grdvolume - Calculate grid volume and area constrained by a
contour
grdvolume grdfile [ -Ccval or
-Clow/high/delta or -Crlow/high or
-Crcval] [ -Lbase ] [
-Rregion ] [ -S[unit] ] [
-T[c|h] ] [ -V[level] ] [
-Zfact[/shift] ] [ -fflags ] [ -oflags
]
Note: No space is allowed between the option flag and the
associated arguments.
grdvolume reads a 2-D grid file and calculates the volume
contained between the surface and the plane specified by the given contour
(or zero if not given) and reports the area, volume, and maximum mean height
(volume/area). Alternatively, specify a range of contours to be tried and
grdvolume will determine the volume and area inside the contour for
all contour values. Using -T, the contour that produced the maximum
mean height (or maximum curvature of heights vs contour value) is reported
as well. This feature may be used with grdfilter in designing an Optimal
Robust Separator [Wessel, 1998].
- grdfile
- The name of the input 2-D binary grid file. (See GRID FILE FORMAT
below.)
- -Ccval or
-Clow/high/delta or -Crlow/high or
-Crcval
- find area, volume and mean height (volume/area) inside the cval
contour. Alternatively, search using all contours from low to
high in steps of delta. [Default returns area, volume and
mean height of the entire grid]. The area is measured in the plane of the
contour. The Cr form on the other hand computes the volume between
the grid surface and the plans defined by low and high, or
below cval and grid's minimum. Note that this is an outside
volume whilst the other forms compute an inside (below the surface)
area volume. Use this form to compute for example the volume of water
between two contours.
- -Lbase
- Also add in the volume from the level of the contour down to base
[Default base is contour].
- -S[unit]
- For geographical grids, append a unit from
e|f|k|M|n|u [Default is meter
(e)].
- -T[c|h]
- Determine the single contour that maximized the average height (=
volume/area). Select -Tc to use the maximum curvature of heights
versus contour value rather than the contour with the maximum height to
pick the best contour value (requires -C).
- -Zfact[/shift]
- Optionally subtract shift before scaling data by fact.
[Default is no scaling]. (Numbers in -C, -L refer to values
after this scaling has occurred).
- -^ or just -
- Print a short message about the syntax of the command, then exits (NOTE:
on Windows just use -).
- -+ or just +
- Print an extensive usage (help) message, including the explanation of any
module-specific option (but not the GMT common options), then exits.
- -? or no arguments
- Print a complete usage (help) message, including the explanation of all
options, then exits.
By default GMT writes out grid as single precision floats in a
COARDS-complaint netCDF file format. However, GMT is able to produce grid
files in many other commonly used grid file formats and also facilitates so
called "packing" of grids, writing out floating point data as 1-
or 2-byte integers. (more ...)
To determine the volume in km^3 under the surface hawaii_topo.nc
(height in km), use
gmt grdvolume hawaii_topo.nc -Sk
To find the volume between the surface peaks.nc and the contour z
= 250 m in meters, use
gmt grdvolume peaks.nc -Se -C250
To search for the contour, between 100 and 300 in steps of 10,
that maximizes the ratio of volume to surface area for the file peaks.nc,
use
gmt grdvolume peaks.nc -C0/300/10 -Th > results.d
To see the areas and volumes for all the contours in the previous
example, use
gmt grdvolume peaks.nc -C100/300/10 > results.d
To find the volume of water in a lake with its free surface at 0
and max depth of 300 meters, use
gmt grdvolume lake.nc -Cr-300/0
- 1.
- For geographical grids we convert degrees to "Flat Earth"
distances in meter. You can use -S to select another distance unit.
The area is then reported in this unit squared while the volume is
reported in unit^2 * z_unit quantities.
- 2.
- grdvolume distinguishes between gridline and pixel-registered
grids. In both cases the area and volume are computed up to the grid
boundaries. That means that in the first case the grid cells on the
boundary only contribute half their area (and volume), whereas in the
second case all grid cells are fully used. The exception is when the
-C flag is used: since contours do not extend beyond the outermost
grid point, both grid types are treated the same. That means the outer rim
in pixel oriented grids is ignored when using the -C flag.
gmt, grdfilter, grdmask, grdmath
Wessel, P., 1998, An empirical method for optimal robust
regional-residual separation of geophysical data, Math. Geol.,
30(4), 391-408.
2019, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F.
Wobbe