Vectors vs Raster

[DeeGee]

Has anyone extended experience of vector charts, in particular their fidelity when it comes to little rocky outcrops and the like, in and around the CI for example.

CMap NT+ and Transas come to mind, in particular.



<hr width=100% size=1>Black Sugar - the sweetest of all

 

[Oldhand]

Interesting that as the vast majority of chartplotters use vector charts, there hasn't been a quick response to your question. The fact that you have asked the question means you have been thinking of how vector charts are produced and the possibility of loss of detail and accuracy in the production process. Having had some experience of digitising techniques, I didn't ask the question, just went straight to raster charts. Perhaps you would be more comfortable if you did the same?

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[Mudplugger]

WHY??, could you explain your response in a little more depth, to allow those of us less knowledgeable to make a sensible comparison...many thanks. Tony W

<hr width=100% size=1>Tony W.

 

[Mirelle]

there is an amazingly good article on this

in the current issue of "Seaways", the Nautical Institute magazine, written by Simon Salter. I wonder if we could persuade YM to give it another airing?

It rather lost me on the mathematics of digitising chart projection conventions, but is clearly the work of an expert who loves his subject - Simon wrote the Admiralty Total Tide programme so I take it he knows about this sort of thing.

It comes down most decidedly in favour of raster.

<hr width=100% size=1>Que scais-je?<P ID="edit"><FONT SIZE=-1>Edited by Mirelle on 31/03/2004 20:36 (server time).</FONT></P>

 

[powerskipper]

The main trouble with Vector charts is over zooming, this means zooming in so far that details are lost, so any object needs to be veiwed in 2 or 3 scales to make sure of all the details you need to know.

As most charts are reproductions of Admiraty charts and said charts for some areas [ those less popular areas] where last survayed by droping a weight on a peice of string over the side of the boat , It is always a good idea to keep a close eye on your depth gadge in anything under 2m depth.

Details/ information of when they were last survayed are normally on the admiraty Charts .

Also GPS positions can vary slighily. This is normally indecated on the more moden sets by how much. So do not use them as your sole way of navigation, but in conjuction with charts and transits work very well.


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IMOSHO of course,/forums/images/icons/smile.gif

 

[rickwat]

I use Sea Pro which uses vector and raster and can switch between the two by choice of preferences by the user. As I understand it the raster is effectively a photocopy of the admiralty paper charts and whilst you can zoom out for a bigger overview the printing becomes distorted. On this system you can't zoom in beyond 1:1 in navigation mode so can easily use normal prudence for safety margins with regard to the scale of the chart. The vector charts used on this system (Livecharts)are claimed to contain all the detail of the admiralty charts but can be zoomed in or out with the printing rescaled to match. Because of this you have to be careful to note the scale of the chart the vector is based on and it is easy to be fooled into zooming in on a large area chart and think you are seeing the accuracy of a more local chart. I prefer the rasters for close to shore but the vectors are great for a scalable overview offshore and have all the toys like lights flashing on lighthouses in the charted cycle. The main thing is to be aware of the scale on either system.

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[Talbot]

Having read all these learnt opinions about raster charts and why they are so great, I felt I had to jump in with both feet in favour of the Vector system. Raster can easily be distorted, and really needs to be viwed at the scale of the original chart. furthermore it is only aligned at a couple of places to the next chart - not to big a problem if you are buying raster charts, but a potential minefield if you are going the cheap route and scanning your own charts. Vector on the otherhand is set up to provide more detail as you reduce the size of area you are viewing. The positions of the charted items are accurate at each scale because they are a digital projection. I have been playing with these things for the last 20 years and much prefer the new C Map NT Furthermore, it is the only system that I know of that uses up-to-date cartography and has a working system in place to get chart corrections to the user.

However, at the end of the day you pays your money and takes your own choice. I personnally have voted with my wallet and have a nice chartplotter with a superwide chip of the whole of the English channel - with data from the Thames round to the Bristol Channel and down to Brest.

