Author Topic: determine horizontal distance from boat to waypoint  (Read 12291 times)

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Offline bassrat

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determine horizontal distance from boat to waypoint
« on: July 22, 2010, 02:14:36 PM »
Greg Walters from HB suggested that I post my question on this forum.  I am trying to determine the horizontal distance from the boat to a waypoint when the units only indicate a linear distance to the object.  I fish in CT and this time of year you need to be accurate when you are fishing a stone wall or old model A on the bottom of Lake Candlewood.

 I also learned that although you may think that you have marked the exact center of the Model A (for example) that it may or may not be accurate due to variations in lake terrain and that you should also mark the "car top" with the 2D sonar due to the narrower cone angle.

I understand that there are members on this forum that use SI units to conduct measurements of objects.  If anyone would be willing to disclose the mystery math I sure would apprec iate it.   


Offline RGecy

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Re: determine horizontal distance from boat to waypoint
« Reply #1 on: July 22, 2010, 04:32:41 PM »
bassrat,

Here is a link to start with.  http://forums.sideimagingsoft.com/index.php?topic=715.0

You may also want to do a search on the forum for slant range and see what other topics come up.  I am at work right now, but will be able to post some more info later tonight.

I think the only error you will find will be when the target is marked.  The depth of the water and the distance from the boat can affect the accuracy of the waypoint.  The deeper the depth, the less accurate. 

As for when you position the cursor over the waypoint, the distance from the boat to the waypoint should be fairly accurate.  The only variable would be the position error of the gps when the waypoint was marked and the position error of the current gps position.

Hope that helps,

Robert
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Offline bassrat

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Re: determine horizontal distance from boat to waypoint
« Reply #2 on: July 22, 2010, 10:03:27 PM »
Robert,

I want to thank you for your reply.  The link to your previous discussion and calculations/diagram was extremely helpful.  Being new to the SI technology, I had incorrectly believed that the HB units displayed  an actual horizontal distance to a waypoint.  However, with the formula it will be easy to make a few calculations for various water depths and slant ranges to come up with a table that would provide a quick reference to locating accurate (at least within a few feet) horizontal waypoints while fishing.  I must admit that having been out of school for a number of years (ok, a lot of years) it did take a bit for me to get my mind around the math.  Again, I want to thank you for all of your help.  :)

Offline RGecy

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Re: determine horizontal distance from boat to waypoint
« Reply #3 on: July 23, 2010, 12:25:00 AM »
Ok, here are a series of snapshots showing how the unit uses the slant range (not the horizontal range) to calculate the GPS coordinates of an object.

Starting at S00099 thru S00102 I marked 4 different waypoints/snapshots. 

The first is S00099 and its on the center line of the boat.  The next is S00100 and its just off the center line, but in the water column.  Even though the cursor was 14.1ft off the center line, the waypoints are marked in the same position.  So we can see that the unit is using the Pythagorean theorem to calculate the distance to the boat and therefore making it more accurate.  As long as the bottom is not sloping significantly, the waypoint should be right on top of the structure. 

ila_rendered
ila_rendered

Then I marked the target S00101 and went back and paused the playback so the boat was near the target and Waypoint S00101.  When I move the cursor from the boat over to the target, the unit is calculating the distance as 36' from the boat.  This is of course is the distance from the GPS receiver on the boat to GPS coordinates of the target.  This number should be fairly accurate

ila_rendered
ila_rendered
ila_rendered
« Last Edit: July 23, 2010, 12:38:17 AM by RGecy »
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Offline felvic

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Re: determine horizontal distance from boat to waypoint
« Reply #4 on: March 13, 2011, 07:35:36 AM »
Ok, here are a series of snapshots showing how the unit uses the slant range (not the horizontal range) to calculate the GPS coordinates of an object.

Starting at S00099 thru S00102 I marked 4 different waypoints/snapshots. 

The first is S00099 and its on the center line of the boat.  The next is S00100 and its just off the center line, but in the water column.  Even though the cursor was 14.1ft off the center line, the waypoints are marked in the same position.  So we can see that the unit is using the Pythagorean theorem to calculate the distance to the boat and therefore making it more accurate.  As long as the bottom is not sloping significantly, the waypoint should be right on top of the structure. 

(Attachment Link)
(Attachment Link)

Then I marked the target S00101 and went back and paused the playback so the boat was near the target and Waypoint S00101.  When I move the cursor from the boat over to the target, the unit is calculating the distance as 36' from the boat.  This is of course is the distance from the GPS receiver on the boat to GPS coordinates of the target.  This number should be fairly accurate

(Attachment Link)
(Attachment Link)
(Attachment Link)

Your waypoint is accurate because it is not deep.

Try at depth 100 ft, it is completely fake (basic trigonometric) ....
François - Montpellier - France

Offline RGecy

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Re: determine horizontal distance from boat to waypoint
« Reply #5 on: March 13, 2011, 10:25:01 PM »
Felvic,

I disagree!  If the object is on the bottom, the waypoint will be accurate.  The "Trigonometry" proves it. 

I have personally marked waypoints in 100' of water and literally dropped the anchor right on it.   I dive on targets all the time from 30' to 100' deep and will tell you they are accurate.  Many times I have dropped the anchor right on top of a target.  A few times was scared to drop it so close for fear it would damage what we were looking for.  Usually sunken boats!

I have my GPS reciever mounted directly above the transducer and this eliminates any error from the setup.  The only error would be in the GPS signal itself.

Robert
« Last Edit: March 13, 2011, 10:26:30 PM by RGecy »
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Offline felvic

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Re: determine horizontal distance from boat to waypoint
« Reply #6 on: March 14, 2011, 04:37:09 AM »
Felvic,

I disagree!  If the object is on the bottom, the waypoint will be accurate.  The "Trigonometry" proves it. 

