Wednesday, October 1, 2014

Lobster on camera

This lobster was captured on camera in 2011 on Jeffrey's Ledge.

Monday, August 18, 2014

Humminbird 698SI survey

Steve and I mounted the Humminbird on a jetski and used it last Friday to check out an eelgrass meadow in Clark's Cove, New Bedford.  It worked well.

Here's the jetski with the Humminbird.  The transducer is on a trolling motor mount so we could easily move it up and down to try and avoid noise.  We could also pop it out of the water for high speed travel.  The set up was fine, though noise is an issue.  With the transducer about 2-3 feet down in the water, the signals were acceptable.  We collected with a 120 foot range and used an enhancement setting of 9.

The processing workflow was straightforward.  I first checked the lines in HumViewer, a free viewer that plays back the sonar data.  This helped me identify which files I wanted to fully process.  Since we were just doing a test run, we didn't keep careful track of when we pressed record!  It also turned out to be very useful because the viewer shows the sounder data as well, and our target habitat, eelgrass, is clearly visible in the sounder data.  I discovered that when we were directly over the eelgrass, it was actually difficult to see in the sidescan image, but it was very easy to see in the sounder data.  The edge of the eelgrass meadow was clear.  The images below are the unprocessed data.  Depth and range are in meters.

The next step was to slant range and beam angle correct the data.  These processing steps remove the water column and make corrections so the sonar image is georeferenced and gain corrected across the range.  I used SonarTRX and it was easy, quick, and resulted in images we could interpret.  I've used Caris HIPS/SIPS in the past, and the batch processing capabilities will probably send us back to Caris, especially for larger surveys.  But the simple and intuitive workflow and easy export to Google Earth (and ArcGIS) made me an instant fan of SonarTRX.

You can see that in some lines it was pretty hard to see the vegetation nearer the sonar -- the best images are at about 1/2 range, or 60 feet.  Even in the 2nd image it might be interpreted as having two beds.  However, as I mentioned earlier, the sounder sonar (downward looking dual beam of 200/83 kHz) clearly showed that there was vegetation under the jetski.

I think the next step is to tweak some of the image settings to try and draw out the vegetation signal more clearly in the sidescan images.

First fusion table: survey map

Here's my first fusion table and Google Map.  If you click on the dot on the map, you'll see information about our latest survey.

Monday, August 11, 2014

Bathymetry and side imaging on a kayak

This webpage has a thorough description of a setup of a Garmin transducer on a kayak.  It looks like the project rapidly advanced to a Humminbird side imaging sonar.  (Thanks to Mike Sacarny, MIT for pointing out the sonar projects on this site; I previously used the same website for information about remote controlled helicopters for mapping.  Just scroll down to the list of projects at the bottom of the home page.)

I liked the idea of using a portable car emergency power pack.  As we learn about operating on smaller vessels, we'll have to move away from the generator.  Also, the generator is heavy and stinky, so for shorter surveys, a battery alternative is ideal.

What mapping software was used for the bathymetry?  I assume something like ArcGIS.  What about the side imaging?  Was it SonarTRX or a higher-end software like Caris or Hypack or was it mosaicing of screen grabs?  Can something like Hugin (which Mr. Illsley provides links to elsewhere in his website) be used to stitch photos together, and then something like ArcGIS or MapKnitter or QGIS be used for the orthorectification?

Thursday, July 31, 2014

Low cost sidescan for eelgrass mapping?

We're trying to map eelgrass in shallow water.  This has been an instrument challenge for years (decades?) that has required experimentation.  Aerial methods are great, but costly.  Remote control aerial is something we're interested in, but we don't have the proper licensing yet.  Processing might still be a real hurdle with aerial photographs too.  So we're focusing on vessel-mounted systems that can be deployed rapidly with data collection tools that result in data that needs minimal processing, or at least a workflow that can be standardized and result in maps in a matter of a day or two.

Most recently we've been using a Biosonics DT-X on a small boat.  Vessel draft and maneuverability is still a concern in shallow waters that frequently have obstructions, especially rocks.  So now we're testing a jet ski, on loan from colleagues in Maine.  Our plan was to mount the Biosonics on that, but our trial run with the jet ski made us realize just how wet it is.  To use the Biosonics, we would have to do some pretty involved retrofitting to have the necessary ruggedness.  If the jet ski proves to be a reliable platform, we might put the effort into the retrofit since we like the almost non-existent processing required of the Biosonics data.  In the meantime, a colleague here at DMF, Mike Bednarski, showed me his Humminbird 698SI.  So we'll take that out and see what sort of eelgrass mapping we can do with it mounted on the jetski.

Here's the whole unit -- transducer, processing unit, and storage (SD card).

Color Palette
Here's an image from the Humminbird website.

We'll be out in a couple of weeks to see if we can image eelgrass and mooring scars in eelgrass.  Some folks at the USFWS have put together some great descriptions and details about how they use their Humminbird for habitat mapping here.

Thanks Mike!

Wednesday, July 30, 2014

Great description of sidescan sonar

Humminbird's website has a nice overview including a great video explaining what sidescan sonar is and how to read a sidescan record.

The website is here.
The link is on the website, or you can access it here.