Accuracy of Plotter Charts
Submitted by Alan James on Tue, 2011-02-01 20:19
99.9% of the time I find the contours and lumps / holes showing on my plotter spot on but there was one occasion a few years back I was looking for a mark off the Murions at Exmouth that on the plotter rose to from 80m to 58m and yet when I travelled over it the sounder showed flat as. I circled a few times without any sign of a lump and then moved on. Anyone had a similar experience?
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NOHA
Posts: 914
Date Joined: 24/06/07
Off Thevenard
My Garmin shows a beautiful elongated rise 40 meters out of 60.. Could not find it.
There was however a rise a little shallower that was all mud/sand. I suspect the feature that is on my chart was there when the survey was done but was a big sand/mudbank that dispersed after a cyclone went thru.
Twin turbo..V8 diesel..Ohh what a feeling!!
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Goodz
Posts: 2332
Date Joined: 20/07/09
Yep on my last trip out down
Yep on my last trip out down south my charts said from 50 up to 35 and 25m but cruised the whole area and only found one lump that came up to 45m. Still was a nice bit of ground, the depths were way off tho.
Ryan C
Posts: 1575
Date Joined: 08/07/10
plotter accuracy
my lowrance/navionics has always been spot on in my boats , other than the fact that when examining a contour marked on the map the depth change or (rise) is not exactly where the middle of the contour is you have to sound around to find the most prominent part of the rise (most contours off two rocks/mindarie hold sambo"s all year round without having to go to the back of rotto.)
DieHard
Posts: 1823
Date Joined: 06/10/08
i know this sounds stupid but
i know this sounds stupid but what are contours :/
DieHard – The Official “Ray & Shark” Chaser!
Alan James
Posts: 2205
Date Joined: 30/06/09
Contour lines are lines on a topographical
map which join areas of similar elevation (height) either above or below sea level. Google it.
DieHard
Posts: 1823
Date Joined: 06/10/08
oh yes i remember now
oh yes i remember now :)
thanks mate
DieHard – The Official “Ray & Shark” Chaser!
Lucky Tim
Posts: 2536
Date Joined: 28/11/07
the quality of the soundings
the quality of the soundings on nautical charts varies depending on the area. Some charts and certain soundings have not been updated since they were first charted and sounded by leadline. You will find, especially in areas that are getting slightly remote that there will be lumps and soundings on the charts that do not exist. This is because when they were surveyed the methods of position and sounding accuracy we have today weren't around.
Metro, high traffic and shallow areas are usually charted very well and most things will be where they say they are but once you get out into less "important" or deeper areas the accuracy can fall away a bit. Charts really exist to assist with safe shipping and navigation so if there are features that do not affect safe navigation (ie deep water reefs) then these areas are going to show less information and 100% accuracy is not as important to the hydrographers publishing the charts. If you look at an actual paper chart it has a Zone of Confidence diagram and shows how each area on the chart was surveyed and how reliable the information is.
It is frustrating heading out to a charted reef and finding it not there, we've gone looking for entire banks 10's of miles long that we could never find. Having said that when you do find reef and the chart says it is meant to be a flat and featureless area it is generally going to be good, especially if you are off the main traffic routes.
Alan James
Posts: 2205
Date Joined: 30/06/09
Good comment Tim
Next time I'm up in Exxy I'll have a more extensive look around the area.
till
Posts: 9358
Date Joined: 21/02/08
Its a bit needle in a
Its a bit needle in a haystack, you can miss a reef by just meters! I remember one day heading out of exmouth gulf, we did 16nm and didn't see a single blip on the sounder.
Alan James
Posts: 2205
Date Joined: 30/06/09
Yes it's a big ocean
The charts don't show this as a small rise so it surprised me that I couldn't find a trace of it on the sounder. The 70m contour rising up from 80m is some 400m long by 150m wide. Might need to set up a trolling pattern of increasing circles.
Another unrelated question. West of the Murions my charts show 3 wells in 160m-190m. I'm sure some of you have bottom bounced these locations. I want to give these locations a try when I'm up there later this year but I wondered if anyone could offer some feedback.
Lucky Tim
Posts: 2536
Date Joined: 28/11/07
I know the wells you're
I know the wells you're talking about but have never been over them. There might be some of the well still on the bottom but I reckon it'd take some finding. Might be a good excuse to have a billy troll out there while looking around.
Alan James
Posts: 2205
Date Joined: 30/06/09
That's what I was thinking Tim
What is on the ocean floor as far as man made structure? I don't know if these explorations wells, production wells, capped or whatever but it doesn't take much structure for fish to make it their home. Come in all you offshore people. What structure are you likely to find on the seabed of a well? When in Exxy I often troll for beakies to the east of these wells along the 100m line. The first well is only about 1nm west so that's a bit more recon to do.
