Hi!
In this attachment you can see my first mapping attempt, the area near my harbour.I am showing you two small ditches I found, but I suspect that they might be made by sonar logs with high speed and therefore different transducer tilt perhaps...Is my fear real? In that case what it can be done to correct it?I am pretty sure that I have overlapped this area many times, are the latter logs correcting a map area or not?

Each new log corect what you have done before but care to the tide if your aréa is with tide!care too of transducer use if not the same use différent sounder profil!
Me I prefert to do an aréea and not comes back on, and do an other next the first(with a part of the first one to correct water level if need.
To make good log to go too fast!
(sorry for my english I'm french!!)

Christodor, to me the trenches don't look right compared with the rest of the areas geomorphology. Looks likely that you have 1 or 2 bad track lines in your dataset (where the features appear). This could have been due to a number of reasons, but the most probable cause would be an incorrect tidal correction (presumably you have been applying tidal corrections to every dataset? If not, you won't get a very accurate contour chart and will get artefacts of varying magnitude depending on tidal range in the area). Another cause might be that your transducer alignment has changed during those 'error' tracks, resulting in a deeper than true reading. Best thing to correct it would be to (if you haven't previously done so), apply tidal corrections for all the tracks on the chart using free software such as wxtide. If it is a transducer alignment problem for an individual track, use the user map display and overlay all your track lines. If there is a single track line overlying the length of Each of the trenches, it is likely that that track line has incorrect depth so remove it from the dataset and re generate the contour chart.

Incidentally, you could perform a simple bar check or 'control point' to help eliminate (or at least minimise) any future erroneous data sets. You can do this by measuring the depth at a known point which is flat (with a tape measure and heavy weight) and, soon after, run over the same point with your boat (at about the same speed you will be recording data). After you add the transducer depth offset to the reading on your depth display reading, you can compare the 2 results. If you use the same 'control point' on another day, you will need to apply tidal corrections to your previously tape measured depth before comparing it with the depth reading on your Lowrance display. If they are way off, your transducer may need realigning for the speed you are travelling.

There are various other things that might affect your readings, and other ways to come up with what's known as propagated error for the data, but you are getting into hydrographic surveying which is getting a bit too deep for your question (excuse the pun)!

My experience is, if there are one or a few extremely missmatching points, these distort the map.

In a map of a little lake in my area I had only one wrong point. This point created a hill in the map.

The best way to correct those points or group of points is, to view the logs in edit mode, not the finished map.

Hi Chris,
I agree with Hermes and Hubert- it does look like you could have an offset problem. The easiest way to check this is to go to the Define Map view and look at your raw track-points. You should be able to see if there is a track that has different depths to the others (tip: you can limit the depth range of the track to increase the colour range). If you do have a fixed offset, you can fix this by right-clicking on one of the bad track-points and adjusting the water level slider until the track-point colours align.
Matt

Hi matt,

Guess chris might need to be careful if he used the water level slider to align the track point offsets if the offset error was caused by transducer angle alignment... For example, I had a track line which showed the same 'artefacts' as chris is seeing and after checking tidal offset and fixed transducer depth offset, discovered that the transducer mounting bolt had come loose and the transducer was not perpendicular to sea floor (it had worked itself to about a 30 degree angle). I had a go at using the rm slider to align the water level, but since the transducer head was effectively pointing further behind the boat, the 'adjusted' depth was for a different location to the GPS position, so errors were still visible in the contour map (hope that makes sense) :-) This same problem might occur if you record different tracks at different speeds (if the angle of the transom and therefore transducer head) changes by much. The best solution I found was to remove the track line which had the error completely.

