Admiralty or Imray charts?

[AntarcticPilot]

Yes and no, plot matching software has been around for a long time and can allow different standards to be overlaid even prioritising the best quality data.
If Taunton thought they could make more money by doing it they would but in reality they leave this to the chart plotter map suppliers who use their data.

Matching different standards and resolutions is fraught with problems; it CAN be done, but whether it SHOULD be done is another question!

If you feel like researching the problem more, look up discussions on generalization;there are many excellent specialized books. "How to Lie with Maps" by Monmonnier gives a good introduction, is easy to read and is regarded as required reading by all cartographers! However, the gist is as follows:

There is a limit to how much information can be represented on a screen or piece of paper, which depends on a) the limitations of the human eye and b) the resoluton of the medium.

People compiling maps therefore have to make decisions about what to represent and how to represent it to take account of this. There are several methodologies for doing this, including symbolization, aggregation and abstraction. ALL of these are scale dependent and in general, irreversible (you can go from more detailed to less detailed, but not the other way).

It gets a little more complex than that; in some cases generalization includes changing the actual concept being mapped; for example a detailed chart might show individual drying rocks, but a less detailed chart will show a reef - different conceptualizations of the same reality.

Therefore, overlaying charts compiled at different scales tends to be unwise unless it is done at the least detailed scale - which probably isn't what you want, and isn't usually necessary anyway.

Note, by the way, that the underlying chart data are not raster (as I think you are assuming), but vector, which makes the problem easier in some ways (positional information is explicitly held for every object), but harder in others because of the issues I skim over above. Basically, producing reliable charts that are centred on arbitrary points of interest requires a uniform map database at the most detailed resolution (ideally in vector form) for the entire area being covered. This isn't the case for charts.

PS, Chart plotters don't usually merge charts; they select appropriate charts according to the display scale. This is very different from the problem of creating charts centred on an arbitrary point of interest.

 

[maxi77]

Matching different standards and resolutions is fraught with problems; it CAN be done, but whether it SHOULD be done is another question!

If you feel like researching the problem more, look up discussions on generalization;there are many excellent specialized books. "How to Lie with Maps" by Monmonnier gives a good introduction, is easy to read and is regarded as required reading by all cartographers! However, the gist is as follows:

There is a limit to how much information can be represented on a screen or piece of paper, which depends on a) the limitations of the human eye and b) the resoluton of the medium.

People compiling maps therefore have to make decisions about what to represent and how to represent it to take account of this. There are several methodologies for doing this, including symbolization, aggregation and abstraction. ALL of these are scale dependent and in general, irreversible (you can go from more detailed to less detailed, but not the other way).

It gets a little more complex than that; in some cases generalization includes changing the actual concept being mapped; for example a detailed chart might show individual drying rocks, but a less detailed chart will show a reef - different conceptualizations of the same reality.

Therefore, overlaying charts compiled at different scales tends to be unwise unless it is done at the least detailed scale - which probably isn't what you want, and isn't usually necessary anyway.

Note, by the way, that the underlying chart data are not raster (as I think you are assuming), but vector, which makes the problem easier in some ways (positional information is explicitly held for every object), but harder in others because of the issues I skim over above. Basically, producing reliable charts that are centred on arbitrary points of interest requires a uniform map database at the most detailed resolution (ideally in vector form) for the entire area being covered. This isn't the case for charts.

PS, Chart plotters don't usually merge charts; they select appropriate charts according to the display scale. This is very different from the problem of creating charts centred on an arbitrary point of interest.

I firstly am not confusing vector and raster charts, I have used both and appreciate the difference. Equally I am also aware of the problems of survey variations, having spent some time navigating in an area where it was inadvisable to select fixing points from both the east and west sides in the same fix. I understand that particular problem was resolved using satellite data.

I do not expect Droggy to do any thing about this as I do not think there is a big commercial return in it. Most electronic charts do advisers users when they over zoom for the scale of the base data.

I suspect in time Taunton will be able to do a print on demand service but I suspect this would simply be a by product of other changes rather than a drive to support a specific market

 

[binch]

Charts

Admiralty Charts can be readily up-dated.
I was a hydrographic surveyor in the Indian Ocean in 1950 to 52.
Admiralty surveys are carried out to extra-ordinary accuracy, but in the pre-electronic period there was a noticeable measure of idiosyncracy. Thus I can to this day detect the "signature" of some of my colleagues work.
Because some countries are chauvinistic, they do not allow surveyors of other countries to survey their waters. I was once arrested in Greece for privately surveying from my yacht, an area of the Aegean which was obbviously faulty. It ended happily: I knew the Greek hydrographer.
The IHB reconcile and digitise all members' charts.
All surveys (Admiralty charts indicate origin of the data) prior to 1965, not excluding my own, are liable to error, but it is not usually great. I once corrected the position of the Comoro Islands, moving them 2,500 yards to the ESE, thereby infuriating the French.
Nice story:
In Limnos, there are four small named hills: Yam Hill, Yrroc Hill, Eb Hill and Denmad Hill. So named by a Lieutenant surveyor who had had his leave stopped for some offence. Read the names backward. His Captain was Captain Corry.

