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PROGRAM:

NAME

grdvolume - Calculate grid volume and area constrained by a contour

SYNOPSIS

grdvolume grdfile [ cval or low/high/delta or rlow/high ] [ base ] [ region ] [ [unit] ] [
[c|h] ] [ [level] ] [ fact[/shift] ] [ -f<flags> ] [ -o<flags> ]

Note: No space is allowed between the option flag and the associated arguments.

DESCRIPTION

grdvolume reads a 2-D binary 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].

REQUIRED ARGUMENTS

grdfile
The name of the input 2-D binary grid file. (See GRID FILE FORMAT below.)

OPTIONAL ARGUMENTS

-Ccval or -Clow/high/delta or -Crlow/high
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
volume between the grid surface and the plans defined by low and high. 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]
Convert degrees to Flat Earth distances, append a unit from e|f|k|M|n|u [Default is
Cartesian].

-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).

-R[unit]xmin/xmax/ymin/ymax[r] (more ...)
Specify the region of interest.

-V[level] (more ...)
Select verbosity level [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).

-f[i|o]colinfo (more ...)
Specify data types of input and/or output columns.

-ocols[,...] (more ...)
Select output columns (0 is first column).

-^ or just -
Print a short message about the syntax of the command, then exits (NOTE: on Windows
use just -).

-+ 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 options, then
exits.

--version
Print GMT version and exit.

--show-datadir
Print full path to GMT share directory and exit.

GRID FILE FORMATS

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. To specify the precision, scale and offset, the user should
add the suffix =id[/scale/offset[/nan]], where id is a two-letter identifier of the grid
type and precision, and scale and offset are optional scale factor and offset to be
applied to all grid values, and nan is the value used to indicate missing data. In case
the two characters id is not provided, as in =/scale than a id=nf is assumed. When
reading grids, the format is generally automatically recognized. If not, the same suffix
can be added to input grid file names. See grdconvert and Section grid-file-format of the
GMT Technical Reference and Cookbook for more information.

When reading a netCDF file that contains multiple grids, GMT will read, by default, the
first 2-dimensional grid that can find in that file. To coax GMT into reading another
multi-dimensional variable in the grid file, append ?varname to the file name, where
varname is the name of the variable. Note that you may need to escape the special meaning
of ? in your shell program by putting a backslash in front of it, or by placing the
filename and suffix between quotes or double quotes. The ?varname suffix can also be used
for output grids to specify a variable name different from the default: "z". See
grdconvert and Sections modifiers-for-CF and grid-file-format of the GMT Technical
Reference and Cookbook for more information, particularly on how to read splices of 3-,
4-, or 5-dimensional grids.

EXAMPLES

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, use

gmt grdvolume peaks.nc -Sk -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 -Sk -C100/300/10 -Th > results.d

To see the areas and volumes for all the contours in the previous example, use

gmt grdvolume peaks.nc -Sk -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

NOTES

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.

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