Remember Me?

 Straight Dope Message Board Remember Me?

#1
07-21-2016, 09:48 PM
 Guest Join Date: Nov 2013 Posts: 950

## Nautical chart - Obstruction

Nautical Chart

Depth numbers in a dotted circle usually means a danger to navigation; in this case at a depth of 10ft. The accompanying note "Obstn" is obvious in it's meaning.

What confuses me is that this 10ft "obstruction" seems to be on a contour line that is all 10ft. I don't understand that and it makes me think I must be off in some way.

As a fisherman, the idea of some sort of underwater structure that differentiates itself from the rest of the bottom is an area of interest, so I will check it out with a line in the water anyway, but I'm curious what this notation is really telling me.

(The magenta teardrop is pointing to a fixed navigation light, not the "obstn")
#2
07-21-2016, 10:40 PM
 Charter Member Join Date: Jan 2001 Location: Brisbane, Australia Posts: 14,259
The line between 10 and 16 is the 16ft line. The bottom is sloping down from right to left, and so towards the 16ft line the bottom is probably nearly 16ft deep. But at the left hand edge of the 10ft obstruction, it would only be 10ft.

I can see what you are saying in that if you were at 10ft of draft you would be safe in the 10ft area, whether over the obstruction or not. But if you were at say 14ft and thought (based on the 16ft line) you could go slightly over to the right of the 16ft line based on the likely slope of the bottom, you would be wrong where the obstruction is.
#3
07-22-2016, 05:19 AM
 Guest Join Date: Mar 2013 Posts: 4,373
Its generally as little as 10 feet deep there... but obstructions are dangerous and variable.

Besides which they are making a map and so they mark interesting features.
#4
07-22-2016, 08:44 AM
 Charter Member Join Date: Jun 2000 Location: Somewhere in time Posts: 6,546
Quote:
 Originally Posted by Ornery Bob What confuses me is that this 10ft "obstruction" seems to be on a contour line that is all 10ft. I don't understand that and it makes me think I must be off in some way.
Something that I can actually answer since I worked at NOAA making nautical charts about 15 years ago.

First, those are not contour lines, they are depth curves and there is a difference. A contour line is supposed to represent how things look exactly in the real world. They are also closed shapes and so do not have ends. A depth curve does not have to be closed and if you notice in your picture a couple of the lines run into the shore and end.

When they make the curves they take any surveys that have and follow the general line of depths. There can be deeper parts behind the curves but there will not be shallower parts outside of the curve. There could be sections that look like this:

5 4 6 5 4
6 8 6 7 5
8 9 7 7 9

and the curve would run between the bottom two lines like:

5 4 6 5 4
6 8 6 7 5
8 9 7 7 9

A contour would run like this:

5 4 6 5 4
6|8|6|7|5
8 9 7 7 9

5 4 6 5 4
6 8 6 7 5
8 9 7 7 9

Quote:
 As a fisherman, the idea of some sort of underwater structure that differentiates itself from the rest of the bottom is an area of interest, so I will check it out with a line in the water anyway, but I'm curious what this notation is really telling me. (The magenta teardrop is pointing to a fixed navigation light, not the "obstn")
#5
07-22-2016, 08:55 AM
 Charter Member Join Date: Jun 2000 Location: Somewhere in time Posts: 6,546
Quote:
 As a fisherman, the idea of some sort of underwater structure that differentiates itself from the rest of the bottom is an area of interest, so I will check it out with a line in the water anyway, but I'm curious what this notation is really telling me. (The magenta teardrop is pointing to a fixed navigation light, not the "obstn")
The Obstn is telling you that there is something there, it really could be anything, it could be part of what looks like the pier right there has sunk, or possibly a large tree has sunk there. I don't believe it means it's a rock though as I thought they had a different symbol for that. It is telling you though that somewhere in that circle is something that could hit your boat if you draw more then 10 feet. It's probably not known exactly what it is though.

I will ask some of my buddies at lunch if I remember exactly how they chart those.

[I also meant to edit my last post but the boards wouldn't let me.
#6
07-22-2016, 09:17 AM
 Guest Join Date: Nov 2013 Posts: 950
Thanks for all the info!

This is a marina that was dredged and built in the 80's, so it's kind of hard to imagine why there would be an obstruction in what looks to be very regular contours of breakwater type construction.

Whatever is going on there, I'll be fishing it tomorrow morning!

#7
07-22-2016, 10:18 AM
 Charter Member Join Date: Jun 2000 Location: Somewhere in time Posts: 6,546
Quote:
 Originally Posted by Ornery Bob Thanks for all the info! This is a marina that was dredged and built in the 80's, so it's kind of hard to imagine why there would be an obstruction in what looks to be very regular contours of breakwater type construction. Whatever is going on there, I'll be fishing it tomorrow morning! Google Maps Satellite View
Interesting. In such a case it's possible that it's been left on there because they have not gotten any information that says the obstruction is gone so it's left. Better to be safe than sorry.

It could be the case that they were unable to take out whatever was there when they dredged the area. NOAA makes well over 1000 charts and they are only updated when someone gives them new data. Odd though, looking at the area it seems that in the LA area they would be updating the charts more frequently. Notice how the pier looks different on that chart compared to the photos.
#8
07-22-2016, 12:32 PM
 Charter Member Join Date: Jun 2000 Location: Somewhere in time Posts: 6,546
Ok, so I asked my buddy what an Obstn is and was told it could be anything, pilings, rocks, anything that creates a hazard. I was also told that it could have been reported by someone that hit something in the water so the obstruction might not have been surveyed at all.

So that obstruction could be anything at all, and it may no longer exist but is kept on the chart until the area is surveyed and they know it's no longer there.

 Bookmarks

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is Off HTML code is Off Forum Rules
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Main     About This Message Board     Comments on Cecil's Columns/Staff Reports     General Questions     Great Debates     Elections     Cafe Society     The Game Room     Thread Games     In My Humble Opinion (IMHO)     Mundane Pointless Stuff I Must Share (MPSIMS)     Marketplace     The BBQ Pit

All times are GMT -5. The time now is 12:44 AM.

 -- Straight Dope v3.7.3 -- Sultantheme's Responsive vB3-blue Contact Us - Straight Dope Homepage - Archive - Top

Send questions for Cecil Adams to: cecil@straightdope.com