duncan Posted November 16, 2004 Report Share Posted November 16, 2004 ...my gratitude! OK you have seen these in mags - and it's always so much theory - so I thought you would all appreciate the option to show your skills. The following is a sonar image recorded running down the tide (West to East) across an area Charlie introduced me too as 'the banks' The overall depth is 83 feet and the highest 'pinacle' comes up a staggering 20ft as you can see. Now, where would you position the boat to fish, how and why? As I have pointed out before I am happy to run these over 'favorite spots' for club members if they wish, and I can direct you to the software that enables you to place your mouse cursor anywhere on the image to get the depth and exact position (lat/lon)............enjoy! Quote Link to comment Share on other sites More sharing options...
Afishionado Posted November 16, 2004 Report Share Posted November 16, 2004 In the picture would the tide be running from left to right or visa versa? OR sorta North to south if you see what I mean. If the current was running on a N/S plane then I would (if I were clever enough) anchor up to fish one of the gullies. If it were running across the ridges then slightly up tide of a high spot and fish with just enough lead to hold bottom and try to work the terminal tackle down tide over the hump and into the slack just beyond it. mad mike Quote Link to comment Share on other sites More sharing options...
duncan Posted November 16, 2004 Author Report Share Posted November 16, 2004 left/right - the boat is running along the line taken by the tide. having the exact position of everything available means that if you are any good you can actually position the boat very exactly relative to the features by working out the angles and back to where you want the anchor 'in' then add a bit uptide ('cos you can let out more easy enough). Fishing it on the flood (right to left on the screen) at anchor would make it interesting as you dropped the bait back - if you got it wrong it's just going to sit in the face of the bank I presume. On the ebb would you try and fish the scour tight in to the face or ? Quote Link to comment Share on other sites More sharing options...
boybilly Posted November 16, 2004 Report Share Posted November 16, 2004 What is the run on the screen? By this I mean what distance does it cover from left to right? BB Quote Link to comment Share on other sites More sharing options...
Bob F Posted November 16, 2004 Report Share Posted November 16, 2004 You've got a whole variety here. The pinicles and troughs are all obviously rock(orange), but on the left it looks like you've also got an area of shallow sand or mud (blue on top of orange). That might be an interesting spot for rays. BF Quote Link to comment Share on other sites More sharing options...
Maverick Martin Posted November 16, 2004 Report Share Posted November 16, 2004 Duncan If it is the mark that most of the older club members know as banks then the shallowest part I have found is 56' at low water. I usually motor uptide of the peak say 100 metres and drop my anchor then allow enough warp out so that the top of the bank is lying just behind the boat. You can easily do this by watching your sounder and noting the depth. The idea then is to trot you end tackle back behind (downtide of) the bank as described by Mike. This should work for some species i.e rays flatfish etc (if they are present) but others such as bass, Pollack may be in other areas of the bank. When the tide turns re-anchor and do it the other way. Some banks fish better on the ebb or flood To be fair this particular set of banks used to fish really well but recent years they seem to be devoid of life other than doggies and the odd ray and occasional bass. I would be pleased if anyone can tell me otherwise as it is an easy mark to anchor and fish. By the way it Quote Link to comment Share on other sites More sharing options...
duncan Posted November 17, 2004 Author Report Share Posted November 17, 2004 yep that ties in - the 66ft above that pinacle was at high water Sunday last. it was here that we lost 6 fish to breaks on the Sunday of the coloured water, and Charlie had 7 conger, so maybe it's an opportunity for after dark some time? also a 15lb bass taken there at the same time - again those conditions were so different............... just thought it an interesting picture to discuss - the gully looks so boring relatively but... Quote Link to comment Share on other sites More sharing options...
Adam F Posted November 17, 2004 Report Share Posted November 17, 2004 Looks at fantastic mark Duncan - its also amazing how clear a picture you get when using a colour sounder....I really am tempted, but Ill wait until they drop down to the 200/300 quid mark (for a decent unit) before I get one... In a way similar to parts of the ledge. I am intreguied to learn more about this software Duncan - do tell? I suppose bank fishing is a little like anchoring a wreck, position of the boat, amount of rope and all that has a big impact on how sucessful one is. Now if I only had the numbers for these 'banks'...... Quote Link to comment Share on other sites More sharing options...
duncan Posted November 17, 2004 Author Report Share Posted November 17, 2004 Adam, Firstly I don't have a colour sounder! My B&W sounder enables me to record the actual input sonar data, combined with the GPS data, and Lowrance provide a software program to view this. That application works just like the unit settings, even down to sensitivity and surface clutter, but is being driven by a 3Ghz PC and displayed on a 42" plasma screen (or at high res). This particular image is a screen grap to paint then convert to gif (small file) for display on the site. Charlie has the same capability I believe. Because of the way it works you can actually import the file into a spreadsheet application and, if you have enough data, it will spit out a 3d map of the bottom - haven't done this though. Re ground - as Billy hints you need to know the boat speed to really understand what's being shown on the screen - Christchurch ledge at 40knots shows as a pinacle! We all get to relate this on our own unit. I am in Edinburgh right now but will give some positions from which distance can be calculated - for reference however the boat speed was 15knots on the button. Quote Link to comment Share on other sites More sharing options...
