# grdmath¶

Reverse Polish Notation (RPN) calculator for grids (element by element)

## Synopsis¶

gmt grdmath [ -Amin_area[/min_level/max_level][+a[g|i][s|S]][+r|l][+ppercent] ] [ -Dresolution[+f] ] [ -Iincrement ] [ -M ] [ -N ] [ -Rregion ] [ -S ] [ -V[level] ] [ -bibinary ] [ -dinodata ] [ -fflags ] [ -hheaders ] [ -iflags ] [ -nflags ] [ -rreg ] [ -x[[-]n] ] [ --PAR=value ] operand [ operand ] OPERATOR [ operand ] OPERATOR= outgrdfile

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

## Description¶

grdmath will perform operations like add, subtract, multiply, and numerous other operands on one or more grid files or constants using Reverse Polish Notation (RPN) syntax. Arbitrarily complicated expressions may therefore be evaluated; the final result is written to an output grid file. Grid operations are element-by-element, not matrix manipulations. Some operators only require one operand (see below). If no grid files are used in the expression then options -R, -I must be set (and optionally -rreg). The expression = outgrdfile can occur as many times as the depth of the stack allows in order to save intermediate results. Complicated or frequently occurring expressions may be coded as a macro for future use or stored and recalled via named memory locations.

## Required Arguments¶

operand
If operand can be opened as a file it will be read as a grid file. If not a file, it is interpreted as a numerical constant or a special symbol (see below).
outgrdfile
The name of a 2-D grid file that will hold the final result. (See GRID FILE FORMATS below).

## Optional Arguments¶

-Amin_area[/min_level/max_level][+a[g|i][s|S]][+r|l][+ppercent]
Features with an area smaller than min_area in km^2 or of hierarchical level that is lower than min_level or higher than max_level will not be plotted [Default is 0/0/4 (all features)]. Level 2 (lakes) contains regular lakes and wide river bodies which we normally include as lakes; append +r to just get river-lakes or +l to just get regular lakes. Append +ppercent to exclude polygons whose percentage area of the corresponding full-resolution feature is less than percent. Use +a to control special aspects of the Antarctica coastline: By default (or add i) we select the ice shelf boundary as the coastline for Antarctica; alternatively, add g to select the ice grounding line instead. For expert users who wish to utilize their own Antarctica (with islands) coastline you can add s to skip all GSHHG features below 60S. In contrast, you can add S to instead skip all features north of 60S. See GSHHG INFORMATION below for more details. (-A is only relevant to the LDISTG operator)
-Dresolution[+f]
Selects the resolution of the data set to use with the operator LDISTG ((f)ull, (h)igh, (i)ntermediate, (l)ow, and (c)rude). The resolution drops off by 80% between data sets [Default is l]. Append +f to automatically select a lower resolution should the one requested not be available [abort if not found].
-Ixinc[unit][+e|n][/yinc[unit][+e|n]]
x_inc [and optionally y_inc] is the grid spacing. Optionally, append a suffix modifier. Geographical (degrees) coordinates: Append m to indicate arc minutes or s to indicate arc seconds. If one of the units e, f, k, M, n or u is appended instead, the increment is assumed to be given in meter, foot, km, Mile, nautical mile or US survey foot, respectively, and will be converted to the equivalent degrees longitude at the middle latitude of the region (the conversion depends on PROJ_ELLIPSOID). If y_inc is given but set to 0 it will be reset equal to x_inc; otherwise it will be converted to degrees latitude. All coordinates: If +e is appended then the corresponding max x (east) or y (north) may be slightly adjusted to fit exactly the given increment [by default the increment may be adjusted slightly to fit the given domain]. Finally, instead of giving an increment you may specify the number of nodes desired by appending +n to the supplied integer argument; the increment is then recalculated from the number of nodes and the domain. The resulting increment value depends on whether you have selected a gridline-registered or pixel-registered grid; see GMT File Formats for details. Note: if -Rgrdfile is used then the grid spacing (and registration) have already been initialized; use -I (and -r) to override the values.
-M
By default any derivatives calculated are in z_units/x(or y)_units. However, the user may choose this option to convert dx,dy in degrees of longitude,latitude into meters using a flat Earth approximation, so that gradients are in z_units/meter.
-N
Turn off strict domain match checking when multiple grids are manipulated [Default will insist that each grid domain is within 1e-4 * grid_spacing of the domain of the first grid listed].
-Rxmin/xmax/ymin/ymax[+r][+uunit] (more …)
Specify the region of interest.
-S
Reduce (i.e., collapse) the entire stack to a single grid by applying the next operator to all co-registered nodes across the entire stack. You must specify -S after listing all of your grids. Note: You can only follow -S with a reducing operator, i.e., from the list ADD, AND, MAD, LMSSCL, MAX, MEAN, MEDIAN, MIN, MODE, MUL, RMS, STD, SUB, VAR or XOR.
-V[level] (more …)
Select verbosity level [c].
-bi[ncols][t] (more …)
Select native binary format for primary input. The binary input option only applies to the data files needed by operators LDIST, PDIST, and INSIDE.
-dinodata (more …)
Replace input columns that equal nodata with NaN.
-f[i|o]colinfo (more …)
Specify data types of input and/or output columns.
-g[a]x|y|d|X|Y|D|[col]zgap[u][+n|p] (more …)
Determine data gaps and line breaks.
-h[i|o][n][+c][+d][+rremark][+rtitle] (more …)
-icols[+l][+sscale][+ooffset][,][,t[word]] (more …)
Select input columns and transformations (0 is first column, t is trailing text, append word to read one word only).
-n[b|c|l|n][+a][+bBC][+c][+tthreshold] (more …)
Select interpolation mode for grids.
-r[g|p] (more …)
Set node registration [gridline]. Only used with -R -I.
-x[[-]n] (more …)
Limit number of cores used in multi-threaded algorithms (OpenMP required).
-^ 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.
--PAR=value
Temporarily override a GMT default setting; repeatable. See gmt.conf for parameters.

