where is it on a down scan

[mfo]

My question is this, when you are using a down scan model of either brand  and you go over a stump and you go to mark a w/p  in lets say in  20ft. of water  where the bottom coverage is at least as wide as your depth if not more how do you know where that stump is in your cone? in 2d sonar  a return shows up darker or less bright if it is off to the side but in downscan how can you really tell if it is 8ft. to the left of the boat or 12 ft to the right of the boat? This is assuming that your gps antenna is right over your ducer even.



thnx
mark

[coldfront]

it's my understanding that this have been factored in...and the difficult math has been programmed in so that when you 'waypoint it'...that's actually 'where it is'...

subject of course to position error variance.

(Fishton made a great point in a different thread:  encouraging anglers to put the EPE-estimated position error- up on the screen like 'depth' or 'temp'...so that you can see how 'good' the epe is before punching in the waypoint...

EPE will bounce about...and it's something that should be able to be managed for best effect to the angler...


Did I get this right Fishton?

[mfo]

hmmm sounds like voodoo to me

mark

[coldfront]

hmmm sounds like voodoo to me

mark

voodoo navionics...

[DougV]

My question is this, when you are using a down scan model of either brand  and you go over a stump and you go to mark a w/p  in lets say in  20ft. of water  where the bottom coverage is at least as wide as your depth if not more how do you know where that stump is in your cone? in 2d sonar  a return shows up darker or less bright if it is off to the side but in downscan how can you really tell if it is 8ft. to the left of the boat or 12 ft to the right of the boat? This is assuming that your gps antenna is right over your ducer even.



thnx
mark

This is one of the problems with Down Imaging you CAN'T Tell where in the coverage the structure or fish are for the "ping" or cell of data displayed. It's could be anywhere in the coverage area of DI. Now Humminbird has three different areas of coverage Narrow about 1/3 of depth. Medium about equal to depth and Wide about 1.5 X the depth area of coverage. So waypointing something  in Narrow will be more accurate than in wide mode do to the coverage area it is looking at. This is why for 2D the QuadraBeam is a great transducer upgrade because it show which side just like Side IMaging compliments DI telling you which side.

These images will show you one of the huge benefits of SI/DI combination because of SI showing which side.

Where are the trees located Left Side or Right Side?




This image has the Blocks marked in color from where they are located in the SI image. I



Here's Example of QuadraBeam Sonar doing the same thing tell you where in the coverage of traditional 2D sonar:

[mfo]

Doug  if I understand you right  , are you saying the bird has selectable beam widths on the DI models? Or are you saying you are SOL unless you buy SI or structure scan?  At the very least you are looking to spend 1 large  which I and alot of other folks don't have in this crappy economy ?




mark

[DougV]

The DI Only - actually with Humminbird it's DI and 2D you have different frequencies and area of coverages you the user can select.

455 kHz / 16° @ -10db - Narrow
800 kHz / 45° @ -10db - Medium
455 kHz / 75° @ -10db - Wide

Plus you still get 2D Coverage: 200 kHz / 25° @ -10db

This is for the 570DI, 596ci HD DI, 597ci HD DI, 788ci HD DI

[Fishton]

High Frequency Sonar - (455/800kHz)

Lowrance LSS-1 (StructureScan) - has 3 pairs of piezo's - left, right and down looking
Lowrance DSI (Down Scan Imaging) - has a single pair of piezo's - down looking

Humminbird 797, 798, 898, 997, 998, 1197, 1198 has 2 pairs of piezo's - left and right looking    Note - current DI is made up using the very outer limits of the left and right beam superimposed to create a 'DI'.
Humminbird DI (Down Imaging) - has a single pair of piezo's - down looking.

[mfo]

Thnx for the replies fellas  so if I am understanding this right  The bird has 2 455frequencies on the d/s models  to select from ? One which is 16 degree beam? if it did that would go along way to answering the initial post which was" how can you be reasonably sure where  the object in question is actually located on the d/s models of both brand"



mark

[DougV]

You have two area of coverages with the 455kHz frequency one is 16 degrees the other is 75 degrees so the 16 degrees will give you very narrow coverage if it shows in the 16 degrees it's very narrow and you can pinpoint much easier.