Bob F Posted November 17, 2004 Report Share Posted November 17, 2004 Duncan, can you confirm what the colours signify? Is yellow a strong reflection (eg from rock) and blue a weaker reflection (like sand)? I just read an article in BFM and it states that it is the other way round, yellow is sand. In which case the "pinacle" is not a rock pinacle but a sand bank that you passed over at fairly high speed??? Or maybe it's just the way your program has converted from B&W to colour??? cheers BF Quote Link to comment Share on other sites More sharing options...
Bob F Posted November 17, 2004 Report Share Posted November 17, 2004 Also, what was the average wave height on the day? We have to figured that out of the equation when looking at the bottom plot. (reminds me of a certain day out in Kimmerridge when there were 30ft pinicles appearing in previously flat terrain!!) cheers BF Quote Link to comment Share on other sites More sharing options...
duncan Posted November 17, 2004 Author Report Share Posted November 17, 2004 no waves - we ran back in at 30 plus knots! colours are Lowrance standards so the info from the sea angler articles on the LCX range will be relevant (LCX used for data) but I can up or down the colour map so without a reference. here is the same sample but in the origional colour format as best I can - had to use a .jpg sample so it's a bit bigger. Also havew give a couple of reference points for those that want (can) work out the scale - ie how wide is that pinacle - 10 ft or 100ft! Quote Link to comment Share on other sites More sharing options...
boybilly Posted November 17, 2004 Report Share Posted November 17, 2004 Not knowing the linear measurement of the picture makes it difficult but from a personal point of view I would be looking to anchor centrally so that I can fish downtide and flick it uptide to find fish if necessary. Sometimes when you downtide you can bounce over depressions. Flicking it uptide means you can drop in to every nook and cranny. You need to use a tad heavier lead fior this but it can be really productive. BB Quote Link to comment Share on other sites More sharing options...
Bob F Posted November 18, 2004 Report Share Posted November 18, 2004 Now I'm not 100% sure about this, but I think 1' in the E-W direction is equal to 400ft. You show 16' between your two westerly plots (29' - 13'), which equates to 6,400ft (16' x 400ft) or 2,100 yds??? Which is just over a mile. Would that make sense??? That's one hell of a long plot if it's true...??? I assume the two X's on the chart are the two cursor plots??? BF Quote Link to comment Share on other sites More sharing options...
Bob F Posted November 18, 2004 Report Share Posted November 18, 2004 Oops!! Just noticed those are seconds (") not minutes, so divide everything by 60. So, the distance between the X's is 35yds (105ft). That's better. BF Quote Link to comment Share on other sites More sharing options...
duncan Posted November 18, 2004 Author Report Share Posted November 18, 2004 seems reasonable Bob - gives that pinacle / bank a base of around 25ft and therefore a seriously steep slope! As suggested by Martin this particular bit would probably fish better by drifting for fish holed up in the shelter than any expectation of bottom feeders 'hanging' to it? Looking at it this way I would look to put my baits further left. On the ebb however I would have thought the flat area (to the right on screen) a reasonable holding / feeding ground. The calculation you refer to can of course enable the calculation of where to have the anchor to get the boat where you want the bait.......... something the professionals seems to know instinctively but always seems to much hassle for the occassional to bother with! Quote Link to comment Share on other sites More sharing options...
Bob F Posted November 18, 2004 Report Share Posted November 18, 2004 I'd still like to explore the ground on the far left of the top picture where it shows a shallow area of blue on top of the yellow/orange. This is either shale on top of sand, or sand on top of rock, depending on how you interpret the colours. I still have a hard time believeing that the pinicle is sand (yellow/orange), but maybe the tide can scour these types of features in harder sand, or could it be a soft rock?? Remember, the formula of 1 minute equals 400ft can only be applied in the E-W direction. For N-S it is a non-linear figure depending on your latitude. Not sure what the average figure for our latitude is. Does anyone have an idea? By the way, great topic Duncan BF Quote Link to comment Share on other sites More sharing options...
