Using Quickdraw in tidal areas - caution needed.

[Larry3215]

In case any of you fish in tidal areas or where water levels change, you need to be careful with Quickdraw. Scanning two areas near each other, when the water level changes between the scans, can create false bottom contours. It can make the bottom look very different from how it actually is.

This is an area in Puget Sound where I was drifting south close to shore and then back north on a path a little further out while the tide was at max flow. This created a scanned area with some over lap. As you can see, the Quickdraw map looks very different from the actual chart. There is no drop off where Quickdraw shows it.

The other issue in tidal areas is that the charts show depths at mean low tide, so if you are scanning at any other tide level, you will also get false drop offs, inaccurate depths, etc where the two charts but up to each other.

[Bass newb]

I wonder if this could be fixed by using GPS position instead of just straight transducer readings.

[Larry3215]

It does use GPS positioning. Other wise it would not know where to put the depth data on the map. The problem is, it cant use tidal data to correct the map. Its just not possible to get 100% accurate tide levels unless you are right on top of a tidal station sensor and even then, the data is delayed. You have no way to access it in real time as far as I know.

On lakes and reservoirs that have water level changes due to dams, irrigation, rain, etc, you can sometimes get fairly up-to date water level data on-line from BLM or other sources. You could use that information to give your Quickdraw maps a better chance of having good bottom contours between scanning sessions.

For example, I boat on Lake Roosevelt sometimes and large parts of that lake are very poorly charted or not charted at all. However, they have a website that lists water level at the dam on a daily basis. If I scan a part of the lake one day, the thing to do would be to take note of the water level at the dam. Lets say it is at 1100 feet. I scan a part of the lake while Im fishing. Then lets say I come back next week. I would want to check the water level at the dam again and note what it is. Lets say its dropped 10 feet to 1090 feet. I would then want to enter that -10 feet water level offset into quickdraw before starting to record for that day. Even that will just be approximate. Lake Roosevelt is many miles long and water levels dont change at the same rate from one end to the other. Its basically really a river with a dam on it.

This will work better on other lakes that are more like real lakes than rivers.

For lakes that dont list the water level, you will want to take note of the water level at the ramp. If it is up or down on your next trip, you will need to enter your best guess for the change in water level and enter that into Quickdraw or your charts will have weird contours where the scans from different days overlap.

[Larry3215]

The bottom line here is that you can NOT trust the area between scans on different days if the water level changes - or even on the same day if there are tides.

If there is an area on your favorite lake you want scanned, and if it has water level changes, you will want to scan the entire area you are interested in at one go. If you do part of it one day and part of it the next, the over lap areas may be very inaccurate if the water level has changed between scanning sessions..

[ShellbackCVA59]

Can't help with the tide changes. But you haven't made nearly enough recordings to develop a map, so your's won't resemble the Garmin chart at all. Just to assume your chart has north at 12:00 o'clock, you will need to make multiple passes north to south or vice versus, working your way offshore to develop the correct contours. Your first drift put the contour lines left to right, then you second pass further to the right allowed Quickdraw to start blending the depth lines.

[LWINCHESTER2]

I've used quickdraw and like it. However there are only 2 times a year that water levels are stable on the lake I fish. Even when it's what I call stable, from day to day the level can change by a couple of feet. The water levels, stated as height above sea level are published hourly. For the next couple of months the water levels will rise by 6" to a foot per day. Is there not a way for me to still use Quickdraw and get accurate results. For example, this morning the level is 1669.49. Tomorrow the level will be 1670.50. In January the level is about 1652. In June 1705. Could I not pick one of these as a reference point and then each time I do Quickdraw enter some sort of offset that would be used to keep everything in sync? If so, which one should be used and how do I do the offset?

[Larry3215]

Can't help with the tide changes. But you haven't made nearly enough recordings to develop a map, so your's won't resemble the Garmin chart at all. Just to assume your chart has north at 12:00 o'clock, you will need to make multiple passes north to south or vice versus, working your way offshore to develop the correct contours. Your first drift put the contour lines left to right, then you second pass further to the right allowed Quickdraw to start blending the depth lines.

Thats true, I didnt do a bunch of scans and the depth lines going the wrong way is another issue for me when you only go one direction. It does look like it takes some overlap inorder to get good depth contours.

However,in the one area where the scans did overlap enough for the program to draw the lines in the correct direction, you can clearly see what looks like a sudden drop off. That drop off does not exist in reality. That entire shore line has a nice steady, smooth change in depth.

Here is another area I scanend on dofferent days with multiple trips through the same area - all at different tide levels. I didnt show this one originally because I dont have a detailed chart to compare it to. However, I know this area very very well. This is the mouth of Gig Harbor which is my home port. I wish I had taken more screen shots on different days, but this "chart" changes every time I go through the passage with Quickdraw on. The actual bottom contour in this area is also very smooth, gradual and even as far as depth changes and yet the Quickdraw map shows a sharpe channel and flats that do not exist in reality.

[Larry3215]

I've used quickdraw and like it. However there are only 2 times a year that water levels are stable on the lake I fish. Even when it's what I call stable, from day to day the level can change by a couple of feet. The water levels, stated as height above sea level are published hourly. For the next couple of months the water levels will rise by 6" to a foot per day. Is there not a way for me to still use Quickdraw and get accurate results. For example, this morning the level is 1669.49. Tomorrow the level will be 1670.50. In January the level is about 1652. In June 1705. Could I not pick one of these as a reference point and then each time I do Quickdraw enter some sort of offset that would be used to keep everything in sync? If so, which one should be used and how do I do the offset?