Lucky Tim
Posts: 2536
Date Joined: 28/11/07
here's some interesting
here's some interesting reading for those that are into this sort of stuff.
http://hydro.gov.au/factsheets/AHS_Fact_Sheet_Accuracy_And_Reliability_Of_Charts.pdf
http://hydro.gov.au/factsheets/AHS_Fact_Sheet_Unofficial_Charts_And_Chart_Plotters.pdf
Tony Halliday
Posts: 2500
Date Joined: 14/06/07
all depends on the number of
all depends on the number of satellites it has a fix on and if you have DGPS signal input / correction. PLEASE NOTE not all GPS units even new ones have DGPS functions or able to receive the DGPS signal.
I know the Furuno units will be with in 5 to 10m accurate most of the time, but with DGPS input or function ( a correction signal land based sent out) it comes down to less than 3m
http://www.amsa.gov.au/shipping_safety/navigation_safety/differential_global_postitioning_system/dgps_fact_sheet.asp
download the fact sheet on the Gov website.
Tony Halliday: ~Meals on Reels ~
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Lucky Tim
Posts: 2536
Date Joined: 28/11/07
GPS and DGPS are accurate
GPS and DGPS are accurate enough nowadays that any normal error in their positions will have you close enough to your spot that you can find it on the sounder. I put the mysterious missing reefs down to old charting methods and position error prior to GPS. A lot of soundings on todays charts are still the original depths recorded when it was measured by lead line and the ships were still sail and steam.
Tony Halliday
Posts: 2500
Date Joined: 14/06/07
yes I have seen where my
yes I have seen where my Bluenav charts from 2004 are diferent to a degree in areas compared to the latest CMap charts.
Garmin has charts that are not as up to date as CMap or Navionics, some times it's only a minor depth or reef curving etc, but on systems like the Swan, Sound and other places where in 10 years changes do occur on the sea bed / river bed, they make a difference.
Tony Halliday: ~Meals on Reels ~
It takes a strong fish to swim against the current. Even a dead one can float with it
"It is always in season for old men to learn." Aeschylus (525-456 BC)
"In a mad world only the mad are sane." Akira Kurosawa (1910-1998)
Reefmonkey
Posts: 711
Date Joined: 22/09/08
Sorry didnt read all the comments but read the topic
But did you mean its showning contours that dont exist? If so I've found that on the latest navionics gold chart, there is a spot about 4 miles west of the centaur marker off trigg shows a raised bottom about 70m in diameter that rises from approx 31meters to 22meters from memory,( knew it didn't obviously exist but sounded it out of curiosity) but yea all 32m flat ground. Have found navionics to be the best yet and detailed far beyond the DPI charts for shallow water but yea... can't fault it as a safety issue fopr navigation but ya scratch your head when you find contours that dont exist off shore
Dave J.
Alan James
Posts: 2205
Date Joined: 30/06/09
Yep contours that don't exist
From reading the responses it would appear the lump I was looking for probably does exist but it is not where the chart is showing it.
seasport
Posts: 115
Date Joined: 15/08/10
The contour lines are quite
The contour lines are quite offten a generalisation, if you spend enough time searching the area you will find the upwelling somewhere within the suggested area. They may only be small, dont expect the whole area to jump up. Ive spent around 30000hrs looking at charts and my sounder and have eventually found everyone Ive looked for, time on the water and keep searching the area , you will find it
Lucky Tim
Posts: 2536
Date Joined: 28/11/07
something to remember is all
something to remember is all of the electronic nav cards for GPS's all get their seabed information from the same place and that's the Navy Hydro branch. A tiny bit may come from places like DPI or harbours but it'd be 99% from Hydro surveys. Navionics, C-Map etc don't do their own surveys of our waters so they just buy the reproducing rights from Hydro. There is no more detail on any card than on a Aus Chart.
One certain brand being better than another really boils down to the age of the info on it and the age of the unit running it. Some have better secondary features like photos, port info, tides etc but the actual seabed info all comes from the same source. Just because your card says it can show contours every metre out there doesn't mean it is anymore accurate than the rest, sometimes I think all this does is gives you a false sense of security. Zooming is super close isn't going to make your chart anymore accurate and if you read the disclaimer at the start that you have to press yes too tells you of all the reliability issues.