Sinbad Posted November 18, 2004 Report Share Posted November 18, 2004 Bob coincidently, its one nautical mile. N50.40 - N50.39 Trouble is by GPS, I work out 1 min of lat. ie. W001.54.000 - W001.53.000 is also 1180metres, which is a bit more than 400ft! - so if I've got this wrong then no wonder I cant find any marks. There's a right answer somewhere Regards Paul Quote Link to comment Share on other sites More sharing options...
Bob F Posted November 19, 2004 Report Share Posted November 19, 2004 Paul, Good man. I think you've put me onto the right road. I screwed up (how unusual). 400ft = 0.1 of a minute (I mis-read the chart). So this is digital, like you have on your GPS, rather than seconds. (e.g. W 01.58.50 to W 01.58.40 would be 400ft) The distance on Duncan's chart is in minutes (') and seconds ("). Difference between the two X's is 29.8" minus 13.8" = 16" (seconds) Convert 16" into digital you get 16/60 = 0.26 of a minute. 400ft x 2.6 = 1,040ft between the X's (or 317m). Which makes the base of the pinacle about 250ft. The rise on the pinacle isn't so steep now. Does that sound more realistic, Duncan? Paul's GPS recons 1min is 1180m. My measurement on a chart using a ruler on a scale says 1 min = 4000ft. 4000ft = 1219m Close enough I suppose. 40m difference. So, perhaps the rule of thumb is 1 minute on the GPS = 4000ft. Or 0.1 of a minute = 400ft (133yds) Come on Shytalk, put me out of my misery. Is this right?? Its not a good idea to be doing this past midnight!! BF Quote Link to comment Share on other sites More sharing options...
Bob F Posted November 19, 2004 Report Share Posted November 19, 2004 So, let's try it in the N-S direction. 1 minute = 1nm = 2,025yds = 6,076ft 0.1 minute (digital) = 200yds = 600ft So, 0.1 = ~200yds in N-S 0.1 = ~130yds in E-W Sound OK????? BF Quote Link to comment Share on other sites More sharing options...
duncan Posted November 19, 2004 Author Report Share Posted November 19, 2004 OK first - I think you will find that the NS is the constant relationship between degrees and minutes with Nautical miles. For EW as you approach the poles each Degree will represent a decreased distance............any chart will give you the relationship at our Lattitude (but I don't have one here at work!) With the increasingly hightech equipement we all seem to be carrying the actual interpretation of the image is rather critical first step to where to fish, how, when and then moving on to where you what to position the boat and finallly where the anchor needs to hit the dirt! This makes fishing and anchoring a wreck seem easy (as it has less steps!). I will look further into the colours for you too Bob. I will let you know more about that area to the left of the origional picture on the evening of the 28th 'cos that's where I plan to fish (weather permitting!) Is there a non secret relatively small feature near Poole Harbour entrance that members would like 'sonar mapped' for more detailed discussion? Lobster Rock for example or the area at the end of Christchurch Ledge (bit big really)? Quote Link to comment Share on other sites More sharing options...
Sinbad Posted November 19, 2004 Report Share Posted November 19, 2004 Bob - well done , at last ! these seem reasonable - a physical check with the handheld GPS paces out approx 2m for every 0.001 of a minute decimal. For my small boat, the position is a bit academic, and as for my fishing technique and boat handling to put me in the right spot also least said the better. However, an underwater view of the ledge end would be fascinating - never been down there myself, so dont know really what it looks like. I've heard of gaps or gullies through the ledge front through which the shoals of bait fish are chased - but never located any to fish. Also personal survey using a diving suit wouldnt help either as a diving mate of mine used to get all excited when he could see his dive buddie only 2 metres away - that was his definition of good visability !!! The admiralty surveys are also so ancient and lack sufficient detail for boat anglers, its a wonder we can find any feature by the chart. So Duncan, Sonar Map - yes please to the ledge !! Paul Quote Link to comment Share on other sites More sharing options...
Adam F Posted November 19, 2004 Report Share Posted November 19, 2004 'Xray' would be great - It has produced some cracking fish for james and I over the last 2 years, but everytime we look around the area all we find is a slight depression of about 5'....I would like to see that. Quote Link to comment Share on other sites More sharing options...
duncan Posted November 19, 2004 Author Report Share Posted November 19, 2004 Adam - Xray is interesting, and the scale of the ground features not disimilar to 'the gully' (not the Ledge one - the other one). Key message being that big features don't always mean fish holding. The specific nature of the ground and the surrounding terrain seem to have a big impact as, for inshore marks must the currents (I suspect xray may be a current thing given it's position......?). Good examples of that are small features in otherwise (large) flat areas. Next time I am over that way I will run some patterns. Quote Link to comment Share on other sites More sharing options...
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