The Official Magazine of the Bluewater Cruising Association

Don't Believe Everything You See On The Screen

Max Shaw

Stevens 47
March 24th, 2019

When you sail in places like the US/Canada; Australia/NZ, or Europe, you can get lulled into believing what is shown on your laptop, handheld GPS or chart plotter is accurate. Looking at a chart plotter that shows your slip in the marina is pretty impressive. Move beyond these areas and four issues are quickly revealed:

  1. Not everything shown on the chart is actually there: We regularly see Aids to Navigation that have long since drifted away
  2. Not everything that is actually there is charted: Many of the vector charts of the Tuamotus are based on some sort of analysis of satellite imagery, but many of the coral heads are either not shown or shown as being at “safe” depths when they are not. In other areas, the passes are stick-like cartoons without enough data to use for navigation or to decide when it is safe to navigate the pass.
  3. Selective display of information in vector charts: lots about this is written elsewhere and was a leading cause factor of the Team Vestas grounding on the last Volvo Ocean Race (report here). As you zoom in and out on vector charts, you can see atolls appear or disappear.
  4. Datum error: most cruising grounds were surveyed well before GPS was available. Therefore, even though the geometry of the features can be accurate, the position of the objects may be incorrect. You can use radar to estimate the datum error, but you need to be aware that each surveyed area will have different datum error. If you want to read more on this, here is one starting point.

Let’s have a look at an example:

1 – Here is a 1:625,000 zoom showing the approach to Tahanea in the Tuomotus. Lots of sea room to the south east, it seems: 2 – Until you zoom only one level in to 1:500,000 and the atoll of Motutunga appears:
3 – Motutunga circled in red.  This issue was one of the cause factors in the Vesta’s crash:
4 – The little grey box gives a hint that an atoll is hidden there:
5 – Of course, once you get there, there may not be a lot of data. This is a close up of Tahanea in CMap: 6 – The charts for the passes are actually pretty good at this atoll:
7 – Navionics has more detail on this atoll than C-Map, but some of the bommies that are marked are not there and some of the spots shown with 16 feet of depth are actually much shallower.  It is suspicious when every bommie is 16 feet deep on the chart: 8 – Okay, we are not sailing over Isla Isabela in Mexico at 7.7 kts. The datum for most of the Pacific coast of Mexico is off by a fair margin. We used the Garmin handheld for our first two years of cruising before we treated ourselves to a little chart plotter:

Tools to Mitigate the Risk

The first and foremost way to mitigate these risks is the Mk1 Eyeball, and careful timing when navigating critical areas. We try to go through passes between 1000h and 1400h and ideally, with the sun behind us so we can see the reefs. During these times, someone is always on the bow watching (clipped on if the pass looks rough). This was our main strategy through French Polynesia, but last year in Fiji – a cruising ground famous for its yacht-grabbing reefs – we also started to use SAS Planet to download, store and analyze satellite imagery.

SAS Planet is certainly not the only tool out there. Many sailors have success with OpenCPN (which we also use) and GEKAP. We also OVITEL Maps on the iPad, as it allows us to have a picture in the cockpit, but it can be a bit finicky to use and the iPad is hard to read in sunlight (and too bright during night watches!).

How We Use SAS Planet

Step 1: Download data

Select the area you want to download. You can do this by scrolling over the area at the desired zoom levels as you would with Google Earth, or you can use the polygon function to select an area.

Once the polygon is selected, you can open the selection manager (the green “check mark”) and pick what zoom levels and which map source you want to use. In Fiji we usually use Google, Bing and/or Nokia. Sometimes a big cumulus cloud is over the critical part of a pass or anchorage, so it is nice to have alternatives.

Once the data has downloaded, it is saved in the cache.

Step 2: Route Planning

The rest of the process is personal preference and based on your risk assessment. Generally I plan routes in Coastal Explorer on the laptop and then export the route as a KML file. Using SAS Planet, the file can then be imported from the “Placemark Manager” and you can see your route and any marks you imported overlaid on the chart data. You can then check the route, at a detailed level remembering it is vector data, to see if there are any hazards.

If there are hazards along the route that regular charts do not pick up, you can then amend the route either directly in Coastal Explorer or by reversing the export/import process from SAS Planet to Coastal Explorer. If particular hazards are noted, they can be marked on SAS Planet and exported to Coastal Explorer and your chart plotter. I do this especially if there are particular hazards in a pass. Also, for very tight anchorages, it is nice to pick an anchor spot ahead of time to ensure adequate swing room.