As Hubert says, if there are individual, or groups of rogue spot depths within a track line causing the 'artefacts', you could correct them by viewing the individual trackline in edit mode. However, if the error is over the entire trackline (as it was in the example I mentioned above), it will be harder to spot in an individual trackline edit display - since there is nothing to determine if, or what the error is! (Once again, hope that makes sense :-)

Bonsoir Nitro 78!J' ai tout compris, il n y a pas problem.Ici a Reefmaster,tout le monde parle la meme langue,la langue de la mer et quelques difficultes a parler bien une langue etrangaire ne sont pas obstacle.
In my mapping areas there is not a significant difference in depth,between uptide and downtide phase,perhaps a value of 50 cm-1 metre maximum.So I don't think that I will need tide corrections.
Hi Hermes!You really made a very good approach to my question!
You helped me a lot.Can newer sonar logs over the "trenches" correct the map?I find this an easier way to fix my map,than searching the dots of almost a dozen logs over this area...
Thank you Hubert and Matt!This map is an educative one,I started it to expertise myself at making a map.Any failures will help me to create better maps for my fishing spots.
Oh, I forgot something.My vessel is a 3,80 m inflatable,consequently I cannot be sure for the tilt of the beam in high and low speed.Most of my logs have been made with a speed of 1-4 knots,but a few ones have been made in high speed, 15-20 knots,returning to my harbour and these trenches happen to be on the same direction!So I guess that we found the problem,that's a difference of transducer angle between low and high speed.

hello agin christodor,

guess you are lucky not having to worry about tidal influence too much - the tidal range can be 8m + where i live, so unless you apply the correct tidal corrections, you cannot make very good charts if you combine different data sets! 15-20 knots is probably a bit too fast to get reliable data - i would be inclined to remove those datasets....

I've included a few screen dumps to help show what i mean about offset errors. As the first chart show, like you, i had a bad artefact caused by a track line that had incorrect depths (caused by the reason i mentioned - the transducer was not perpendicular to the sea floor and at a 30 degree angle). To solve it, first thing i did was to identify the 'bad track'. This wasn't too easy as i had lots of different tracklines in the area, - easiest way i found was to put all the tracklines into a user map and change the colour of each - as screen dump 2 shows, the black trackline was the bad dataset. In the user map, I then looked at the number of points in the trackline, then found the corresponding trackline by looking at the number of points in each of the individual tracks. Once I had identified the 'bad track' i tried adjusting the offset as Matt mentioned, but it didn't get a good result, so i just removed the bad track from my dataset and re computed the contour map - result is shown in final screen dump.

Incidentally matt, I've mentioned it before, but it might be a useful upgrade to reef master to have the trackline name associated with the trackline display in the user map - it can be quite tricky to find out which track is which - only way I've been able to use is by identifying the number of points in each and then finding the corresponding number in an individual track - maybe there is already a better way??.

hope this helps. (please excuse the jpg quality - emailing using a satellite link on a ship which is v v v v slow)!

Hi Hermes! Thank you for your help!
So I guess that I have to search for the bad sonar log and erase it from the map.8 meters is not a tide, is Noah's flood!!!Hahahaha!

I just found the bad log, removed it and this is the result....

looks much more real... Looks like you have a couple of other not so prominent features which might be affecting your contour map (i guess those might have been when you were going fast and recording data). You wrote it was a practice map - in which case (if you can't remember which tracks you were going fast on), maybe you could start from scratch and just record the data when you go slower. Like i wrote before, Ideally get a flat, known position and use a measure to find the depth and try to run over it every time you record data - this will help you spot any error 'blunders' and improve final chart quality.

Calibration run.

Something I try and do with any set of soundings is to produce at least one known run of soundings against which I can calibrate any set of soundings that cross it. I would do this on a perfect day, under perfect conditions then adjust it carefully to the datum of the chart I'm working on, and then adjust all the other runs based upon the intersections.
This may mean that you need to spend a day criss-crossing your bay at 3 kts to produce a proper calibration. Something I learned many years ago..."if it doesn't look right, it probably isn't". There are still, as Hermes points out, some odd inconsistencies in your plot. A sidescan swath would help greatly here.

T