I cannot answer for the channel islands, but as far as the solent is concerned it seems to be as accurate as the best charts, and I tend to go pretty shallow when against the tide.

 

[Cotillion]

I have no knowledge of chart plotters but work daily with software that uses both Vector and Raster images. Vectored images can be used same size or blown up to any size infinitum without loosing detail whereas raster images work on dpi so anything larger than S/S will pixelate causing the image to appear. Try enlarging any jpeg and you will see what I mean.

The important thing is how big was the image when first drawn. If it was large and then reduced to fit on the screen then it will be more accurate. If drawn small and then enlarged not so accurate.

I know what I mean, if you do too then I hope I have been of some help.

kim

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[Cotillion]

I have no knowledge of chart plotters but work daily with software that uses both Vector and Raster images. Vectored images can be used same size or blown up to any size infinitum without loosing detail whereas raster images work on dpi so anything larger than S/S will pixelate causing the image to appear bitmapped (Lots of dots). Try enlarging any jpeg and you will see what I mean.
The important thing is how big was the image when first drawn. If it was large and then reduced to fit on the screen then it will be more accurate. If drawn small and then enlarged not so accurate.


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[kdf]

I have both on my laptop - Cmap NT, S57 (both Vector) and Maptech and SHOM (both Raster). I agree with others that when you zoom raster charts they can be distorted - you do need to view them at their intended scale. In their favour raster charts are more familiar to us. We've grown up with their detail (conspic spire, tower etc) most of which is missing on the vector equivelent. My personal preference is for vector for the long passages and raster for close in, detailed work.

Another point re Vector. I would not touch any of the "proprietary" formats offered by some companies. In this bucket I would put Livechart, passport, transas. In the course of your sailing life the investment in charts is going to be considerable and I'd be reluctant to put this kind of money into formats that are restricted and lack the financial cushion of the larger organisations.

My two cents/pennies etc.

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[pandroid]

We used C-Map NT in the Swedish archipelago (loads of rocks) last year alongside the Batsportkort paper charts.

I can honestly say that there was no fidelity difference between them. They were both good and bad in the same places. To prove it, I used both to get into a small rock pool, and ignored the (larger scale) chartlets in the Lasr Graneth books. Suffice to say, in a place where both chart systems showed clear water, we hit the deck. The books chartlet showed a (1.7m) rock.

The paper charts were 1999 vintage, the C-Map bang up to date. The only bouyage errors we found were incorrect on both

We've also got ARCs (raster) on board, but not for Sweden. I prefer ARCs cartography to the angular style of the vector, but ARCs is only seriously useful at about 2 zoom levels, and gets round this by having a myriad number of overlapping charts at different scale levels - all more money to the Admiralty



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[DeeGee]

Interesting responses. The reason I posted the question is because I, too, have both. I have been using scanned charts for some time, and, as you say, it is just a question of having enough charts. Now I am expanding my cruising area significantly, and maybe going further, the convenience and cost advantage in vector charts led me to MaxSea and CMapNT+.

I have hardly used these vector equivalents yet, and was a bit surprised how much less I appreciate the picture. I can live with the different representations, but I was wondering about the detailed bits, the rocks, and that was why I wanted to hear of others' experience. I don't like the mention of that 1.7m rock!!



<hr width=100% size=1>Black Sugar - the sweetest of all

 

[Mirelle]

It may be worth noting that ship systems are going in just the other direction; with the so-called "dual fuel" systems expected to run raster charts most of the time, and S57 vector charts when in narrow waters such as the Channel and North Sea, Singapore Straits, etc.

This is because the cost of re-charting the globe onto S57 vector cartography would be out of all proportion to the benefits.

However, please note the "S57" specification of these vector charts....

<hr width=100% size=1>Que scais-je?