 

[prv]

Admiralty Charts can be readily up-dated.

As I'm sure you know, but to avoid misleading any novices reading the thread, Imray charts can also be updated. They take the Admiralty notices and package them up according to which Imray charts they affect, you can then download a sheet of corrections from each chart's page on their website (they also have a scheme for sending them out by post). It's not quite such a highly-developed system as the Admiralty one which gets corrections out to ships anywhere in the world, and they don't have agents correcting charts to the day you buy them, but it's more than adequate for yachtsmen.

Pete

 

[dt4134]

On the other hand, the multiple Scilly charts in their 2400 pack seem pretty comprehensive.

Pete

It wasn't convenient at the time to nip out and buy those.

Had it been, I'd've gone for the small craft edition of the BA chart (883?) which is what the small red lettering on the Imray chart C7 said anyway.

 

[Steve N]

Looks like this thread has got a little off topic, but thanks for the replies anyway.

I think I'll just get the Imray chart folio for now and will get a single Admiralty chart for comparison at a later date.

 

[prv]

I think I'll just get the Imray chart folio for now

OK.

Don't forget that charts are already out of date when you buy them[1], sometimes by several years. The proper thing to do is to update them when you buy them, then regularly thereafter. It's true that not everybody does this (I do), but I think at the very least it would be worth looking at the current corrections sheet for the pack for anything that's particularly likely to affect you (eg major buoyage changes in the entrance of somewhere you don't know well but may visit). You can then mark the chart accordingly, whether as a proper correction in the traditional purple pen, or just a note in biro.

Corrections for the 2100 pack are here: http://www.imray.com/corrections/IC2100-4N.pdf
(at the time of writing; I don't know if the URL changes when they add each batch of updates).

Pete

[1] Unless you buy pukka Admiralty standard (not leisure) ones from an old-fashioned Chart Agent that spends all day hand-correcting all his stock, of course.

 

[GrahamM376]

I had a discussion with the Hydrographic dept some years ago. Having bought a new chart, can't remember the scale (but not too small) found there was no buoyage on it. Their excuse was that there iss a sub-chart for the area so I was supposed to buy that as well! Been using Imray ever since.

 

[doug748]

......Channel Islands to Morbihan was going to be about 12 to 15 sheets and cost a fortune. Imray cover it in about 6 sheets for less than half the price......

Excellent point, and exactly the one I made yesterday - or rather it would have been, but the computer went down and I lost it all -(
Anyway. good point

Mind you, I always use Admiralty. I find them less cluttered. Lots available secondhand nowadays.

 

[Steve N]

OK.
Corrections for the 2100 pack are here: http://www.imray.com/corrections/IC2100-4N.pdf
(at the time of writing; I don't know if the URL changes when they add each batch of updates).

Great - thanks for the link!

 

[Uricanejack]

Anything but American. They are hard to see with so much detail in fine print, use odd colors. but worst of all use lower grade paper which does not stand up to spilled coffee, tea or beer stains.

 

[david_bagshaw]

I wonder if the UK and Europe was ever available or is genuinely "no longer available" as a sop to the Hydrographic office from the NGA. ...................
.

they were available and a group on open cpn were calibrating them for use as kap files that work in most programs.

Sadly it was stopped as a group effort as NGA cited copyright issues or something similar.

 

[Summer Sailor]

I've got a sneaking feeling I might be inciting a religious war here but can you tell me the pros and cons of these two paper chart formats, or why you favour one over the other?

I'm looking to buy charts for the SE coast. Imray do a convenient pack (#2100) for £37.50, but I'd need two Admiralty packs (SC5605/6) at £44.30 each, meaning that the Admiralty charts would cost me cost an additional £50. What do I get for this extra dosh?

I prefer Imray. They are waterproof and much easier to store.

 

[KellysEye]

We bought some Admiralty charts and then saw the Imray charts which we preferred and used them for the UK, Caribbean and Med.

 

[ZEBEDEE]

As Chart Agents we sell both if you are comparing the folio packs you will find that the Imray 2400 in the west country covers a larger area but SC5602 goes up the rivers like the Tamar and Truro river Imray seem to have forgotten what rivers are. For interest, Admiralty folios outsell Imray in our shop by a ratio of about 10 to 1