Calculate grid volume and area constrained by a contour
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].
- 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.
- Also add in the volume from the level of the contour down to base [Default base is contour].
- For geographical grids, append a unit from e|f|k|M|n|u [Default is meter (e)].
- 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).
- -Rxmin/xmax/ymin/ymax[+r][+uunit] (more …)
- Specify the region of interest.
- -V[level] (more …)
- Select verbosity level [c].
- 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[,…][t[word]] (more …)
- Select output columns (0 is first column; t is trailing text, append word to write one word only).
- -^ 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.
- Temporarily override a GMT default setting; repeatable. See gmt.conf for parameters.
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. (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
- 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.
- 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.
Wessel, P., 1998, An empirical method for optimal robust regional-residual separation of geophysical data, Math. Geol., 30(4), 391-408.