Author Topic: 798 quality scan...  (Read 5444 times)

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Offline xSilmarilSx

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798 quality scan...
« on: May 06, 2012, 12:26:10 PM »
I just got my first day on the water with a 798HD.

Now, I have questions for the seasoned guys here..
From what I have deduced, quality wise, you have a couple of choices to make a good recording.
You have to choose a speed ans chart speed that match closely. This is the easy part.
But next you must choose the width you want to scan and the depth you will be scanning.

The big problem is that the width and depth will change the unit ping rate and the speed the
screen update.
So, for example, with a speed of 5 mph and a chart speed of 5 (which people here seem to agree)
will produce different results in 5 ft or 50 ft of depth, and 50-200ft of width.

So, the deeper you get, and the farther you want to scan, the slower the boat need to be to
produce the maximum number of pings par distance travelled.

I checked quickly, and to achieve a ping of around 100ms ( 10x a second) you must scan at no
farther than 75 feet each side, in max 50ft of depth.

With a little math here, if we choose the above setting of 10x pings par second and want to resolve
a image with a resolution of 1 in par ping, we must travel a max 10" per second. (10 pings of 1" each)

10 in/s is 0.56 mph

At 5 mph scan speed, you can't resolve detail smaller that 10" in scan resolution.
Chart speed have no effect on ping speed as I can see.

I will check next on the beam width to see what are the resolution the 455 kHz beam can theorically
achieve.

What are your thoughts on this one?
Any tips on resolving the max detail on side imaging?



Offline xSilmarilSx

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Re: 798 quality scan...
« Reply #1 on: May 06, 2012, 12:41:32 PM »
Ok, so with a little research, I found that the 455 kHz SI transducer should obtain a beam width around
1.7.

With a little math, we can obtain that on a 75ft wide scan (each side, 150ft total), the maximum resolution the beam can achieve, is around 2-1/4"

So at 2.25" per pings, with a speed of 10 pings per second. For maximum image quality we need a speed of 22.5" per second.

Which translate to 1.3 mph.

Beam resolution:

50 ft = 1.5"
75 ft = 2.25"
100 ft = 3"
150 ft = 4.5"
200 ft = 6"


I will investigate the ping rate of each side range to check the best speed for each scan width.

Offline xSilmarilSx

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Re: 798 quality scan...
« Reply #2 on: May 06, 2012, 07:46:34 PM »
I have found the data on the ping rate of the unit.

50ft scan = 85ms avg
75ft scan = 105ms avg
100ft = 125ms avg
150ft = 165ms avg
200ft = 206ms avg

With the remaining data posted above, the best theoretical speed for optimum image quality is:

50ft = 1.0mph
75ft = 1.27mph
100ft = 1.36mph
150ft = 1.53mph
200ft = 1.7mph

With a speed between 1 and 2 mph will produce maximum image quality for almost every settings.

Offline newkid4si

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Re: 798 quality scan...
« Reply #3 on: May 06, 2012, 09:32:37 PM »
Looks like time to call in the BIG guns.  Rickard , please report for duty. LOL

Offline ITGEEK

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Re: 798 quality scan...
« Reply #4 on: May 07, 2012, 05:19:36 AM »
xSilmarilSx:
This is big time stuff here.

Like newkid4si said:You definitely need the brainiacs to verify your findings.
And next to you, Rickard is the next biggest other brainiac I know. :)

Offline newkid4si

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Re: 798 quality scan...
« Reply #5 on: May 07, 2012, 07:54:36 PM »
 xSilmarilSx
     I appreciate your effort. I spent two hours today recording at 2-3 mph. Seems like it takes forever to cover much area.
     Haven't looked at the SD card in Humviewer yet, but I'm not expecting much as I think my transducer is failing.
     New transducer will be here tomorrow. I am experiencing the dreaded "intermittent problem".

     Normally scan at 4-5 mph until I see something interesting, then rescan target from several different angles at slow speed.
         
                Mike

Offline xSilmarilSx

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Re: 798 quality scan...
« Reply #6 on: May 11, 2012, 01:37:57 PM »
Since we have no replies here, I will add some more information that I have found analyzing the HB data.

Everyone can agree that HB use the 200 kHz signal to derive the depth.
And with the depth data, HB adjust the ping rate of the 2D AND SI beams.

So, to collect the maximum data per unit of length, you need 2 things: fast Ping rate and slow speed.

With the above calculation we can find the best speed for the best imaging.
Now, with my researches, I discovered that the Ping rate and depth calculation form HB can greatly
improved with a little setting in the unit.

From a decision from HB engineers, the ping rate is set to 4 to 7 times the depth found by the unit.
This process enable the unit to have data well below the real bottom and provide a clean image in the
case the screen changes ranges. If you check a Lowrance/Garmin video, when the unit change depth, you will seen a blank screen in a section when the unit changes range.

The problem with the HB setting, is that the ping rate is greatly reduce because the unit must wait to receive data from 7 times the real bottom.

Here are the data I collected.

depth = ping depth
10ft = 75ft
20ft = 75ft
30ft = 90ft
40ft = 140ft
70ft = 230ft
100ft = 380ft

So if you are standing in 100ft of water, the unit slow down to wait for signal coming back from 380ft of water.


Now, the simple way of bypassing this "semi-problem" is to set the MAX DEPTH RANGE in the unit to a range just over the depth of the lake you plan of fishing/mapping.

So in a 100 ft lake, just set the MAX range to 130-150ft.
You will see a boost in speed from the Live view, screen refresh, Si refresh just by doing this simple change in your settings.

You can even try this in simulation mode!

Offline ITGEEK

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Re: 798 quality scan...
« Reply #7 on: May 11, 2012, 03:25:07 PM »
This is great info.
Thanks.

Offline SonarTRX

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Re: 798 quality scan...
« Reply #8 on: May 11, 2012, 03:36:02 PM »
On a side-note: It is also beneficial to select the smallest possible SI Range for your needs.

Maintaining a minimum SI Range results in:
- Smaller size recordings & memory requirements
- Faster to import, view & process with the various imaging / viewer applications

If you are surveying parallel to shore (of a river, coastline or marina), set the range just big enough to overlap with the shoreline.
If you are making multiple parallel passes to create a map, set the SI Range to provide the minimum "safe" overlap between each pass.

Tore

Offline newkid4si

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Re: 798 quality scan...
« Reply #9 on: May 11, 2012, 08:51:13 PM »
This post is another example of why this site is an outstanding source of HB information.
The fact that members have the ability and willingness to post technical knowledge on this level helps us all.
Thank you all for helping us learn to operate our units better.

                 Mike

Offline xSilmarilSx

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Re: 798 quality scan...
« Reply #10 on: May 11, 2012, 10:25:31 PM »
To have a smaller range in the SI recording will produce a smaller file, but not necessary. Since the shorter the range, the faster the unit pings and collect data..

It all came down to how much ping you can squeeze in 1 sec. This will determine the file size.

But the Ping speed is calculated with the SI range, 2D depth, and the MAX depth setting.
Each of these things will change the ping speed of HB units and the file size per hours of recording.

Offline SonarTRX

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Re: 798 quality scan...
« Reply #11 on: May 11, 2012, 10:40:36 PM »
Good point, but at least the sonar will not be "waiting for echo returns" that you are not interested in.

The higher ping-rate also results in higher resolution in the direction of movement
(, unless you instead prefer to speed up to shorten the duration of the survey.)

Tore.

Offline xSilmarilSx

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Re: 798 quality scan...
« Reply #12 on: May 12, 2012, 07:37:45 AM »
Exactly!


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