If you can be confident in the depth change between scans, I think you can still get good contours of your lake.

Quickdraw has two different "offsets". I cant copy and paste from the manual, so it would be best to read what it says in the newest version that covers Quickdraw.

I havent played with these yet, but it sounds to me like "Recording Offset" is the difference in your sonar readings and the actual depth. I am assuming that is the depth under water that your transducer is set. I dont know how else you would set that unless you had a known depth to compare your readings to. For example, if your existing chart said the depth was 20 feet at your exact location, but your sounder was reading 18 feet, then this offset would be 2 feet. Whats unclear to me is if this offset is a one time thing or if it needs to be adjusted for each body of water. The wording in the manual is unclear on that.

The other setting is "Display Offset". This one sounds like the one to use when the water level changes from day to day. The manual is also unclear here about how to do it exactly, If your lake water level drops, do you use a negative number or a positive number?

Hopefully Todd will chime in with some clarification or someone will need to call customer service and ask questions :)

[TroyBoy30]

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There is also an option to change the Display Offset for the Quickdraw Contours, which allows for adjusting the contour depth labels based on the lake level. This is similar to the LakeVu HD Ultra Lake Level feature, but the two are independent of each other. Depth Range Shading is also supported on Quickdraw Contours.

[Larry3215]

Thanks Troy - I knew I had read something about this somewhere. Do you know if you enter positive or negative numbers when the level goes Up/Down?

[LWINCHESTER2]

Thanks Larry. I'd like to get right before spending a lot of time. When Quickdraw first came out I used it for 3-4 hours. All that is down the drain. All the mapping that I would do would have to use the full pool elevation for reference. My guess is that I would enter a plus amount when recording when not full pool. Current level is 1670.65, Full pool 1705.00. So if I was mapping right now I would figure to add 34.35. But the logic could be completely the reverse. It could be how much under full pool. In that case it would be -34.35. There is good reason for that logic as many lakes could be in flood stage and the plus numbers used to represent recording during flood stage.

[Larry3215]

The plus or minus thing could make the difference between wasted time or good maps.

I wonder if you could do a quick test - enter the offset as a positive then do a short mapping run and see if the indicated depths were adjusted in the correct direction?

On the other hand, that may not work either. It depends on how and WHEN they apply the offset. It could be that if you enter an offset from full pool on the first scan, then all your depths will be off by that amount from then on.

Lets take your example. The lake is down by 34.35 feet. If you enter that offset now - and then go out and do a scan - all your maps will show the depth as either 34.35 feet too hi or too low. They will never give you the correct depth as it is right now.

Im still guessing but I think you will need to just go do a scan the first time - but make a note of the depth relative to full pool. Then the next time you go out you will need to enter the offset based on the second scan. Say the water level goes up by 10 feet the second time you show up. You will need to add the 10 feet offset - not the new level relative to full pool.

Maybe. Maybe not.

We really need some better instructions on how to do this or we could be wasting tons of time and making bad maps.

[Larry3215]

There is also the other situation where you may not have any official depth level to give you a reference. In those cases you will just have to make your best guess.

That still leaves the question of when to enter the offset - on the first scan or the second?

[LWINCHESTER2]

Ok. I called Garmin Tech Support. Call answered immediately, by Josh. No waiting. He seemed to understand what I was asking and suggested the following for my situation.
1. Make my calculations for the offset relative to full pool. The internal mapping is relative to full pool. For me that is 1705.
2. Each trip out verify the current lake level. Subtract that from full pool to get the offset. As I write this, the current level is 1671. 1705-1671=34
3. Enter that as a negative number. So if I was using Quickdraw right now I would enter an offset of -34.
4. He, in realtime, verified that the 7610 will accept as big of an offset that my situation requires.

Simple enough. A+ on tech support.

[Larry3215]

Excellent! Thats actually pretty logical - nice to run into that sometimes :)

[LWINCHESTER2]

Ok. I called Garmin Tech Support. Call answered immediately, by Josh. No waiting. He seemed to understand what I was asking and suggested the following for my situation.
1. Make my calculations for the offset relative to full pool. The internal mapping is relative to full pool. For me that is 1705.
2. Each trip out verify the current lake level. Subtract that from full pool to get the offset. As I write this, the current level is 1671. 1705-1671=34
3. Enter that as a negative number. So if I was using Quickdraw right now I would enter an offset of -34.
4. He, in realtime, verified that the 7610 will accept as big of an offset that my situation requires.

Simple enough. A+ on tech support.

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.

[Larry3215]

Interesting. Using Quickdraw in areas where depths change is turning out to be a little complicated.

If you record with the wrong offset initially, can you fix it later? In your case, if you go back again and do more recording with the correct offset, how will that integrate with the recording you already did?

[LWINCHESTER2]

Pretty sure that once you record it wrong you're out of luck. We're not far enough along in the development process to have external utilities for data manipulation. Delete the files and start over.

[Larry3215]

Yeah, I suspect thats why the delay in opening up the data sharing website.

I wonder if - as in your case above - what would happen if you just continued your future recordings with the wrong offset? Could you correct that bad an error just with the display offset setting? Probably better to get it correct from the start though.

Could be some people get several sessions of recordings done only to find out they have to scrap them due to wrong offsets from the get go.

[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.

I verified that the MFD manual had the recording offset stated correctly: