Surface area values from grdvolume with and without -Cr0

Q&A
1

Hi!

I want to calculate area and volume of water in a small bight in the Gulf of Bothnia (Northern Baltic sea). I want these for upscaling oxygen respiration measurements in water column and sediment to the whole basin. It is my first trial with grdvolume and I am confused with the resutls.

I extracted a grid for my basin from a larger scale Baltic bathymetry dataset freely available from EU EMODNET Bathymetry project and run grdvolume on that grid. I get different area values with and without -Cr0. I have no way of understanding which one is the correct.

gmt grdvolume OF-EMODNET-Bathymetry-2020-MASKED.nc -Se
0	65382296.2528	-910966771.429	-13.9329271628
gmt grdvolume OF-EMODNET-Bathymetry-2020-MASKED.nc -Se -Cr0
0	60764930.933	892930713.498	14.6948363108

Then I tried to set all depths to zero using grdmath mygrid.nc 0 MUL=mygrid-times-zero.nc and run grdvolume again to get the area of flat surface with no bathymetric topography, with the same results:

gmt grdvolume OF-EMODNET-Bathymetry-2020-MASKED-times-zero.nc -Se
0	65382296.2528	0	0
gmt grdvolume OF-EMODNET-Bathymetry-2020-MASKED-times-zero.nc -Se -Cr1
0	60802750.3812	60802750.3812	1

I expect seafloor area to be the same or slightly larger (maybe few percent) due to presence of slopes. However, the area values from grdvolume suggest it is the other way around: the seafloor area is ~7% smaller than the grid area in zero depth plane. This is confusing.

My questions - what is the meaning of the area values? is grdvolume a correct approach to get area of a sloping topography (bathymetry) at all? I also found this Calculating area of a sloping surface , is that applicable to my problem?

links to my two grids:
http://koljofjord.marine.gu.se/files/OF-EMODNET-Bathymetry-2020-MASKED.nc
http://koljofjord.marine.gu.se/files/OF-EMODNET-Bathymetry-2020-MASKED-times-zero.nc

Thanks in advance for any explanations
Mikhail.

2

OK, it looks like EMODNET data resolution is way too crude and many grid cells are lost when running slope calculation for a small shallow basin with many coastline details.

3

How did this go?

I’ve been trying a bit of grdvolume lately, and it looks like it does not take into account slope:

$ gmt grdmath -I1 -R-100/100/-100/100 X = tri.nc

$ gmt grdvolume -C0 tri.nc
0       19999.96        999996.020931   49.9999010464

Which is roughly equal to A = 100*200 = 20.000.