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!
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
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
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
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
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)?
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
'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.
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.