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post #10 of Old 08-10-2009
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Originally Posted by tap View Post
The raster charts are scanned copies of the "real" charts? Where do the real charts come from then? Each revision is made by hand with pen and ruler?
I'm sorry. What I should have specified was the real chart is the master chart that chart copies are made from. The raster chart file was scanned from either the master or the copy. Each revision may be made to a copy of the master, but I'll bet the master has to be revised too. I didn't think I eluded to the paper master chart being made by hand...or did I?

Originally Posted by tap View Post
Obviously, NOAA's been making charts with computers for decades. The raster charts are made on a computer and serve as the masters for the paper charts that are printed out. There is probably the same GIS database behind both the RNC and ENC charts.

I have used a GPS that uses raster charts, Megellan's Triton hand held units. Of course you can't just load any GeoTIFF you happen to have and use it with the GPS, that would be too useful. All GPS manufactures seem to hate their customers, it's not just Garmin. But the Tritons can load raster topo maps from the National Geographic Topo program. (The maps don't come from NG of course, rather the USGS. Funny how all these GPS companies sell maps funded by the US taxpayer for big bucks).
It's called free enterprise.

Originally Posted by tap View Post
Anyway, using a raster map with a GPS sucks. Doesn't help that the Triton software is a POS either....

The problem is that the raster map was made to be printed on paper at something like 300 DPI. Your typical GPS has a screen that's more like 50 DPI. If you make the map the same size as the paper version, it's unreadable because of the loss of resolution. Small text is just a blob. Thin lines disappear. If you zoom in until you can see the detail properly, thick lines become huge and all the larger text on the map has become giant size and you need to scroll around just to read stuff. If rotate a chart made with East up so that North is up, all the text gets rotated too. Makes track up display pretty bad too, but that's not a problem on the Tritons because they can't do track up...
I agree with your explanation. Even if they could make a raster work good with a GPS or plotter, you would still be left with only one layer of information. I hardly ever use rasters when preplanning or when I happen to use my laptop while sailing (to check against my plotter), but I have them for reference in case the vector does not show certain information I'm might be looking for...or I get out the real (copy in this case) chart.
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