Gnomonic charts - Area coverage

[Monique]

Hello fellow sailors,

I want gnomonic charts for our upcoming travels. Found various places to order but nowhere a picture of coverage. Descriptions galore but no piccy...

A photo would be useful so I could get only the ones I want.... for instance, from Gib to Barbados but from what I've read, Barbados is not shown on the 5095 chart.

Any info?

 

[rubbishman]

Hello fellow sailors,

I want gnomonic charts for our upcoming travels. Found various places to order but nowhere a picture of coverage. Descriptions galore but no piccy...

A photo would be useful so I could get only the ones I want.... for instance, from Gib to Barbados but from what I've read, Barbados is not shown on the 5095 chart.

Any info?

I would look in NP 131

 

[jdc]

This doesn't answer the exact question posed, but what are you using them for?

If it's only to calculate and plot great circles there is another method which I have grown to prefer. Starting with the two end-points (as one would do with a chart anyway) I calculate the latitude for each whole number of degrees of longitude along the track, and the course to steer. I then mark these points on the normal Mercator projection chart (where they appear as a curve). Calculating these intermediate points and the associated CTS isn't hard, besides I've made a simple program to do it for me (which runs on iPhone, iPad, Android or Microsoft/Nokia smartphone or tablet, or on a normal laptop.
http://www.awelina.co.uk/sextant/GreatCircle_latest.html)

I also tend to plot faintly in pencil a few other great circles radiating from my destination, so if wind etc dictates a different path one still has the great circles on the chart. This way one is not reliant on electronics while on passage.

 

[Monique]

Thanks for the method. I've decided that the GC provided by my GPS will be good enough. At worst, I can read the LAT/LONG off the GPS and transfer to a Mercator.

Seems only the big ships use these now ... only when they face an inspection I've been told.

 

[capnsensible]

Worth remembering that the further south you go, the less use they are. You may wish to cross as low as 10 or 12 to get some wind (had to do that twice).

Personally, never used them, the wind messes you about enough as it is!

Have a good trip.

 

[KellysEye]

> Gnomonic charts - Area coverage for instance, from Gib to Barbados but from what I've read, Barbados is not shown on the 5095 chart.

A gnomic chart is not needed for a passage from Gib to (say) the Canaries there isn't a great circle route but it is handy for the Atlantic crossing because it gives the great circle route. We bought the gnomic north Atlantic chart from Imray and it covered an area including the Canaries, Cape Verdes and the whole Caribbean island chain including Barbados. It is the chart number you mention.

 

[bjl]

Not exactly what you want but Google 'Imray North Atlantic Ocean Passage Chart. You'll see images of the chart number 100 which is in Conical projection. It shows both Gibraltar and Barbados. Might that do?

 

[AntarcticPilot]

Not exactly what you want but Google 'Imray North Atlantic Ocean Passage Chart. You'll see images of the chart number 100 which is in Conical projection. It shows both Gibraltar and Barbados. Might that do?

No. Conical projections don't have the property that great circles are straight lines, which is why you use a gnomonic projection.

Gnomonic projections have the severe disadvantage that they distort scale vastly; you can't measure distances on them. I'd never think of using one; the calculations for great circles aren't that hard, and there are plenty of utilities to do them (I would use geod or similar packages).

 

[bjl]

No. Conical projections don't have the property that great circles are straight lines, which is why you use a gnomonic projection.

Then maybe Imray have it wrong when they print on their chart cover "Conical projection - Great Circle routes are plotted as straight lines"?

 

[KellysEye]

On Gnomonic charts waypoints are plotted in a straight line. A straight line seems odd but the distance of the straight line on the chart is the same as the distance of the great circle route. If you buy the Gnomonic Imray north Atlantic chart the straight line distance for the Canaries to St Lucia is about 2,700 miles.

 

[AntarcticPilot]

No. Conical projections don't have the property that great circles are straight lines, which is why you use a gnomonic projection.

Then maybe Imray have it wrong when they print on their chart cover "Conical projection - Great Circle routes are plotted as straight lines"?

