Sonar chart offsets

Skinny RunningSkinny Running Posts: 66 Greenhorn
You can set your transducer offset to correct for the depth the transducer is actually at within the water column. Sailboats or offshore rigs with deep drafts and transducers that may be 3 or 4 feet below the waterline can use that so their unit will read the actual water depth instead of just the depth under the transducer. But it's available for use with any offset amount so you can get the reading on the screen you prefer to see. Some people want to see the actual water depth and some people want to see how deep it is under their keel. It looks like you can use an offset of 6 inches or several feet if you wanted to make the units read deeper or shallower depending on the boat.

If the unit offset is say -1 or +3 is the assumption the transducer is really running 1' or 3' higher or lower than the waterline and there would be an adjustment in the back end processing based on those settings? What if there is no offset used but the unit is really on the transom and 2' below the water when the data was recorded? How would it take a situation like that into account to correct for operators that don't know the proper setting, or just don't use an offset? It seems to get the accurate contours drawn it would have to know for sure exactly where the transducer was in the water column at the time the recording was made. It seems there is no way it could know that unless it assumed the offset settings were made correctly.

I have some friends that run around and details like this are just not something they have likely every thought about. They are just plug and play types. Some don't even know what Waas is let along turn it on. I cant imagine they would set up a transducer offset and think through these matters. Is it safe to assume the processing of the data works fine but only if the offsets are actually used and set up properly and with readings taken at idle? If you want to map a decent size area it could take a pretty long time to idle miles and miles recording it all. Is that what you are supposed to do or can you take readings running faster? If you run faster than idle then the transducer's depth in the water will change. I use no offset because I want to know what the depth is under the boat. In 2' of water at rest it shows 1 foot and on plane it shows 2'. Is there some specific offset setup settings you are supposed to use for this?

If I record at idle the transducer is really 1' below the water. What setting am I supposed to use to get the correct map back? If I record running faster than idle the transducer will be shallower than 1'. And for a person uploading if their offset was inadvertently set to the wrong thing wouldn't that also be a problem in getting the correct contour output? Sorry for all of these questions but if I am going to do this I want to make sure its right so I don't waste my time.

Replies

  • mikenavmikenav Posts: 647 Officer
    Hey Skinny! Sorry it took so long to get to you. This is actually the second offset question I've gotten this week. None of the settings that you plug into the machine will actually affect the sonar logs. Those settings are for your comfort and your safety on board. The sonar logs take into account so much information that comes in independent of your specific settings, and are then run through a crazy long algorithm by Navionics that your settings will not affect how they turn out.

    Don't worry about asking too many questions. Let me know if you have any more.

    Thanks!


    mike
  • Skinny RunningSkinny Running Posts: 66 Greenhorn
    mikenav wrote: »
    Hey Skinny! Sorry it took so long to get to you. This is actually the second offset question I've gotten this week. None of the settings that you plug into the machine will actually affect the sonar logs. Those settings are for your comfort and your safety on board. The sonar logs take into account so much information that comes in independent of your specific settings, and are then run through a crazy long algorithm by Navionics that your settings will not affect how they turn out.

    Don't worry about asking too many questions. Let me know if you have any more.

    Thanks!


    mike

    I appreciate that info Mr Mike. But it doesn't seem possible that that any software could determine exactly where the transducer is in the water column if offsets are not taken into account. If that is not the case, the log would just show the indicated depth to the bottom from the transducer. There is no way the unit could know it's mounted 3' below the water line or at the water's surface or in between. That is what the offset is for. It has to collect the data somehow or this algorithm you mention to process the log has to use an average that someone just programs in that it applies to all the logs no matter the settings. Perhaps there is a different average used for different size boats. But if offsets really are used for this and they are set up improperly it could be trouble. If they use an average in the process and the average is different from what was actually the case when the data was collected the could also be trouble as the output would be off by as much. Since you advise it doesn't matter the offset settings, they have to be imputing an an assumed average for all offsets. What is it exactly? Some people using this are running rigs that draft over 3'. That means their logs could be up to 3' off one way or the other.

    No offense but I'm not quite buying into the just trust us it somehow works answer. Makes no sense. You will have to do much better than that to sell me on trusting this technology for any shallow depth contours it produces. It doesn't sound like your tech folks gave you these details. Maybe they don't want people to know all of it? This is not an issue that can be sidestepped. It is what it is. There is a clear and precise answer to this question. It makes pretty pictures but it seems in some cases it could be up to several feet off due to some programming assumptions. If that is not correct, please let us know exactly how it works. If they are not going to educate with the fine details you they certainly are not going to tell me. It's hard to get them on the phone most of the time. I called up there and got a similar general response and had the feeling they didn't really know or didn't want customers to know. They also gave me the trust me it works answer and didn't want to discuss offsets.

    Seems like a a good tool for reefs and deeper water but probably not so great for people like me that run around proximate to shore in shallow areas. Doesn't matter if its a foot or 3 off out on the reef. But up at Fernandina Beach or Mosquito Lagoon or Crystal River a foot off can ruin your day.
  • mikenavmikenav Posts: 647 Officer
    Hey Skinny! As a Pro Staff guy and someone who is on the water a lot, I totally get where you're coming from. And as someone who has been around the marine industry for a long time, I understand your skepticism when someone tells you to "just trust me", to use your words. All I can tell you (remember, I'm just pro staff), is that Navionics will not release the way their maps are created. They have a very in-depth process which takes into account a wide variety of factors, but going into exactly how it all gets put together would get into proprietary stuff I know nothing about.

