We used the Garmin handheld for our first two years of cruising before we treated ourselves to a little chart plotter: The datum for most of the Pacific coast of Mexico is off by a fair margin.

It is suspicious when every bommie is 16 feet deep on the chart:Ĩ – Okay, we are not sailing over Isla Isabela in Mexico at 7.7 kts. This is a close up of Tahanea in CMap:Ħ – The charts for the passes are actually pretty good at this atoll:ħ –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. This issue was one of the cause factors in the Vesta’ scrash:Ĥ –The little grey box gives a hint that an atoll is hidden there:ĥ – Of course, once you get there, there may not be a lot of data. Lots of sea room to the south east, it seems:Ģ – Until you zoom only one level in to 1:500,000 and the atoll of Motutunga appears:ģ – Motutunga circled in red. Let’s have a look at an example: 1 – Here is a 1:625,000 zoom showing the approach to Tahanea in the Tuomotus. If you want to read more on this, here is one starting point. You can use radar to estimate the datum error, but you need to be aware that each surveyed area will have different datum error. Therefore, even though the geometry of the features can be accurate, the position of the objects may be incorrect.

Datum error: most cruising grounds were surveyed well before GPS was available.

As you zoom in and out on vector charts, you can see atolls appear or disappear.

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).

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.

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.

Not everything shown on the chart is actually there: We regularly see Aids to Navigation that have long since drifted away.Move beyond these areas and four issues are quickly revealed: Looking at a chart plotter that shows your slip in the marina is pretty impressive. 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.

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