not all paths are so meticulously documented.

When visiting more remote locations, you may notice that regular chartplotter charts vary in accuracy by region. On one map, one thing can be marked, and on another map of the same region, a completely different one. Chartplotter brands are increasingly tied to a specific chart provider, so in some waters you may not have the accuracy you're looking for. In other areas, there are no updated maps at all.

This is where satellite imagery comes in handy. If the nautical charts of an area are old or of questionable quality, then the satellite image will almost certainly be much more recent. If you look, for example, on Google Earth, you can check whether there have been changes in the location of the sandbars, the layout of the harbor or channels.

This is very useful if you have a good internet connection. But offline, things are different. And, of course, it is very convenient if you can see your boat on the screen while moving on the satellite image, so that you know exactly where the coral reef is or something more interesting.

Paid solution to the problem

In some chart formats, such as Navionics Platinum+ and C-MAP Reveal, satellite photos can be selected as an additional layer. You are left with the information you would normally get on a map, but you can compare it to reality on a satellite image. Below you can see the difference between a "fresh" map (first image) and a satellite map (second image). A new berth or spit will not be the most pleasant surprise, will it?

This requires a multifunction display that supports this feature and an appropriate mapping chart. All of these options are mostly paid.

But there is also open (and almost) free software for working with maps. They are very popular among cruisers because these sources of information can be used to plan routes and visit "off the beaten path" places.

Data from open sources

The two most popular solutions are OpenCPN and Guru Maps. OpenCPN is an open source mapping software that can be loaded with digital maps from various free and paid sources. OpenCPN runs on computers running Linux, MacOS, and Microsoft Windows. There is a paid Android derivative for tablets and phones. Guru Maps is an application that can be downloaded to your tablet or phone. Below you can see an example of a route laid out on a satellite image "by eye".

After downloading the mapping software, the next step is to acquire georeferenced satellite imagery so you can import it into your mapping application. This sounds like a daunting task, but luckily there are detailed guides on how to do it online. In the picture below you can see the same section of the route, only laid on the navigation map.

There are various sources of satellite imagery: Google Earth, Bing, ArcGIS and others. You will need different sources because the anchorage or island you are looking for may have been hidden under the clouds when the Google Earth satellite passed over it. 

Another source may have a more accurate depiction of the same area. One very useful tool is SAS.Planet. This is a program designed to view and download high-resolution satellite images. All downloaded images will remain on your computer and you can view them offline.

You will find all detailed instructions on the use of a particular tool on the relevant sites. We wish you good luck in your search for new unknown paths. Remember - yachting is not only about adventure. Yachting is about safe rest!

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19.07.2022