Raster vs. Vector chart discussion

[Roger Long]

There is quite an extensive thread starting over in the "Points East" magazine forum

http://www.pointseast.com/forum/index.php

about charting issues. Since I wrote the original article, I'm going to keep most of my comments and follow up over there on this issue. Please drop by and post your thoughts.

 

[kloudie1]

Vector charts in my Garmin are a little wonky sometimes too.. A couple of years back, I was in the North Shore Channel in eastern Lake Ponchartrain ( look in http://demo.geogarage.com/noaa/ just east of the I-10 bridge around 89 46.17 W and 30 11.45 N ) Vector charts, as displayed, had the channel about 150 feet to the northeast.. As I motored through heading southeast, and cautiously leaving the greens to starboard, my track on the screen showed me clearly out of the channel to the southwest and on the wrong side of the greens, which the plotter displayed as off to port.. cute ! The other beef I have with the vectors is the bridge info. There is a bascule bridge about 7.5 miles south of the northshore of Lake Pontchartrain on the Causeway. This bridge does not even appear unless the screen is zoomed almost all the way in until the bridge is about 50% of screen.. Several local bridges show up that way (or rather don’t show up that way!).. They clearly show on the raster charts .. I haven’t been in the channel since then so I don’t know if Garmin has corrected, but I am very cautious with channel entrances after that wake-up call..

 

[druid]

I'm not sure if that's a "raster vs vector" problem. I've had MANY quirks with my Garmin G3 charts that make me glad I only use them as backup. Like suddenly moving 100yds or so east, or sailing on dry land... There's also often a "jag" when I go between charts.

But the worst was years ago when I was trying out a chartplotter and looked at Secret Cove area: they had the stbd marker for Welcome Passage pretty much where the port marker is, and the marker for the entrance to Secret Cove was in the middle of Turnagain Island!

druid

 

[sailingdog]

Good article. I'd point out that when using any charts, electronic or paper, raster or vector, that the chart is an interpretation of the real world that is not going to be 100% accurate, since it was made by a human being, using data that is a snapshot in time...and that the real world changes.. so you really have to take the charts with a grain of salt...

 

[Gary_H]

by the time charts are assembled, published, distributed and you get hold of it they are out of date. channels and waterways can change over very short periods of time. Even electronic charts aren't updated often enough. I have made my way through inlets where I was obviously in the middle of the channel and the chart plotter had me sailing across dry land. Use them as an aid to navigation but don't put blind trust into them.