trying to understand charts content of digital mapping

[pcatterall]

Thanks for all your help in trying to resolve my chart requirements.
Thanks to your help I now have Cm93 charts and Open CPN running and am playing about with this on my laptop.
I would still like to use my Garmin 451S for navigation and want to buy the G2Vision type mapping for this.
The area of interest is the French coast and into Spain and the Balearics but possibly moving over to Italy next year.

First question....... As the plotter is on the boat in France can I use the G2Vision charts on my PC here for planning I had some vague impression that the card had to be referenced to the plotter to enable this.

Second question ( which applies to all these digital chart offerings) ...when you buy 'area coverage' how do you know which charts are used in the 'digital folio'? one extreme would be to just use one small scale digital chart the other extreme would be every small and large scale chart within that area. In the various adverts I read there is no explanation. When I buy paper charts I know just what areas and at what scales I have and so have confidence ( or not ) in my navigation data.
Thanks ( again)

 

[antaris]

I use a software called Homeport *that Garmin sell, you pop-in the card in a card reader on your PC and voila! you can do all your planning at home (with a mouse, so you don't go nuts). This way you may also check on your trails, your waypoints, etc on your PC.

As far as I am aware, the maps come in large chunks: for instance I've got the "Garmin Bluechart G2 HXEU015R Aegean Sea & Sea of Marmara" that covers the whole of the Aegean and Ionian Sea (and more) in great detail. I am not aware of other maps for the same region and I believe that the same applies to other digital cartographers - due to the Zoom function you can essentially have many detail levels in one map... and that is reflected on the price, too...

* see here https://buy.garmin.com/shop/shop.do?pID=64242

 

[pcatterall]

I use a software called Homeport *that Garmin sell, you pop-in the card in a card reader on your PC and voila! you can do all your planning at home (with a mouse, so you don't go nuts). This way you may also check on your trails, your waypoints, etc on your PC.

As far as I am aware, the maps come in large chunks: for instance I've got the "Garmin Bluechart G2 HXEU015R Aegean Sea & Sea of Marmara" that covers the whole of the Aegean and Ionian Sea (and more) in great detail. I am not aware of other maps for the same region and I believe that the same applies to other digital cartographers - due to the Zoom function you can essentially have many detail levels in one map... and that is reflected on the price, too...



* see here https://buy.garmin.com/shop/shop.do?pID=64242

Many thanks for that, very useful. My question about scale still stands though, this 'zooming in' is one of my concerns ( as former hydrographical surveyor) if 'zooming in' changes the chart for one of the appropriate scale then great, if it simply enlarges the scale to one which no longer reflects the surveyed accuracy of the chart then there is an issue. Generally you see by the 'style' of the chart what order of scale it is but some users may not realise this. ( I think that chart plotter displays should have a box showing the charts scale and the scale of the image on the plotter.)

 

[antaris]

There is an option in Homeport to show/hide chart edges (or similar...) - I believe that this is what you are referring to.

Also, occasionaly you may notice at specific zoom levels that there is different density of information, usually divided by straight lines. Thus, I'd presume that essentially we are looking at tiled maps... so to speak.

In addition, I have also noticed discontinuous depth contours around those same edges - but I presume taht you'd understand that better than me! I can only imagine that the "tiles" do not actually match 100%... which makes sense at that level of detail that we've achieved.

See the following example, this is off OpenCPN with Cmap cartography - similar things happen on Garmin. The area is near Piraeus / Athens (to my best knowledges the actual data for this area looks the same on Navionics, Cmap and Garmin Maps - another interesting issue, where do they all get their data from!!???):

 

[pcatterall]

My first reply seems to have gone somewhere!!
Yes Antaris, that does look like 2 charts, there are some linear discrepancies and the plotted sounding density does seem to jump, in some cases this can be cartographic licence where one stile may reduce plotted soundings where the sea bed is flat ( there are clear rules for this on Admiralty charts. I will look at that area on CM93, it does look strange.
I am going to order 'homeport' and chart EU454s best prices I have seen to date are 26.95 and 140 ( TCS marine) would appreciate any info on cheaper sites.

 

[BrianH]

My question about scale still stands though, this 'zooming in' is one of my concerns ( as former hydrographical surveyor) if 'zooming in' changes the chart for one of the appropriate scale then great, if it simply enlarges the scale to one which no longer reflects the surveyed accuracy of the chart then there is an issue. Generally you see by the 'style' of the chart what order of scale it is but some users may not realise this. ( I think that chart plotter displays should have a box showing the charts scale and the scale of the image on the plotter.)

The following are two CM93 chart images produced from OpenCPN that prove two separate charts are invoked. They are of the harbour of Umag in Croatia where zooming in shows a less up-to-date detailed chart of the harbour without the outer marina walls as shown in the smaller scale, clearly an earlier cartography from before the wall was added.

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[antaris]

And here is what a Jepessen Plan2Nav map consists of - I thought it was quite interesting...

The individual map listing is very very long...