Using Quickdraw in tidal areas - caution needed.

[Larry3215]

Now Im confused - again :)

When do you use which offset?

"Display Offset" says it is to be used to compensate for depth changes or errors in recorded maps. Are Recorded Maps the maps pre-loaded on the MFD or just quickdraw maps?

Reading through the description for both offsets again makes it sound like lwinchester was using the wrong offset when he started his recording?

Should he have used ANY offset when he did the initial recording? What should he do/which offset should he change on subsequent recordings if the water depth has changed?

When would you use the Recording Offset? That one sounds to me like it is intended for adjusting the difference between what the sounder reads and the actual depth under the boat - at that moment. In other words, a correction for your transducer being mounted well under the water on a low point on the hull or just giving the wrong depth reading. On my boat the 'ducer is almost 1/2 meter under the water when we are going slow. Some boats would be off more than that depending on how the 'ducers are mounted.

Part of my confusion comes from not knowing the exact definitions of the terms used in the Manual.

What exactly is "sonar depth"? Is that the true depth of the water measured from the surface OR the actual depth of the water measured from the sonar transducer to the bottom OR the depth the transducer is mounted below the water surface (this is what it sounds like) OR?

Also what is "contour recording depth"? Is that just the depth that is shown on the contour that ends up getting recorded?

Help?

[Todd Driscoll]

Well I don't know about this. Today I decided to so some mapping with Quickdraw. The area I was in iwas about 100 ft in the deepest part of the channel. . I checked with TVA and we are 21 ft from full pool. So I entered a recording offset of -21. As I watched the mapping the depths recorded were around 80 ft. There is no way to make this work. The recording offset has to be +21. Then it will add 21 to the depth measured, making the depth correct at full pool. Then when using the created map the Display Offset will be right for the rest of the year by entering a negative number for what amount we are at less than full pool.

Now Im confused - again :)

When do you use which offset?

"Display Offset" says it is to be used to compensate for depth changes or errors in recorded maps. Are Recorded Maps the maps pre-loaded on the MFD or just quickdraw maps?

Reading through the description for both offsets again makes it sound like lwinchester was using the wrong offset when he started his recording?

Should he have used ANY offset when he did the initial recording? What should he do/which offset should he change on subsequent recordings if the water depth has changed?

When would you use the Recording Offset? That one sounds to me like it is intended for adjusting the difference between what the sounder reads and the actual depth under the boat - at that moment. In other words, a correction for your transducer being mounted well under the water on a low point on the hull or just giving the wrong depth reading. On my boat the 'ducer is almost 1/2 meter under the water when we are going slow. Some boats would be off more than that depending on how the 'ducers are mounted.

Part of my confusion comes from not knowing the exact definitions of the terms used in the Manual.

What exactly is "sonar depth"? Is that the true depth of the water measured from the surface OR the actual depth of the water measured from the sonar transducer to the bottom OR the depth the transducer is mounted below the water surface (this is what it sounds like) OR?

Also what is "contour recording depth"? Is that just the depth that is shown on the contour that ends up getting recorded?

Winchester has it right in his comments I quoted. As he pointed out, he just had his recording offset set in the wrong direction.

[Larry3215]

Im still confused.....

[LWINCHESTER2]

I'll try to help. Forget, for the time being about Display Offset. Only consider Recording Offset. So we are going to make a contour map of an area using Quickdraw.
Is the lake at full pool? If it is, then just go ahead. No adjustments necessary. The actual depth measured at the transducer is the depth recorded for mapping purposes. Maybe the lake is not at full pool. Like in my case the lake was 21 ft below full pool. The recorded depth needs to be the full pool depth so 21 ft needs to be added to the measured depth. On another day the lake may rise so that it is only 10 ft below full pool. So that day add 10 ft as the Recording Offset, again providing recorded data for full pool depths. That puts all your recorded data with the same reference point. The reference point is full pool. According to the help desk Garmin's maps are referenced at full pool. So that makes everything match up.

Now consider the display offset and assume it's a year later and on that day the lake is 15 ft below full pool. You are not recording. You are fishing and you want to see the maps display accurately. Garmin's maps and your maps recorded with Quickdraw are all reference to full pool. So the display offset has to be -15. That's negative 15. The unit will subtract 15 from the Garmin and Quickdraw recorded full pool data depths for display purposes. So now it's all in sync.

Last case. Today the lake is 17 ft below full pool. You want both to record some Quickdraw maps, adjusting for being below full pool and display the depth accurately at the same time. The Record Offset would be plus 17 to make the recording correct. A Display Offset of minus 17 would then be required to compensate.

Now, that should be clear as mud.

[Larry3215]

That helped a LOT.

Im still wondering why they didnt say anything in the manual about how to use Recording Offset relative to lake levels when recording? If you dont do it correctly, your maps are toast and you have wasted a ton of time.

The way the manual is worded, it sounds to me like Recording Offset is to correct for transducer mounting height relative to the water surface. My transducer for example is almost exactly .5 meters below the water line. On larger boats they could be much deeper.

When they say "sets the distance between the sonar depth and the contour recording depth" - how do you get to lakes at full pool?

And what exactly is "sonar depth" is it the depth of the sonar transducer or the water depth? If its the water depth - why not say so?

[Larry3215]

never mind

[LWINCHESTER2]

Todd,

Am I about to get access to a Garmin map of Fontana Lake, NC? I'm homebound right now recovering from a knee replacement, but I got a call from a guy on the lake that said he saw a boat with markings "Garmin Survey Mapping" on it.

[Todd Driscoll]

Todd,

Am I about to get access to a Garmin map of Fontana Lake, NC? I'm homebound right now recovering from a knee replacement, but I got a call from a guy on the lake that said he saw a boat with markings "Garmin Survey Mapping" on it.

Yes, MaxDef mapping of Fontana Lake will be available with the 2017 map version.