If that's what they say, they are definitely wrong. The only projection that has the property that Great Circles are straight lines is the gnomonic projection, which isn't a conic projection (you can think it as being projected from the centre of the Earth onto a plane tangent to the Earth's surface). There are many conic projections, but none of them has (or can have) the property that Great Circles are straight lines.

 

[bjl]

Imray are a very reputable publisher of charts and other things nautical. You can be confident that a straight line on their North Atlantic Passage Chart 100 is a great circle course, just as they say.

 

[jdc]

So my thought experiment and logic is as follows, can anyone see a flaw?

1. The earth is a sphere more or less, and for the purpose of this discussion we'll assume it is a sphere.
2. A great circle is the intersection of the surface of the earth with a plane through the centre of the earth (and passing through the start and destination points of the journey)
3. A simple conical projection is one where the earth just fits in a cone, and positions, including lines of lat and long, are projected from the centre of the earth onto the cone, and the cone is then 'unwrapped' and laid flat.
4. A gnomonic projection is one where the positions, including lines of lat and long, are projected onto a plane just touching the earth.
5. A plane, projected onto (which is the same as intersecting) another plane will be a straight line, so great circles are straight lines on gnomonic projections.
6. So a plane, projected onto any other 3-dimensional shape, such as a cylinder or cone, can not be a straight line. QED

Steps 1 - 5 are uncontroversial I think. In Step 6 I've glossed over the unwrapping and laying flat. However I don't think that can convert an arbitrary intersection of a plane with a cone into a straight line when laid flat.

However as said, Imray are reputable (even if their proof-reading of their pilot books is frequently sloppy), so I'm now intrigued and would like a look at the chart in question to understand how it works / what its actual projection is.

 

[AntarcticPilot]

Imray are a very reputable publisher of charts and other things nautical. You can be confident that a straight line on their North Atlantic Passage Chart 100 is a great circle course, just as they say.

If it is a Conic Projection, then I am afraid they are in error; a great circle is NOT a straight line on a conic projection, and cannot be. I agree entirely that Imray are a reputable publisher, but even Homer nods. I suspect that they may have mis-named the projection, in which case the property of the chart is correct, but the name of the projection is not.

I speak from well over 30 years working on map-making, implementing and using a vast array of map projections. I refer you to publications such as "Map Projections - a working manual" by Snyder (USGS Special Publication 1395), and the documentation for proj4 (ftp://ftp.remotesensing.org/proj/OF90-284.pdf) is a comprehensive account of map projections and their properties.

PS. I have just had a look, and the Gnomonic Projection can be regarded mathematically as a limiting form of the Perspective Conic projection. However, it is a limit in that it is no longer a conic projection when that limit is reached; the other limit on the perspective conic projection is a cylindrical projection. I mention this as a possible reason why the chart could have been mislabelled if Imray happened to use this method to produce the projection.

I should note that labelling a chart as "conic projection" is not actually useful; there is a vast array of possible conic projections, with an equally wide range of properties.

I will also note that even the UKHO doesn't always label charts fully and correctly; I had to request more information than was present on the chart in order to use data from a chart of South Georgia.

 

[capnsensible]

> Gnomonic charts - Area coverage for instance, from Gib to Barbados but from what I've read, Barbados is not shown on the 5095 chart.

A gnomic chart is not needed for a passage from Gib to (say) the Canaries there isn't a great circle route

Of course there is a great circle route. The correct thing to say is that it has limited use for a yacht with a large north/south component. Gnomic charts only really have value for east/west or vice versa passages. For ships with engines..... Or aeroplanes!

 

[VO5]

> Gnomonic charts - Area coverage for instance, from Gib to Barbados but from what I've read, Barbados is not shown on the 5095 chart.

A gnomic chart is not needed for a passage from Gib to (say) the Canaries there isn't a great circle route but it is handy for the Atlantic crossing because it gives the great circle route. We bought the gnomic north Atlantic chart from Imray and it covered an area including the Canaries, Cape Verdes and the whole Caribbean island chain including Barbados. It is the chart number you mention.

Traverse Tables....enter arguments D Long and D Lat and read off the vector at the top or bottom of the page and then apply as a course as appropriate.:encouragement: