Anyone know this lump
Submitted by Alan James on Sun, 2011-06-12 12:49
On the chart below there is a rise about centre screen approx 3.8nm NW of North Murion that rises from 90m to 60m (58m when zoomed in). Is anyone familar with this lump? I've been over and over the area and circled and haven't been unable to find any rise at all ...... ???
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Tomcat
Posts: 614
Date Joined: 24/02/11
Lump
I looked for it 3 weeks ago not there but did well on reds not far away but that for you to find look for small flare ups not lumps
meglodon
Posts: 5981
Date Joined: 17/06/10
not a lump i suggest
Hi Allan & Tomcat,
I hate to throw cold water on your quest to find a nice lump...but the chart symbol you are indicating is not a marking of a lump.If you look on the chart imediatly to the north of the 98 number you will see the figure 104.Both the 98 and the 104 numbers appear between conture deapth markings ie: 98 appears between the 90 and 100 metre conture lines, the 104 appears between the 100 and 110 conture line.
None of the isolated numbers are an indication of isolated objects (rocks lumps poles etc)and do not appear in the chart 5011 (which is actualyis a book) and the following web site show all the approved current indicators of submerged items and the nature of the sea bottom.
http//mapserver.mytopo.com/mapserver/nautical_symbols/contents.html#k this is a great online reference for chart symbols etc.
I don't know what country is resopnsable for the chart shown in your comment, however if you look at it there is quite a number of numbers located on the chart that do not have a symbol against them. I strongly that the figures represent the location of the end/start of the decline/incline between the two conture lines.. Looking at the 98 figure this shows a difference of two metres between it and the 100 metre conture line.
Thats my interpritation of those figures, if anyone has a reference from a chart authority on what they mean I would love to hear from you, as I say that is just my own interpretation as I can find no authoritive explination and don't know who published the chart.
Hope I haven't confused you but I'm guessing you are hunting a ghost
Alan James
Posts: 2223
Date Joined: 30/06/09
Not sure I fully understand
but thanks for your comments Meglodon. I fully understand your comments about the 98 and 104 numbers. They are depth markings between contours. The lump or rise that I am referring to are the small oval 80m, 70m and 60m contours 2nm WWS / SW of the number 98. I have seen no indication on the sounder that this rise exists. My thinking is that the 4 small circle contours (90m, 80m, 70m and 60m) immediately to the west of the S.CO annotation are incorrect and the 90m contour that the S.CO sits on should run parallel to the 100m contour north of it ............ ???
till
Posts: 9358
Date Joined: 21/02/08
I think he misread the red
I think he misread the red current arrow as you trying to point out to that stray number ;)
Adam Gallash
Posts: 15644
Date Joined: 29/11/05
rise
Yes, there isn't any rise there, but I've hooked 5 marlin on that 'spot' in one hour of power before. Good areas around there for all sorts.
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Alan James
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Date Joined: 30/06/09
Thanks Adam
but can we trust you locals to give us outsiders correct info
harro
Posts: 1959
Date Joined: 07/02/08
try further north east
look between the 70m & 80M area
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Tomcat
Posts: 614
Date Joined: 24/02/11
Locals
I'm not a local and I'm sure the other comments are to help you not hinder
Lucky Tim
Posts: 2536
Date Joined: 28/11/07
it may not exist or be away
it may not exist or be away from the marked area, it depends on how and when the survey was done. Some charts still have areas that haven't been surveyed for decades or even centuries. The ZOC diagram on the proper AUS chart will show how a area was surveyed.
Megladon the 5011 book you use is abbreviations and symbols for nautical charts. These computer based electronic chart programs and the likes of CMAP and navionics cards don't run the same symbology so you won't see the symbols from 5011 on them. That's why they do not meet standards for survey on commercial vessels and they need to carry the relevant paper charts instead. Using them is another story though.
meglodon
Posts: 5981
Date Joined: 17/06/10
wooops
Yes you are right about me misreading the chart position you were seeking answers about - sorry for all the blah blah blah about the wrong item. Lucky Tim I haven't seen any cmap or navonics e charts . I run the transas navigator chart system and I haven't had a look at that area on the chart , transas navigator charts are the aus series of charts so it will be interesting to compare what it shows and the above chart. I have been told that the navonics chart are far superiour to the aus naval charts for detai.It will be good to compare both for detaill
Lucky Tim
Posts: 2536
Date Joined: 28/11/07
Transas runs raster copies of
Transas runs raster copies of the Australian Hydrographic Service issued nautical charts. Apart from ENC's these are the only ones that meet survey requirements.
As for companies producing cards like Cmap and Navionics, they contain no more legitimate info than what an AUS series chart does. They do not do their own surveys and get all of their chart info from either the Hydro Office or other authorities like the DPI charts of WA. The way a survey works is the Navy send their Hydrographic ship out to survey an area. This info is then assessed in the Hydro office and reproduced onto a nautical chart which is then published for use. Some info that is not relevant to navigation is left off the published chart. On your cards like Cmap and Navionics you can turn features on and off like contour lines which gives the impression they are more detailed than the Hydro charts but I can tell you they contain no new information.
