Wednesday, April 15, 2015

Transect Design: Consideration of Scale

One of the most frequently asked questions by BioBase EcoSound users is, "how far apart should I space my transects for creating maps?"  Although as always, the most appropriate response is: "it depends," we still offer solutions below that cover the most common use case scenarios.  We thank our partners at NC State University Department of Crop Science for contributing useful data from Waccamaw Lake in North Carolina USA.

Full Bay/Lake Map on a Large Lake
Transects can be designed in ArcMap using the Fishnet Tool in the ArcToolbox (see BioBase Support & Resources).  In this case, 200 m transects perpendicular to the longest shore with one nearshore loop was created.  The total length of transects in this instance equaled 189 km (117 mi) and would take 23 field hrs to map driving 8 km/h (5 mph).  ESRI Shapefiles or Google Earth .kml path files can be converted to .gpx for your Lowrance Chartplotter.  Convert the Trail to a Route and let out Outboard Autopilot do the mapping for you!
Aquatic Vegetation biovolume from raw sonar data from a Lowrance HDS, processed by BioBase, and imported and converted to raster with ArcMap.  In BioBase, the map buffer around each side of 200 m transects were increased to 100 m which produced a complete map and increased the resolution of each local prediction (i.e., a grid cell) from 5-m x 5-m (default) to 25-m x 25-m.  In this case, a coarse-resolution full lake map was the objective for more precise targeting of follow up surveys.
Follow up, intensive surveys in areas of interest
Zoomed in area near boat launch where an invasive aquatic plant species was located during initial BioBase mapping surveys.  Follow-up intensive surveys were carried out to more precisely map infestations.
Coarse-resolution aquatic vegetation BioBase biovolume map created from 200-m transects (left) and detailed biovolume map created from follow up intensive BioBase surveys (right).  Overall vegetation statistics are generated for this NW area of Waccamaw.  Note only subtle differences in the overall numbers, but great difference in the precision and detail between maps collected at different survey intensities.  Scale of inference and tradeoffs of alternative approaches should be considered when designing surveys and implementing management actions
Mapping small bays and fingers? Try concentric circles

For mapping small bays, inlets, fingers, canals, try a concentric circle approach to mapping.  Create a new trail on your Lowrance chartplotter and start logging data while traveling as close to shore as navigable.  Methodically work your way inward as you encounter your previous trail while avoiding overlap.  When you get to the middle, stop recording, you have a complete map! 
Situational awareness - mapping vegetation edges
Use a perpendicular to shore approach if you are interested in mapping the edge of aquatic plant growth in a deep lake.  Or if using a concentric circle design, ensure that you travel in a weave-like fashion over the bed edge.

Situational awareness - creating the smoothest bathymetry map
Get the smoothest bathymetry with a concentric circle transect design
Situational awareness - dealing with extremely steep slopes

Traveling along very extreme slopes can present mapping challenges because how rapidly depths change and because the acoustic beam intercepts the bottom at an angle. Indirect sonar returns increase the likelihood of erroneous bottom typing.  In these cases, the best bathymetry is achieved by avoiding the slope and mapping along the top and bottom of the shelf.
Situational awareness - mapping small patches


Patch of hard bottom (brick red on map) in an otherwise soft-bottom lake (light tan).  Surveyor noticed double echo on sonar chart while mapping and diverted course to methodically cover hard patch and thus more precisely map its extent.  
We hope this gallery of images and explanations can help BioBase users make the most efficient use of their time on the water mapping and produce the best possible aquatic habitat map!  For more information search this blog, aquatic resource mapping user forum, or contact us at info.biobase@navico.com.

2 comments:

  1. For any conversion needed to make gpx file to kml format or vice versa to have a better visualization of routes or tracks, I suggest to check out this free converter http://gpx2kml.com/ that generates good results.

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