ECDIS use of Mercator Chart Projection is a Result of Path Dependence

ECDIS has made Mercator charts obsolete, at least in the same sense that Gnomonic, so-called great circle charts are rarely used to find graphical solutions for GC navigation.

The Gnomonic chart fell out of use when cheap nav calculators (Casio) and nav programs for PCs came into use. Instead of drawing the track on a Gnomonic and then transferring track-line points to Mercator chart the calculator was used to directly calculate the GC.

Likewise the purpose of the Mercator chart is to enable simple graphical solutions.


in ECDIS the solution of navigational
problems and the measurements of distances and directions on the electronic chart can be conducted analytically on the surface of the reference ellipsoid without any
graphical work on the nautical chart, as it is done in traditional navigation.

ECDIS has made Mercator charts obsolete and ocean mariners will follow …

I understand your point, even if only in relation to paper charts.
However every Electronic Chart that I have ever seen Displayed, by all the ECDIS systems I have ever used, is a Mercator chart.
A click of the mouse may change from RL to GC and the line of pixels changes accordingly, but the visual presentation remains Mercator.
Semantics and nit-picking, I know, but mercator is not going away for a while…

(tbh, closest “true-ish” display of earth on screen is something like Google Earth)

yep. It won’t be until 3D ECDIS is developed/released and projects on a large oblate spheroid in the middle of the bridge that Mercator will really be obsolete.

Personally I can’t decide between leaning over a holographic spheroid (partial) like a large yoga ball, and a VR arrangement that I stand inside pointing at the surface. (But inside-out navigation might be a bit disorienting…)
I’ll keep watching reruns of Star Wars/Trek and the others until I’m sure :flying_saucer::globe_with_meridians::rocket::world_map::compass:

Although it is perfect for celestial nav.

If you’re using triangles and dividers to get bearing and distances off the ECDIS then you’d have to stick to Mercator. Otherwise no need.


When working with bits of paper we learn to put up with this stuff. Paper charts require
precise projections so that accurate bearings and distances can be plotted using a pencil and simple instruments (dividers, protractors and parallel rule). Not so the computer displayed chart – the calculation of accurate bearings and distances can be handled entirely separately from the display of the chart. The display of the chart should focus on speed and being reasonably distortion free.

Good 'ol Simon Salter,
know him well.

FYI he sold Chersfot a year or so back to Welbeck in Denmark, Admiratly distributors

I find the premise of this paper slightly strange. It opens by saying

It is surprising that the international standards on ECDIS and on ENCs do not provide for specific requirements or recommendations on the employment of map projections.

It then goes on to detail why projection choice is largely irrelevant to ECDIS navigation:

The lack of standardiza-tion on the use of map projections does not cause any real problem, since in ECDIS the solution of navigational problems and the measurements of distances and direc-tions on the electronic chart can be conducted analytically on the surface of the reference ellipsoid without any graphical work on the nautical chart,


For medium and large scales, according to numerical tests and evaluations conducted by Pallikaris (2010), Pearson (1992) and Maling (1973), different map projections provide practi-cally the same visual results on the screen

So the only case in which it really matters is when you have the screen set to small scale and use the relative position of contours and objects to fashion a detailed plan or gauge how things are going. IRL, how often is that? During passage, the ECDIS should be used to plan anchorages in the South Pacific islands somewhere in the distant future, everybody knows that…

Our author then goes on to analyze a number of projections with great emphasis on minimizing visual distortion, which to me is entirely unimportant. In fact, I prefer to have my ECDIS set to orthographic, because it gives the impression of spinning a globe when you pan around in small scale. It is the projection with the highest distortion, but it is a sort of distortion that makes sense. Not that it’s really relevant, but some of you may have noticed that Google Maps changed its projection to ortho recently.

All in all, this article seems like a solid effort to minimize a non-existent problem.

Didn’t mean to imply there was a problem, just thought it was an interesting question.

The Mercator is still very much in use but is that because of past practice or is it in fact the optimum projection?

What has changed of course is the shift to ECDIS lifts the constraint that navigation problems have to be solved graphically. The paper says that for large scale charts the projection doesn’t matter much. Another point is that something around 8% distortion is not noticeable to the user so minimum distortion is not a requirement.

The author reports for low latitudes “Mercator modified at latitude 15°” is suitable for ECDIS. A internet search mostly turns up results related to political controversies over the use of Mercator.

Otherwise this:

If it is desirable to restrict the number of map projections
that are used in ECDIS, only two projections can be used:
i) The Miller cylindrical modified at latitude 30° and ii)
The Azimuthal equidistant. These two projections provide
better visualization results than the Mercator and the
Azimuthal Stereographic that are used in many commercial systems

Also I thought this was an interesting graphic:


According to the paper on small scale charts for planning it would be useful if the GC was drawn as shown by the blue line and rhumb lines drawn as shown in red.

For a lot of people including mates the GC track seems counter-intuitive .

UTM, the world is flat, so easy to use ( inside one zone)

UTM zone 19N, home sweet home.

This is from the paper in the OP:

Most of the early ECDIS and ECS systems in the 90’s used one map projection only “the plate carré”.

According to Wikipedia plate carre is Equirectangular projection, invented in AD 100

The distortion looks like this:


Compare to Miller Cylindrical


And Mercator:


This BTW is a world-wide chart done in UTM ( Transverse Mercator projection)


I don’t think traditional paper-based navigators would be very happy with that.

I always plot my courses with a French curve. :slight_smile:

It’s all in the wrist.

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I’ve changed the title, downgrade the subject to a curiosity. The use of Mercator is similar to the persistence of QWERTY in word processors.

Transverse Mercator is commonly used for coastal charts of scale1:100,000 or :200,000. The early transit satnavs like Magnvox could only store 8 waypoints and all courses and distances were GC.
Using a genomic chart I established what island or reef it was planning to steam us over . My old programable

HP calculator then calculated the intermediate points so I could give the courses a bit of a tweak.
Leaving from the South Island of NZ to Cape Horn always involved a composite GC.