grdvolume — GMT 5.1.1 documentation

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# grdvolume

grdvolume - Calculate grid volume and area constrained by a contour

## Synopsis

grdvolume grdfile [ -Ccval or -Clow/high/delta or -Crlow/high ] [ -Lbase ] [ -R[unit]xmin/xmax/ymin/ymax[r] ] [ -S[unit] ] [ -T[c|h] ] [ -V[level] ] [ -Zfact[/shift] ] [ -f[i|o]colinfo ] [ -ocols[,...] ]

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 (bellow 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.

-^
  Print a short message about the syntax of the command, then exits (NOTE: on Windows use just -).
+
  Print an extensive usage (help) message, including the explanation of any module-specific option (but not the GMT common options), then exits.
-?
  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. When reading grids, the format is generally automatically recognized. If not, the same suffix can be added to input grid file names. See grdreformat and Section Grid file format specifications 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 grdreformat and Section Grid file format specifications 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.

## See Also

gmt, grdfilter, grdmask, grdmath

## References

Wessel, P., 1998, An empirical method for optimal robust regional-residual separation of geophysical data, Math. Geol., 30(4), 391-408.

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