Not trying to be a spoil sport but I think there is a misconception out there that these cards are super detailed and have secret spots the others don't.
fishy fingers
Posts: 1719
Date Joined: 28/04/07
I said this ages ago
I was in a discussion with grayzee a while ago about the quality and detail of memory map charts on the pc I said back then the
electronic charts like navionics etc all get their info from hydro charts (which memory map uses) and any extra contours or detail where put in by the producers themselves they dont do their owns surveys.
grayzeee
Posts: 2283
Date Joined: 09/07/09
this may be the case but
this may be the case but navionics is still more detailed in shallow area's than MM as i showed.
so are you saying it's just guess work when they put in the extra contours around rottnest or 3 mile?? i'll still take the charts with more detail over less in these well charted area's
If I spent half as long fishing , as I do reading this bloody forum , I'd be twice the fisherman I am.
fishy fingers
Posts: 1719
Date Joined: 28/04/07
I dont know
I'm unsure where they get it from but going on the fact they get their info from the hydro charts I
just dont know where they get the extra detail from if no one else does the surveys. maybe the navy dont feel the need to draw the contour lines in, what would be interesting to see if there is any depth difference between the contours that exist on your navionics that dont on the hydro charts.
grayzeee
Posts: 2283
Date Joined: 09/07/09
what i was origionally saying
what i was origionally saying was the MM chart is less detailed than the DPI version produced by MCS (the shop bought printed charts which also look like my navionics charts) in the shallower waters (close up of rotto and 3 mile under 30 meters), which i must then assume is as Tim says extra survey that "may have come from places like DPI or harbours "
it's the most likely explanation i think considering the printed map chart is indeed DPI
i'm just assuming navionics have used the extra DPI detail where MM must not have.
If I spent half as long fishing , as I do reading this bloody forum , I'd be twice the fisherman I am.
fishy fingers
Posts: 1719
Date Joined: 28/04/07
Memory map do
use DPI charts as well as hydro and yes the dpi are more detailed than Hydro but dont cover the whole state,
If I compare my dpi paper to the dpi on MM they are the same, are you saying the dpi are the same as navionics
but not the hydro?
Reefmonkey
Posts: 711
Date Joined: 22/09/08
Interesting
They must get their detail somewhere beyond the DPI charts I always navigated on DPI charts with a parrallel ruler and a basic GPS for the last 10 years, recently just upgraded to a chartplotter with navionics, Navionics is easliy more defined in shallow water than the DPI charts are, only in depths tho not danger, marks or underwater structure
Dave J.
grayzeee
Posts: 2283
Date Joined: 09/07/09
to prove my point again what
to prove my point again what i am/have said is you are going to have to humour me and actually pull out the DPI paper chart of rotto (412 stark bay to parker point)
exactly the same as navionics gold (just checked in the tub again)
my memory map (M15 aus west) is most certainly definately unquestionably less detailed at highest zoom.
i showed you that pic huggy put up off navionics and you agreed with me then.
whats changed??
look at any reef around rotto at any zoom you care to
please try this or once again come round and i'll show you.
i can't say any more than that
If I spent half as long fishing , as I do reading this bloody forum , I'd be twice the fisherman I am.
Ashram
Posts: 134
Date Joined: 23/10/09
Chart cartridge
Good morning all.
Check out this link to Jeppesen web site. These charts are new bathymetric charts which at the moment only have contour information. I got to look at demo one last year and they are pretty damn good. I got some to go in the Furuno gear but noticed that they are better utilised if your plotter has 2 card readers. That way it will overlay on the chart. there were 2 areas for WA a south and a north and the detail of contours was huge. I checked out cape naturaliste area and it really does open your mind to what is there.
Anyway here is the link
http://www.jeppesen.com/company/newsroom/articles.jsp?newsURL=news/newsroom/2010/High_Res_Bathy_Charts_Available.jsp
Cheers Rhyss
Alan James
Posts: 2205
Date Joined: 30/06/09
Interesting site there Rhyss
I have a Navnet vx2 with Navionics gold. I'm not particularly electronics savy so my question is can the jeppeson product be used in conjunction with the set up I have?
marble
Posts: 772
Date Joined: 03/09/09
Bring one up here to try out
Bring one up here to try out Rhyss , , go on you know you wanna do it :-)
PMY 25 Centre Console DF300 Suzuki
Ashram
Posts: 134
Date Joined: 23/10/09
Bathy chart card
Hi Alan, you will have to check that out with navionics they are one of the other major players in charting.
Hey ben I already have one permanently in my briefcase so when I come up for Gamex we shall chuck it in your navnet and you can have a look. Are you fishing Gamex or AIBT?
Rhyss
marble
Posts: 772
Date Joined: 03/09/09
Rhyss we not fishing either
Rhyss we not fishing either this year :-( Theres too much else happening this year and it all costs money . Was gunna use our boat for a fly team for the AIBT which would have been interesting . Be good to have a look at that, I`m sure a social day out catching reds could be organised if you got time while your here. Gotta test it in real world conditions after all . . .
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