 

[Oldhand]

I use MaxSea for the same reason. I'm not sure why "up-scaling" is so important to people when if you buy the right raster chart pack for the area you are sailing in, all the necessary detailed charts are usually included and perfectly adequate at their intended scale, just as the appropriate paper charts are.

"Up-scaling" any chart, whether raster or vector doesn't improve accuracy at all. In the case of the latter, they are digitised from the paper charts in the first place, losing detail as they draw straight lines between the digitised points. All they do is increase the digitised point density in places such as marinas so the people that use them think they are highly accurate because they show their boat to be in their marina berth. This is, ofcourse, a great way to sell chartplotters and thus get people to buy the charts.

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[Nick_Fairfax]

I have only just got in to electronic charting (vector c-map) and my first impressions have been very positive. I cannot comment on the benefits of vector v raster, but I would like to add some food for thought on the good old paper chart from which they are all derived.
I hate to think how many paper charts I have bought over the years, but comparing old edition to so called new edition British Admiralty paper charts, it is very interesting to note the differences over the years. I have noticed that more and more harbours and rivers are now being shown as un-surveyed at 25,000 scale - a useful and cost effective scale for yachtsmen. For instance I use to have Portsmouth harbour on my old 25,000 scale chart - now on the new edition it is un-surveyed. Why is this ? Is it the BA trying to get more revenue or can anyone think of another reason for this trend ?
Also bear in mind that the amount of surveying being done now is very small and all those nice cruising destinations are the lowest priority on the BA survey schedule. Just check some of the survey dates on your paper charts !
As I become more experienced and start to rely on my electronic charts, I will remember what they are created from and not expect too much from them in those nice little bays and coves that I love to explore.

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[Guest]

BUT with Raster against Vector

Raster - you are stuick with details on the chart that you want or don't want.
Vector being in layers etc. can have detial on / off / set at what you prefer and they zoom in / out much better without loss of resolution.

I agree thjat Vector takes some getting used to after using paper charts - raster being a copy of the paper version ....

But once got used to - I like them ....... (I still use Seaclear though which doesn't cater for Vector charts ......)


<hr width=100% size=1>Nigel ... and of course Yahoo groups :
http://groups.yahoo.com/group/gps-navigator/

 

[Guest]

Note that you can amend electronic

You can mark / annotate electronic charts with overlays / etc. to put on those little things you find that are NOT marked etc.


<hr width=100% size=1>Nigel ... and of course Yahoo groups :
http://groups.yahoo.com/group/gps-navigator/

 

[Guest]

Personal preference

I like the look of Raster - as it fits comfortably with Paper Charts and what you expect to see ....... (style wise I mean)

But for electronic use I prefer Vector for the many reasons given by myself and others ..... they zoom, they are less overcomplicated by large lettering etc.

The problem comes that many look for what the appearnace is rather than the content. I have many examples of scans and prints of charts of same areas in Vector and Raster form - they all as far as can be seen carry same information but in different style / font etc. The Vector tends to be plain and surgical, the Raster tends to be 'colourful' ......

Hundred years ago or so many shouted down the advent of the motor-car advocating the horse etc. Sorry but I think this is a similar sitaution .... where everyone is used to one system / style and a new one comes along and is poo-poo'd .....
Remember the Ford Sierra ...... 'upturned Jelly-mould' it was branded, must admit I thought that as well, but look at cars today - they are all following that style - in fact a lot are just plain ugly jelly-moulds now !!

So look at content, and base it on use .... not cosmetics and lack of use.

Commercial shipping is embracing the both and funnily enough tending over to Vector as time moves on .....


<hr width=100% size=1>Nigel ... and of course Yahoo groups :
http://groups.yahoo.com/group/gps-navigator/

 

[rickwat]

S57 sounds good

Is this a vector charting system incorporating all the details of ALL the raster charts for a specified area viewable seamlessly - zoom in/out,left/ right ? This sounds like the future.

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