The island of Yadua in Fiji is a good example. Below, a route is drawn with Open CPN into the western anchorage. It looks good until you export it to SAS Planet and find that you would drive over a reef. In Google Earth you can see the entrance but not the the southern anchorage. With Nokia it is clearer on this occasion.

1 – The western approach drawn in Open CPN: 2 – Oh, except when you export it to SAS Planet you see you would have driven over a reef:
3 – By using Nokia instead of Google in this case, the southern part of the anchorage is visible as well:

The other side of Yadua provides another example. It is a beautiful anchorage, near to a friendly village and excellent spearfishing. The entrance and anchorage are a bit tight.

1 – The entrance to the eastern anchorage in Coastal Explorer. Note that the two red Aids to Navigation shown on the chart are not actually there: 2 – When you look at the SAS Planet data, you see two coral heads in the pass that are not marked on the chart:
3 – In this screenshot, you can see two danger marks on the coral heads as well as a possible spot to anchor with sufficient swing room: 4 – And then this can be imported back into Coastal Explorer and the chart-plotter:


  • No tool replaces the Mk1 Eyeball and commonsense.
  • Just because Google Earth shows there is open water, it does not mean it is without hazards. To save data, some areas are shown as open water where there are actually reefs.
  • This is certainly not the only way to do this – there are lots of other tools.

Always look where you are going! If this was a pass transit, Liz would have her PFD/tether on as well.


  1. Nancy Carlman says:

    Excellent article! The importance of MK Eyeball cannot be exaggerated. We’ve had experiences with electronic charts that don’t show bays unless you zoom in and in, as well as Isla Isabella being more than a mile off in longitude. It’s a good idea to have paper charts as well, at least for planning, because you can easily get an overview. Even if paper charts are not GPS correct, the relationship between land and rocks doesn’t change.
    Nancy Carlman

    1. Max Shaw says:

      Thanks Nancy. Articles like this are hard to write as I am a bit of a boat geek I could go on for pages and pages … We do carry paper charts and all the gear for celestial not so much for mitigating the errors in electronic charts but rather to have another layer of redundancy. A good point though that the traditional coastal navigation practices with a sufficiently large scale paper chart are good mitigations to datum errors as the geometric relations remain correct. It does not help though if the reefs etc are not actually plotted on the charts. In the end, the most reliable tool is the Mk1 Eyeball coupled with a good dose of patience and planning.



  2. Jean Baillargeon says:

    Hi Max. Hope your latest passage was a safe one. We met in Suva Suva and again at Musket Cove, Shamata.
    Enjoyed your piece, adds to my understanding to these tools and their limitations. I also wanted to suggest a great read on the matter of charting in the digital age. The book is from famed marine writer NIGEL CALDER. It’s called: « How to read a nautical chart » Although the title sounds very basic, Calder in his regular fashion, dives into the intricacies of charting in the digital age. By examining the descrepancies between paper, raster and digital charts, he shows us why there are such variances. It’s a bit technical, but it’s a great read and education on the subject. Highly recommend it if that is something of interest to you. Fair winds.

    1. Max Shaw says:

      Thanks Jean. A bumpy passage back to Majuro as always. We should try sailing downwind in the trades some day.

      Definitely would also recommend Calder’s book. In fact I would recommend any book he has written. I have three onboard: Mechanical etc, Diesel Engines and Refrigeration. I read the Nautical Chart book years ago and though he did himself a disservice in the title as it certainly goes beyond “how to read” with all the background GIS info. He also has articles regularly in Professional Boatbuilder.



  3. Murray, SV Calico Cat says:

    Excellent presentation! Is it your CF training or the RYA skills and knowledge that makes your documentation so appealing? In either case I enjoy your sharing of knowledge and discoveries in the Pacific Ocean. I shall continue to follow your BCA submissions with appreciation of your hard work putting together all this data in an easy to understand format.
    SV Calico Cat

    1. Max Shaw says:

      Thanks Murray. Good question. While I was impressed with the quality of the RYA training I would credit any skills in documentation to all the great mentoring and experience in the Canadian Forces.

      I have a few ideas for future articles but after a great two month tour in the outer atolls and a growing “to do” list before the passage to Alaska they are seriously on the back burner. After two months away from internet I do hope to update the blog in the next week or so.



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