I have personally marked waypoints in 100' of water and literally dropped the anchor right on it.   I dive on targets all the time from 30' to 100' deep and will tell you they are accurate.  Many times I have dropped the anchor right on top of a target.  A few times was scared to drop it so close for fear it would damage what we were looking for.  Usually sunken boats!

Robert You say yourself : "Then I marked the target S00101 and went back and paused the playback so the boat was near the target and Waypoint S00101.  When I move the cursor from the boat over to the target, the unit is calculating the distance as 36' from the boat.  This is of course is the distance from the GPS receiver on the boat to GPS coordinates of the target.  This number should be fairly accurate

The distance on the unit is not from the GPS on the boat to the GPS coordonates of the target (top of water) but from the GPS on the boat to the target itself.
The depth is 11,5', distance of the target is 36'. This is the distance  from the sonar to the target.
Using trigonometric correction, the target is really at 35' which is pretty accurate.

Now take the same object at depth 100'.
The distance from the sonar to the target will be 107' but the object is still at 35' which is not accurate at all.

I'm surprised from you to say that, you said the contrary at the beginning of the post : http://forums.sideimagingsoft.com/index.php?topic=715.msg3259#msg3259
 ;)
« Last Edit: March 14, 2011, 04:38:40 AM by felvic »
François - Montpellier - France

Offline RGecy

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Re: determine horizontal distance from boat to waypoint
« Reply #7 on: March 15, 2011, 04:49:56 AM »
Felvic,

I think you are confusing the "VISUAL" Range displayed on the screen, which is actually the Slant Range, to the Calculated Horizontal Range the unit uses to calculate the GPS position.

No the range displayed is not an accurate representation of the Horizontal Range to target, especially in deeper water, but when the unit calculates the GPS position, it uses the Slant Range and Depth to calculate the actual Horizontal range which will make the waypoint very accurate.

Here is a post I did some time back that talks about this. 

http://forums.sideimagingsoft.com/index.php?topic=946.0

Since this has been brought back up and Humminbird has recently added the SI Contour View, which corrects the Slant Range Distortion, maybe its time to do an article specifically on this subject.

Robert

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Offline felvic

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Re: determine horizontal distance from boat to waypoint
« Reply #8 on: March 15, 2011, 05:40:25 AM »
Robert,

I apologize, DeepView made me wrong.

Hummviewer use the same correctio than the units, the waypoints are accurate.

But Deepview don't do the correction.

I find the problem when using "slant range correction" in DrDepth ...

François
« Last Edit: March 15, 2011, 06:35:44 AM by felvic »
François - Montpellier - France

Offline bobn

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Re: determine horizontal distance from boat to waypoint
« Reply #9 on: March 15, 2011, 08:10:50 PM »
Robert,
             Have the hb sidescan units allways used slant range correction to determine the latitude and longitude of waypoints established on the sidescan view?

Offline RGecy

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Re: determine horizontal distance from boat to waypoint
« Reply #10 on: March 15, 2011, 08:33:38 PM »
To my knowledge yes.  There was some discussion on this early on back in 2005, but I cant say for sure that it was any different.  If it was, it was quickly fixed in 2005 with one of the first updates to the 987 and 981.  All units after that I am sure had it from the begining.

Robert
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Offline felvic

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Re: determine horizontal distance from boat to waypoint
« Reply #11 on: March 16, 2011, 12:03:12 PM »
I think this is accurate on a flat bottom but not on a slope.

Let us take an object  32 m of distance 10 m of depth on flat bottom:



The distance unit calculates grab the waypoint at 30 m.

Now  same object same distance on a hard slope:



Distance is the same, but the depth has changed to 30m. The waypoint is then calculated by the sounder in 11 m, that is an error of 19 m...

True?
François - Montpellier - France

Offline bobn

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Re: determine horizontal distance from boat to waypoint
« Reply #12 on: March 16, 2011, 02:38:04 PM »
 The situation you now describe is called layover , it is a problem for sidescan sonar in general. The pythagorean therom is based on a right triangle , which is no longer present , so the slant range correction error increases with slope .  In commercial  sidescan operations coregistered bathymetry determines the actual depth of a target and what the slope is. But  recreational units don't have this info and make corrections based on a flat seafloor.  Therefore waypoint accuracy decreases , most users  have waypoints on fairly flat bottoms and their waypoints are very accurate. 
  For portions of the seafloor sloping up from your transducer a target will appear closer than it really is.  If the bottom slopes away or down  targets appear further away than they really are. Waypoints marking these targets are not accurate. 

Offline felvic

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Re: determine horizontal distance from boat to waypoint
« Reply #13 on: March 16, 2011, 03:24:30 PM »
Thank you Bob, you seems to know very well the subject.

Can you explain me why, on this picture, we can see the arches of the bridge:



The boat is straight at the vertical of the bridge, depht is 38m.

Thanks
François - Montpellier - France

Offline RGecy

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Re: determine horizontal distance from boat to waypoint
« Reply #14 on: March 16, 2011, 10:09:10 PM »
The bridge is not directly under the boat.  You can tell because there is a shadow of the bridge on the left and not on the right.  The bridge outline and the arches/piers are visible in the shadow.

The sonar overlaps slightly under the boat and you will see some of the bridge on the right as well.

Robert
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Offline felvic

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Re: determine horizontal distance from boat to waypoint
« Reply #15 on: March 17, 2011, 06:13:45 AM »
Thanks Robert ;)

I thought like you but in this case the left screen and notably shadows and arches should be like that:



Am I right?
« Last Edit: March 17, 2011, 09:18:58 AM by felvic »
François - Montpellier - France


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