I've been out before looking for a charted bank that was about 10nm long and never found it. Areas that do not pose a threat to navigation or are in a low traffic area may not have been resurveyed since the original that may have been done with a lead line and less accurate means of position fixing. That's the reason some lumps and features either don't exist or may not be where the chart says they are.
just dhu it
Posts: 1081
Date Joined: 14/05/09
tried, not found
yea allan i have been over this area twice and now assume the info supplied to the chart company was incorrect, but it does attract some interest
Bodie
Posts: 3758
Date Joined: 05/11/07
yep tried it too...nothing
yep tried it too...nothing there! head further north tho, some good lumps up there :)
Alan James
Posts: 2223
Date Joined: 30/06/09
Thanks for the feedback guys
I'll stop looking for that particular rise then. Shame, it does look very fishy.
You seem to have a good knowledge of the subject Tim. How common is it for features to either not exist or be somewhere else? The chart in the pic shows contours every 10m. Are other charts / software settings available that show the contours in smaller increments?
Lucky Tim
Posts: 2536
Date Joined: 28/11/07
Alan it depends on the area
Alan it depends on the area being surveyed. If there are no nav dangers or it isn't a commercially important area then it may be decades/centuries between surveys. Generally the closer to a port or shipping channel or the proximity to danger the better an area is surveyed. Gradually Australian charts are getting updated but it is a slow process and areas like the one in your pic are probably way down the list or priorities.
As for contour lines on other programs and cards, you can generally set up the program to show selected or all contours and depth soundings. This doesn't make the program any more accurate than the paper chart though as a lot of the "in between" contours are just joining up spot soundings or fitting them in between the main contour lines. Paper charts do not show all the contours becuase it just crowds things up with information you don't need. I'm running a HDS10 with Navionics and the contours are not really super accurate, especially up here where the card was based on a chart that covered a large area already.
hlokk
Posts: 4290
Date Joined: 04/04/08
Does anyone thats run over
Does anyone thats run over that area by any chance have a lowrance or humminbird, and have recordings of any tracks over that area? I have come across some interesting software that may shed some light....
I know someone with a garmin combo unit, I think it is, and that thing has user selectable maps where you can choose nav charts of fishing charts, and at least around rotto and the 3 mile it seems to have 2m contours which is certainly interesting. I dont know where they got that data from though. The navionics on my phone has the exact same chart as the one above (from the data Tim mentioned presumably).
Playing around with some software and having a look into interpolation techniques, one interesting thing is that a lot of features can appear a lot more defined than they actually are. Depends how many points you have. So they're really only a rough indication. What is interesting for that lump is that there are quite a few spot soundings given. If it was just one, maybe it was an error in one reading then they expanded it to make a lump (e.g. a 60 when it should have been an 80). But theres a few of them, which is interesting. I wonder if the spot soundings were in the original data or interpolated themselves (you would think not though).
There are a few methods to get really accurate data with todays technologies, but I guess there is not really a big push to map out the seafloor for fishing purposes. Anyone here been on Whiteys boat at Jurien though? ;)
till
Posts: 9358
Date Joined: 21/02/08
Ahh I hear the Garmin charts
Ahh I hear the Garmin charts are just made by extrapolating the f**k out of the same data everyone else has, and the extra detail is almost always completely bogus.
hlokk
Posts: 4290
Date Joined: 04/04/08
Hmmm, I was wondering that.
Hmmm, I was wondering that. In that case, I can get charts down to the millimeter scale if anyone wants some for some megabucks, haha. Jason said they were free in an update at least though, so I guess you cant complain too much.
Maybe it'll give some people more to think about when they think contour lines are features.... (unless they're actually close together or your using it for a rough depth, contour lines dont mean much at all on the actual seafloor)
Adam Gallash
Posts: 15644
Date Joined: 29/11/05
clarify
Can you clarify the last part of your comment Matt.. Contour lines don't mean much at all on the actual seafloor?
I've got contour lines that equate to 25m drop offs that result in fish..
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hlokk
Posts: 4290
Date Joined: 04/04/08
Was referring to a lone
Was referring to a lone contour by itself not meaning much. Obviously contour maps are very useful things when fishing and finding ground, but a contour (particularly a lone one) does not guarantee ground. (Note the disclaimer in above post :p)
Yeah, slightly confusing. I'll explain it out fully so its clear (for those who can be bothered reading), and so some people who are new might understand it better (but i'm sure quite a few will allready know all this)
I've heard a lot of people that think say the 20m contour line is a feature, but a contour line by itself doesnt necessarily mean much. Contour lines are merely the outside edges of slices of the seafloor (or of a mountain or whatever). So say we have a gentle sloping sea floor, the contour line is just where the seafloor happens to intersect at 10m cross sections. If you did it in yards or fathoms, you'd get the contour in different places. The seafloor doesnt know what measurements we use. So exact placements of contour lines are somewhat arbitrary by themselves. There might happen to be a feature or dropoff on them (its slightly more likely depending on the size of the dropoff), but not necessarily if its a slowly sloping bottom.
