The danger of over-zooming electronic charts

[jackdale]

I got this in my LinkedIn feed. While it applies to ENCs and ECDIS, it is also appropriate for recreational users.

There is a common misconception by some Mariners that zooming in beyond the compilation scale of the ENC allows for greater accuracy – however, this is not the case. In reality zooming in beyond the intended maximum display scale of ENCs may be misleading and dangerous, particularly for ‘isolated dangers of depth less than the safety depth’.

The risks associated with over-scaling the ENC within ECDIS are two-fold:

Firstly, the symbol selected by the cartographer to represent a real-world feature may no longer fully cover that feature.

Secondly, but most importantly, because the text and point symbols stay the same size within the over-scaled ENC, any sense of appropriate distance from a potential danger is no longer intuitive and can result in a false sense of safety that does not reflect reality.

Mariners are strongly advised not to zoom in ECDIS beyond the compilation scale to a point where the ECDIS over-scale indication or pattern are triggered.

Some ECDIS allow the operator to turn off over-scale warnings.
This is not recommended under normal circumstances.

https://blog.geogarage.com/2020/12/i...tion-over.html

[Franziska]

Here is one method to avoid that.

OpenCPN 5 you can have two different chart windows open.

Both can be set to follow the boat and can display the same or different charts at the same or different zoom levels.

Since version 5 we open two chartwindows, one pretty zoomed in for close range and another more as overview.

OpenCPN can also display the scale, you can even adjust the boat symbol to the size of your vessel....

[rslifkin]

I typically mitigate concerns around this by displaying 2 types of charts in any unfamiliar waters. With my current electronics, it's typically Navionics on the plotter and NOAA RNCs or ENCs on a laptop with OpenCPN.

[psk125]

Why would that ever be a problem?

https://www.youtube.com/watch?v=1gmnWEZCEiw

[CaptTom]

I agree that it can be dangerous for inexperienced navigators. But I don't like apps or plotters which prohibit over-zooming.

Say the scale of the available chart is relatively small, and the cove you're anchoring in isn't shown well. Once you've sounded it out and settled in at anchor, you want to zoom WAY in on your anchor location to make sure you're not dragging.

Basically, any time I don't need the details on the chart, and just want to see relative positions of where I am and where I've been, I should be able to over-zoom. Put up a warning if you want, just don't prevent me from doing it.

[jmh2002]

Originally Posted by psk125

Why would that ever be a problem?

I was going to post that same video, thanks for adding it - it's a very scary and graphic reminder of just how serious such a mistake could be and always gives me the chills...

Originally Posted by Franziska

Here is one method to avoid that.

OpenCPN 5 you can have two different chart windows open.

Both can be set to follow the boat and can display the same or different charts at the same or different zoom levels.

Since version 5 we open two chartwindows, one pretty zoomed in for close range and another more as overview.

OpenCPN can also display the scale, you can even adjust the boat symbol to the size of your vessel....

Yup

[Franziska]

Yep, Vestas was an eye opener for the issue.
Certainly not an inexperienced skipper. He might have been tired, and possibly just not aware of the issue.
Thanks to his mishap we are now...

Originally Posted by psk125

Why would that ever be a problem?

[jmh2002]

Originally Posted by Franziska

Certainly not an inexperienced skipper. He might have been tired, and possibly just not aware of the issue.

It was the Navigators fault. He plotted the course directly over the reef because he didn't zoom in sufficiently to be able to see it on the chart.

Easily done, but a fundemental and unforgiveable error nonetheless.

The crew on deck don't have any chart plotter and don't do any actual navigation there, they simply steer to the course(s) given. They were after all supposedly on an ocean leg at that time.

On commercial level ECDIS systems the route can actually be automatically checked by the computer after being plotted - it will take depth, draft, and obstacles on the chart, etc, into account and warn if the route is not safe.

Having said that, commercial vessels also have these types of accidents too on a somewhat regular basis

[Franziska]

Exactly what I said. Even pros make mistakes, when, perhaps tired and overworked. Thanks to his mistake we might be able to avoid the same one.

Noone is error free, we are all human. So, cut him some slack, **** happens.

Originally Posted by jmh2002

It was the Navigators fault. He plotted the course directly over the reef because he didn't zoom in sufficiently to be able to see it on the chart.

Easily done, but a fundemental and unforgiveable error nonetheless.

On commercial level ECDIS systems the route can actually be automatically checked by the computer after being plotted - it will take depth, draft, and obstacles on the chart, etc, into account and warn if the route is not safe.

Having said that, commercial vessels also have these types of accidents too on a somewhat regular basis

[Chotu]

Raster charts all day every day.

A lot easier to see if you’re leaving the intended scale