[mfo]

thnx doug





mark

[Fishton]

If it did that would go along way to answering the initial post which was" how can you be reasonably sure where  the object in question is actually located on the d/s models of both brand"

This will help -

Sonar / Accuracy
1. Relationship between frequency properties and depth –
The deeper you go, the more difficult it is to get an accurate waypoint .  This is because a sound wave is not as focused as a laser for example.  The wide coverage area of the 83 and 50kHz cone shaped sound waves are great for finding fish, but not good for taking a waypoint of structure or cover.  The 200kHz frequency soundwave is much better for taking waypoints from, as it has a much smaller coverage area, thus improving your accuracy.   With the introduction of high frequency Side Scan, accuracy has been greatly improved.  This is because when you pass over an object of interest, and you see the object perfectly mirrored in the water column, you know that you are directly above the object and it is safe to take a waypoint from the 200kHz screen, or the 800kHz DownScan.  If the object appears slightly stronger either on the left or right, do NOT take a waypoint, but rather go back targeting either left or right whichever had the strongest return.

If you are using a Humminbird remember to set your DI mode to 'Narrow' and not 'Wide', as 'Wide' is merely an overlay of the left and right beams.  The HB DI units coming out later this year will have high frequency transducers facing down, thus offering the user True Down Imaging.

[mfo]

ya fish I am talking about the bird  dedicated d/s models.




mark

[Fishton]

yo mofo, hb di bro, no d/s yo.

[coldfront]

You have two area of coverages with the 455kHz frequency one is 16 degrees the other is 75 degrees so the 16 degrees will give you very narrow coverage if it shows in the 16 degrees it's very narrow and you can pinpoint much easier.

Doug,
My understanding/thoughts are that the downimaging is great in that it really depicts the objects below the transducer much, much more realistically...and in that I wouldn't be using a 'traditional' sonar image to locate with pinpoint accuracy the gPS coordinates...I won't be doing this withi the down imaging either.

On the other hand, the side imaging...this is where you can locate accurately stuff off to the side without actually having to go over them...

here's the question:  in fact, can you 'waypoint' a side image location with higher accuracy than possible with a down image?  (is the opportunity for position error greater/lesser with Side Imaging vs Down Imaging)?


From a practical use standpoint...the largest value of the technology to ME remains the side imaging...  down imaging is very helpful as well... and I'm sure it'll grow on me...but my main focus continues to be to 'FIND' spots that previously would have taken a lot more time/effort...


Thoughts?

[DougV]

DI and SI coverage area front to rear distance is very narrow so this will help reduce adding any additional offset to the waypoint compared to 2D with a cone shaped coverage area. THe thing about SI is every Ping of the sonar it records GPS Location X,Y,Z Axis. Since SI is very harrow and the only distance you are measuring is straight line to the Point of Interest the Algoryhtms Humminbird Uses are very Accurate. I can tell from my experiences that I can mark the exact location in SI and then use DI or SI and Navigate to that spot and be dead on the exact same piece of structure.

[mfo]

Either Fish is loosin it or that above post is some kind of code!




mark

[screwballl]

Here is a little math:

Look at the degree angle of the specific frequency.

First, width of the beams are 0.07 feet per 4 degrees.
Second, the high frequency is the most accurate but narrowest beam.

So, the 16º is VERY narrow, take the depth multiplied by 0.28. At 10 foot depth, the 16º beam is 2.8 feet across. At 20 feet deep it is 5.6 ft wide. So a stump at 20 feet may be anywhere within that 5 foot circle, but considering most boats are in the 5-6 foot width, any lure dropped in the area should be close enough for the fish to see/feel and bite if they are hungry enough. This is typically why in clear water, it is better to cast out 20-30 feet and let the lure swing down into position, so the fish doesn't see it dropping straight down above them and possibly scare them away.

Now if you add in the side scan capability of some fishfinders, you can get a bit better picture as to whether the stump is left or right of your current position.

Next, relating to GPS, even the most accurate GPS signal (with WAAS) is still only accurate to about 10 feet radius at best. Without WAAS, it is in the 35-45 foot radius (less accurate).
So you can set the GPS waypoint, and the next 5 times you come back, you may be anywhere within the 10 foot circle.

This is why it helps to use a combination of the GPS to get you close, and the sonar to get you positioned above or near the stump. Most of the time as long as you are within 10 feet it should be easy enough to cast around.