Contour lines close together are a completely different story. If you get two close together it means you have a sharp feature there. So a 25m dropoff, should show at least three contour lines close together. If you see contours close together, they indicate features. The map may not accurately show the contours though so you can still get big dropoffs without contours close together (but due to surveying, the dropoff probably wont be exactly in the right spot). Quite a few instances of pinnacles with huge rises not appearing on maps too obviously. Theres obviously lots of features not on maps, or features that fit perfectly to maps, or ones that are right near a contour at certain spots. But just because theres a contour on the map doesnt guarantee a feature.
If you have a big dropoff, you have a much higher chance that a contour line will intersect it however. Assuming its perfectly mapped, then it will correspond to the feature. A 9m rise is unlikely to not interesect a contour line (it would have to fit perfectly inbetween two just by random chance). But lets consider a gently sloping sea floor that has a sharp 2m drop at the edge of some semi-submersed limestone. Obviously a 2m sharp drop might be worth fishing. On a perfect chart however, the contour may not show up. E.g. say the dropoff is from 14 to 16m. The 10m and the 20m contour could be a fair distance away from the feature (the sharp dropoff). Both contours could be largely featureless. Imagine the same dropoff from 20.5 to 22.5m. The dropoff would be pretty close to the 20m contour, so if you fished the contour you might be close enough anyways (but not perfectly accurate). However, it could be that its a flat reef and it extends for 50m from the contour to the dropoff. So if you trolled the dropoff according to your chart, you might miss it. Though, there could be the case where the dropoff is 19-21m, in which case it should be smack bang on accurate (if the charts are perfect).
Some features correspond to structure. E.g. trolling the 20m contour for mackies at Mindarie, but some dont, like the 30m contour which is pretty much very slowly sloping sand for the most part.
Obviously if you want to find fish, you use the combination of your charts and your sounder, so if the chart looks good with a few contours, you can find the exact dropoff with a sounder. Just because theres a contour on your map, doesnt guarantee theres anything interesting there, but it may warrant a look perhaps.
Phew, now back to my (now cold) pie, haha.
81macca
Posts: 270
Date Joined: 02/07/09
FARKUP!!!!!
FARKUP!!!!!
I actually fish.
Bodie
Posts: 3758
Date Joined: 05/11/07
Alan this seems to be a quite
Alan this seems to be a quite a common occurance around exmouth, especially north towards or around the murions.
Just south of where the map you put up finishes has some more that dont seem to be there, from memory in deeper water, out near the 100 mark that comes up to 70 or 80.. and i think even deeper 120m that comes up to 90. Never found them tho!
Alan James
Posts: 2223
Date Joined: 30/06/09
I agree Bodie
I have spent some time studying the charts looking for 'out of place' spot soundings. I've transferred these coordinates onto my sounder to check out. The first mark you refer to above is probably the 66m spot sounding shown between the 80m and 90m contours. So that is not there either, I haven't checked that one yet. Seems I need not bother. There is another 'cracker spot' if it was real but I don't believe it is. Look direct west of NW Reef, there is a 59m spot sounding on the 120m contour. For those that may have wondered the 46m spot sounding approx. 1nm SW of the nonexistent rise in the chart shown isn't real either.
81macca
Posts: 270
Date Joined: 02/07/09
Most of these "bogus" spots
Most of these "bogus" spots hold bills, alot are on our trolling paths and alot of fish have been raise/tagged near by? Strange!
I actually fish.
Alan James
Posts: 2223
Date Joined: 30/06/09
I wish I hadn't read that comment
Repeat after me 'It is only coincidence ....... it is only coincidence .......... it is only coincidence'.
81macca
Posts: 270
Date Joined: 02/07/09
Is this the lump they call
Is this the lump they call the tennis court?
I actually fish.
harro
Posts: 1959
Date Joined: 07/02/08
alan
pm me, i can give u some ground not far from there,
that is "off the chart"
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marble
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Date Joined: 03/09/09
Can we all do that Harro ??
Can we all do that Harro ??
PMY 25 Centre Console DF300 Suzuki
Alan James
Posts: 2223
Date Joined: 30/06/09
No, haha
meglodon
Posts: 5981
Date Joined: 17/06/10
chart information
lucky tim your spot on you would't be ex hmas moresby by chance would you
Lucky Tim
Posts: 2536
Date Joined: 28/11/07
ha no Navy for me. Just
ha no Navy for me. Just always been interested in charts and surveying. There's some good info here for those with the time and interest
http://www.hydro.gov.au/factsheets/factsheets.htm
particularly the "Accuracy and Reliability of Charts" and the "Unofficial Charts and Chart Plotters". The latter will answer a lot of your questions and answer why Navionics and Cmap and the others only offer extra gimmicks rather than factual info of the sea floor.
harro
Posts: 1959
Date Joined: 07/02/08
lol
put it this way,
dble headers rankins/reds, and much much more,lol
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Lucky Tim
Posts: 2536
Date Joined: 28/11/07
what a coincidence, I need a
what a coincidence, I need a double header of reds.....
harro
Posts: 1959
Date Joined: 07/02/08
lol
i meant to say monster rankins and monster reds,lol, we also caught big rosy,s there, think they were anyway,
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