Bathymetric Mapping on a College Student Budget
My first experience with undergraduate research started with Dr. Chris Houser, a coastal geomorphologist and at the time, the assistant dean to the college of Geocscience at Texas A&M. I began studying Geographic Information Systems (GIS) later in my academic career and coupled it with a minor in oceanography. I focused my studies where the two fields met. Hydrography is the science which focuses on the geological formations of the surface and subsurface of the ocean floor. What interested me was the use of various remote sensing tools like acoustics, LiDAR, and photogrammetry to explore the deep blue.
Dr. Houser, actively uses hydrography in his research to understand the building of sandbars and movement of sediments in the nearshore environment. He assisted in creating a syllabus for a directed studies course focusing on the history and methodology of hydrography and the various tools used to preform surveys. I then replicated the procedures using software and instruments that were cost feasible and titled my project, 'Near-shore bathymetry: Mapping on a College Student Budget'. My first survey was conducted on Lake Bryan, a small reservoir near my university.
Directly below is a report describing the history and methodology of hydrographic surveys.
The survey was conducted using a sea-kayak equipped with a Lowrance X-15 fish-finder, GPS receiver, 50/200 kHz transducer and a 12-volt car battery to power the equipment. The fish-finder had a LCD screen and navigational aid which allows users to input way-points or tracks which I created using Google Earth and exported them as a .KML file. The GPS was an attachment created by Lowrance with DGPS capabilities which saved the GPS locations every second, and coupled it with bathymetry data obtained by the transducer. The transducer's function is to convert electrical energy into acoustical waves which propagate downwards through the water column. With a built in receiver, the system is able to estimate the depth by subtracting the time the wave is created from received time, with the speed of the wave in water as a constant.
The GPS and bathymetry data were then exported from the fish-finder and stored on an SD card; to utilize the data in ArcGIS I created a python script which I call 'fishBelly.py' (after a fish I saw try to swim through the reeds on its belly). Lowrance records all the GPS information to a proprietary projection that no mapping software is familiar with. After spending some time on a few fishing enthusiasts forums I was able to find mathematical formulas to convert the lat/lon data to decimal-degree. The script takes in a pre-editted file (descriptions in source code), cleans it of unnecessary information, converts the GPS data and outputs a .csv file that can be directly imported into any mapping software. From there the points were generated to create a raster file based on depth using Inverse Distance Weighting (IDW) method, and then used in ArcScene to create 3D models of the lake bottom.
Below is the fishBelly code, followed by the completed bathymetric survey displaying the rasters on top of satellite imagery. I also included a short video of the 3D model created using ArcScene.
Overall the project was a success which made me confident about my ability to plan and execute a workflow on my own. The following summer I was planning to join the Texas A&M Geography department on a study abroad trip to Costa Rica where our studies would emphasize field mapping and produce mini-theses on micro-climates within the cloud forest. At the time Dr. Houser and his Ph.D. student Sarah Trimble, were studying the sediment transport and formations of rip-currents in Playa Cocles, a beach located in Puerto Viejo, Costa Rica.
After the results of the mapping project were turned in, Dr. Houser recommended I try a similar methodology on their beach while abroad. As an undergraduate research project, I was to obtain in-situ bathymetric data using the method described above to compare against bahthymetric data derived from images obtained from the WorldView-2 satellite which utilized the coastal-band (400-450nm). By using steroscopic-photogrammetry, two images of the same scene taken at different angles can produce z-values (depth) and create an accurate bathymetric profile.
With Dr. Houser's recommendation, I was offered a George Bush Foundation Scholarship which helped me afford the expenses of getting to Costa Rica to preform fieldwork. Click here to see the project page for the work I completed in Costa Rica.