The Modern Navigation Trap

[goboatingnow]

Originally Posted by StuM

FWIW, if you come across any more old french charts ( pre WW1), the Paris meridian is 2°20′14.03″ E of the Greenwich meridian.

The Paris meridian sounds much more exotic then some place squeezed between blackheath and Romney road !

Dave

[hpeer]

[QUOTE=wdkester;1469419]

Originally Posted by hpeer

Screen shot of chart plotter. Lets see if this is clear. Presuming it is, you can see my track through the headlands.

But, when using Fugawi 4.5 in split screen with GE, and doing an overlay, the headlands and harbor line up.
......Damn.....
Image is not attaching, says "Rotate Image"
Started to happen a couple of days ago.

Try this PhotoBucket link to the same image.

Francois Bay Photo by hpeer | Photobucket[/QUOTE

I don't have Fugawi and can't comment directly on the coincidence of GE overlayed on the chart.

I'm wondering what would happen if you were to import the track into GE? I think it would plot right on top of the channel into Francis Bay. This is why I think so; I just plotted the cursor position (from your screen shot) on OpenCPN using CM93 charts. It plots about .5 nm SW of the apparent Fugawi cursor and is about .2NM due East of West point (and lands right on the ferry track). I mentally moved moved your track .5nm West and a little South and it looks like it might be OK. Which says the CM93 chart might be OK.

I then moved the cursor position to GE and it plotted .2nm East of West point. Which says that for that point CM93 and GE were similar.

Yes, I think you are right.

This is starting to get complicated to explain.

The screen shot is off my Standard ?Horizion CP using CMap.

Apparently CMap used the larger scale chart. I have a paper copy.

The Fugawi raster chart is of a smaller scale, but without the errors. Thus it lined up in Fugawi, but does not have certain detail such as the buoy.

Fugawi actually presents GE side by side with no overlay. The overlay is ONLY at this location and an adjacent harbor. clearly someone has "checked" the accuracy at these two locations.

Due to this exercise I now have a better understanding and I THINK someone checked the charts against GE.

BTW, I went in their in dense fog using radar. It's a little disconcerting to sail into a headland on your CP. I found the buoy by easing up to it, ( coulda been a rock on radar) that gave me confidence i was in the right crack in the rock, ( they all look the same on radar) then ran in on radar.

I had known of the potential for chart errors from reading the paper chart. Had I not, but blindly gone in, things could have been very different.

[valhalla360]

I'm not interested in either "probably" or "commonly" where rocks are concerned, because folks who run into rocks are both probably and commonly amateurs.

I'll give you 100-1 odds I get better data:
- I have data from 100 amatures going thru the area in the last year with accurate modern GPS units
- Over a profesional who went thru one time 100 yrs ago with a lead line and sextant.

Either way, you are dealing with probabilities but my probabilities are a lot better.