Thursday, August 20, 2015

How is eelgrass mapped?

  • First we look at aerial photos (Google Maps/Earth, Bing, those flown for the state).
  • In some places, we run acoustics to look at deeper waters or cover time periods that we don't have aerial photos for. 
  • Then we get in the water with cameras to confirm (groundtruth) that we're identifying eelgrass and not algae.
  • These steps are illustrated below.

Is the dark shading in the aerial photo eelgrass in the center of Cohasset Harbor?

Step 1: Go out in a boat with sidescan and underwater camera.

The water is shallow where eelgrass grows and instruments are fairly sensitive to waves.  So use a small boat or jetski on a calm day.  Here we're using a Humminbird 698SI (200 kHz) sidescan. The transducer is mounted on a pole attached to the boat with a trolling motor bracket.

This is the Humminbird 698SI sidescan unit. The  photo to the right shows the whole unit as it comes out of the box.  The next photo down shows the transducer on the pole and the processing (and GPS/nav) unit is the smaller computer screen mounted to the table in front of the boat driver.

This is the GoPro Hero2 camera on a line with a shackle for weight.  We set it up to automatically snap images every 2 seconds.  Before each station, we write the station number on a white board, take an image of the board, then toss the camera overboard and lower to seafloor then bring it up slowly. We record the lat/lon of the station on paper and using the waypoint feature on the Humminbird sidescan unit.  This works to about 20 feet.  Then more weight is needed.  Deeper than about 30 feet might need light too.

Step 2: Drive survey lines with the sidescan and drop camera overboard at stations throughout the survey area.  We can cover 150-200 acres in 4 hours.

The survey lines are in yellow.  We space the lines approximately 200 feet apart to achieve about 150% coverage of the sidescan.  We do that by eye using the chart on the Humminbird sidescan unit.  The drop camera stations are the white dots.  These were distributed on the fly in the field to achieve 6-10 stations per planned survey line.  In this survey we added survey lines to the northern section late in the day, on the fly.  We didn’t bring the deeper drop camera, so that area wasn’t included in the photo groundtruthing.

 Step 3: Process sidescan data in SonarTRX or CARIS HIPS-SIPS software, create mosaic.

Here is the mosaic, I don’t know why there are two blank sections –one in the middle toward the top and one in the northwest corner.  The data is there and when I zoom in and out it blinks on and off.
Step 4: Examine groundtruth photos and compare to sidescan and aerial photos.  Identify patterns in aerial photos and sidescan that are consistent with eelgrass (or other habitats like gravel).

The image below has the grass stations coded in green, sand in yellow, and sand-gravel in orange. The green outline was drawn by DEP on aerial photos from 2006.  All data files are available for download as kml/kmz files for opening in Google Earth.

Sidescan sonar mosaic (200 mb)
Lat/lon of photo stations with notes (2.2 kb)
Photos geotagged to the correct location (several per station) (514 kb)
DEP eelgrass polygons, statewide, 2006-2007 (107 kb)  metadata

No comments: