Geoscience Reference
In-Depth Information
10.4 Materials and Methods
Initial bathymetrical data were processed and the maps were developed using the
ArcGIS 9.2 software. The digital bathymetric and slope maps generated are based on
the 500-m cell size raster model of data organization (GRID). A raster bathymetric
GIS layer, serving as the baseline layer, was generated by reading depths of a total
of 6,390 points from nautical charts of scales 1:25,000, 1:50,000, and 1:100,000
another.
The depth data used can be fully relied upon, as the depths were measured,
corrected, and mapped strictly in accordance with the procedures adopted by the
Hydrographic Survey of the Russian Federation's Ministry of Defence, the only
institution licensed to draw and distribute nautical charts of the Russian territorial
waters. Under the procedures in question, the depths measured by an echo-sounder
are corrected for the following: instrument error; deviation of the actual sound
velocity from the reading; echo-sounder vibrator submergence; between-vibrator
distance; the vessel's drag in shallow water; seafloor slope; and the offset with
respect to the sea surface level. As a result, the standard deviation-based uncer-
tainty factor of the depth reading does not exceed 0.1, 0.3, 0.5, and 1 m for the
depth intervals of 0-10, 10-30, 30-50, and 50-100 m, respectively, and the uncer-
tainty for depths larger than 100 m does not exceed 1% of the depth read-out.
Important depth points (local minima and maxima, inflection sites) are marked on
the nautical charts for a better representation of the seafloor bathymetry. The proce-
dures described are strictly adhered to at all stages of bathymetric surveys and map
development.
The initial data set was large enough for us to use the nearest neighbour interpo-
lation technique in the bathymetric model development. The main advantage of this
technique lies in the absence of high distortions in the input depth values.
The seafloor slopes (bathymetric surface gradients) were mapped using the
ArcGIS 9.2 slope function which calculates the maximum depth gradient between
adjacent cells. The output raster slope GIS layer was calculated using degrees as
units. The raster bathymetric surface gradient model obtained made it possible to
identify areas with both relatively high and relatively low gradients. The low gra-
dients correspond to fossil wave-cut platform surfaces, whereas the high gradients
correspond to the maximum inclination of ancient coastal cliffs. Axial lines corre-
sponding to areas with the maximum slopes were drawn on the map as linear vector
GIS layers.
The submerged wave-cut cliffs were located with the aid of echograms produced
by a single-beam Simrad EA 400 SP echo-sounder (38 and 200 kHz) used dur-
ing several cruises of RV PROFESSOR SHTOKMAN in 2007-2008. Hydrographic
data collected during the cruises with a CTD were used to correct for the sound
velocity error. Thus corrected, the echo-sounder readings were processed by the
ArcGIS 9.2 software to produce seafloor topography profiles. All the GIS layers
and the final maps were developed in the WGS_1984_UTM_Zone_34 N projected
coordinate system.