    I am looking for a clearer way to explain the process without divulging industry secrets and when I have that I will gladly pass it along. In the meantime -- and this goes with or without Navionics -- if you are in water THAT shallow or where there is THAT much of a worry, take extreme caution and be careful.
  • Skinny RunningSkinny Running Posts: 66 Greenhorn
    My take away from this....
    The output data are subject to inaccuracies of up to several feet depending on the area and location. There are too many variables to factor in that all have to line up perfect to get a perfect result. It would not line up very well at Navionics to be selling 1' contours at the same time caveating the data as being subject to several feet off one way or the other. When you have places with huge tide swings of up to 7' like up near the FL / GA line and other places with 25 miles between tide stations and all of the other factors mentioned that need to line up it is pretty clear that is the reason for the obfuscation on the detail.

    What is interesting is that they obsfuscate the truth about it when its just a matter of time before someone actually videos stick measurements and then shows the output on the map they get back. Eventually, the truth will be documented. All of the mfg. will be in the same boat like this.

    So, I will use the contours to get read on where there is likely changes in bottom elevation but not rely so much on exactly what the depth indications are.
  • mikenavmikenav Posts: 647 Officer
    Your takeaway is what it is. My takeaway is I have scanned places I know intimately and the charts have turned out perfectly. Despite my skepticism. The simple truth is you are going to believe what you believe until proven otherwise. And that is absolutely fine. My suggestion is you scan a place or two you know well and see how it turns out. If it's off, you have your answer. Otherwise, you will then know that this algorithm Navionics uses works.

    I have nothing to sell. I make no commission on anything. I'm here to answer questions and help people as best I can. I understand your doubt, but the best thing I can tell you is to give it a try and see what happens.
  • Knot LeftKnot Left Posts: 72 Greenhorn
    mikenav wrote: »
    Your takeaway is what it is. My takeaway is I have scanned places I know intimately and the charts have turned out perfectly. Despite my skepticism. The simple truth is you are going to believe what you believe until proven otherwise. And that is absolutely fine. My suggestion is you scan a place or two you know well and see how it turns out. If it's off, you have your answer. Otherwise, you will then know that this algorithm Navionics uses works.

    I have nothing to sell. I make no commission on anything. I'm here to answer questions and help people as best I can. I understand your doubt, but the best thing I can tell you is to give it a try and see what happens.

    If its a foot or two off in 15' or 20' of water no one is going to notice and they wont likely care either. Most are just seeking changes in bottom but are not concerned much of exactly how deep it is. For boaters who run in shallow water it makes a difference. They are advertising 1' contours but they are not necessarily accurate to 1' in terms of the depth indication but Navionics seems to infer they are. Far too many variables in play for the output to consistently draw contours exactly to the correct foot. The OP is correct. Two different boats recording in 2' of water in two different areas with one running its transducer at a foot below the surface and the other running at the water's surface are not showing the same result and if one place is in between tide stations that are separated by say 10 miles, it's even subject to more variance. Collecting this data with all of these different variables in play and advertising perfect output results is stretching credibility. It's pretty obvious, they can't be honest about it. They won't even give you the real answer. There is a reason for that and it has nothing to do with proprietary secrets. Remember, these are the same people that are of the opinion that the grey gloomy and fuzzy pictures of Homosassa and so many other areas are "hi res". If they would just caveat the output accordingly and state the average accuracy or the standard deviation of the variances it would be a refreshing change. There is nothing accurate exactly to the foot on a Navionics map. They may accurately show 1' changes in the bottom with contours but those contours are not going to show consistently accurate depth indications. That is an important distinction when you are running around in shallow water. That being said, the tool is very insightful and and useful for showing elevation changes in deeper water. Its pretty obvious from looking at the demo videos that is where it is being used. You just wouldn't want to count the depth indications on it running around close to shore at Steinahatchee.
  • SeaBoss180ccSeaBoss180cc Posts: 89 Greenhorn
    Seems to me that people who frequent the real skinny waters are real familiar with areas they fish in and hardly ever even look at the GPS. My buddy Kyle (ShallowSightings) knows the Chassowitzka backwaters like the back of his hand. He didn't get to know it by looking at his GPS. Nothing replaces experience... GPS is a guide to make it back to the dock in the fog, to get you over the reef...
  • Skinny RunningSkinny Running Posts: 66 Greenhorn
    Seems to me that people who frequent the real skinny waters are real familiar with areas they fish in and hardly ever even look at the GPS. My buddy Kyle (ShallowSightings) knows the Chassowitzka backwaters like the back of his hand. He didn't get to know it by looking at his GPS. Nothing replaces experience... GPS is a guide to make it back to the dock in the fog, to get you over the reef...

    Navigating in the shallows by boaters relatively unfamiliar to an area and even Chassowitzka can be done successfully on a GPS if the GPS has well documented chart features to follow. Plenty of people have their own waypoints and routes etc. and can run them in the dark in some crazy places. I don't think you can do it successfully solely on a stock Navionics chart. I agree it is a general guide. A bit of irony I suppose given the name. But a GPS can be well more than just a general navigation guide for any area. In theory, if they really could get these contour depth indications consistently accurate in less than 3' of water, it would be very helpful in that regard but it appears its just not quite there yet for the reasons previously mentioned. They won't admit it but I think that is the current reality of it. Once transducers can auto sense how much water is below them and above them then this uploaded information will be far more reliable and they won't have to use an assumption for where it is in the water when the reading was recorded. Just a matter of time I would say based on how the technology has advanced just in the last couple of years.

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