## Operators¶

Choose among the following 209 operators. “args” are the number of input and output arguments.

Operator args Returns
ABS 1 1 abs (A)
ACOS 1 1 acos (A)
ACOSH 1 1 acosh (A)
ACOT 1 1 acot (A)
ACSC 1 1 acsc (A)
ADD 2 1 A + B
AND 2 1 B if A == NaN, else A
ARC 2 1 Return arc(A,B) on [0 pi]
AREA 0 1 Area of each gridnode cell (in km^2 if geographic)
ASEC 1 1 asec (A)
ASIN 1 1 asin (A)
ASINH 1 1 asinh (A)
ATAN 1 1 atan (A)
ATAN2 2 1 atan2 (A, B)
ATANH 1 1 atanh (A)
BCDF 3 1 Binomial cumulative distribution function for p = A, n = B, and x = C
BPDF 3 1 Binomial probability density function for p = A, n = B, and x = C
BEI 1 1 bei (A)
BER 1 1 ber (A)
BITAND 2 1 A & B (bitwise AND operator)
BITLEFT 2 1 A << B (bitwise left-shift operator)
BITNOT 1 1 ~A (bitwise NOT operator, i.e., return two’s complement)
BITOR 2 1 A | B (bitwise OR operator)
BITRIGHT 2 1 A >> B (bitwise right-shift operator)
BITTEST 2 1 1 if bit B of A is set, else 0 (bitwise TEST operator)
BITXOR 2 1 A ^ B (bitwise XOR operator)
CAZ 2 1 Cartesian azimuth from grid nodes to stack x,y (i.e., A, B)
CBAZ 2 1 Cartesian back-azimuth from grid nodes to stack x,y (i.e., A, B)
CDIST 2 1 Cartesian distance between grid nodes and stack x,y (i.e., A, B)
CDIST2 2 1 As CDIST but only to nodes that are != 0
CEIL 1 1 ceil (A) (smallest integer >= A)
CHICRIT 2 1 Chi-squared critical value for alpha = A and nu = B
CHICDF 2 1 Chi-squared cumulative distribution function for chi2 = A and nu = B
CHIPDF 2 1 Chi-squared probability density function for chi2 = A and nu = B
COMB 2 1 Combinations n_C_r, with n = A and r = B
CORRCOEFF 2 1 Correlation coefficient r(A, B)
COS 1 1 cos (A) (A in radians)
COSD 1 1 cos (A) (A in degrees)
COSH 1 1 cosh (A)
COT 1 1 cot (A) (A in radians)
COTD 1 1 cot (A) (A in degrees)
CSC 1 1 csc (A) (A in radians)
CSCD 1 1 csc (A) (A in degrees)
CURV 1 1 Curvature of A (Laplacian)
D2DX2 1 1 d^2(A)/dx^2 2nd derivative
D2DY2 1 1 d^2(A)/dy^2 2nd derivative
D2DXY 1 1 d^2(A)/dxdy 2nd derivative
D2R 1 1 Converts Degrees to Radians
DDX 1 1 d(A)/dx Central 1st derivative
DDY 1 1 d(A)/dy Central 1st derivative
DEG2KM 1 1 Converts Spherical Degrees to Kilometers
DENAN 2 1 Replace NaNs in A with values from B
DILOG 1 1 dilog (A)
DIV 2 1 A / B
DUP 1 2 Places duplicate of A on the stack
ECDF 2 1 Exponential cumulative distribution function for x = A and lambda = B
ECRIT 2 1 Exponential distribution critical value for alpha = A and lambda = B
EPDF 2 1 Exponential probability density function for x = A and lambda = B
ERF 1 1 Error function erf (A)
ERFC 1 1 Complementary Error function erfc (A)
EQ 2 1 1 if A == B, else 0
ERFINV 1 1 Inverse error function of A
EXCH 2 2 Exchanges A and B on the stack
EXP 1 1 exp (A)
FACT 1 1 A! (A factorial)
EXTREMA 1 1 Local Extrema: +2/-2 is max/min, +1/-1 is saddle with max/min in x, 0 elsewhere
FCDF 3 1 F cumulative distribution function for F = A, nu1 = B, and nu2 = C
FCRIT 3 1 F distribution critical value for alpha = A, nu1 = B, and nu2 = C
FLIPLR 1 1 Reverse order of values in each row
FLIPUD 1 1 Reverse order of values in each column
FLOOR 1 1 floor (A) (greatest integer <= A)
FMOD 2 1 A % B (remainder after truncated division)
FPDF 3 1 F probability density function for F = A, nu1 = B, and nu2 = C
GE 2 1 1 if A >= B, else 0
GT 2 1 1 if A > B, else 0
HYPOT 2 1 hypot (A, B) = sqrt (A*A + B*B)
I0 1 1 Modified Bessel function of A (1st kind, order 0)
I1 1 1 Modified Bessel function of A (1st kind, order 1)
IFELSE 3 1 B if A != 0, else C
IN 2 1 Modified Bessel function of A (1st kind, order B)
INRANGE 3 1 1 if B <= A <= C, else 0
INSIDE 1 1 1 when inside or on polygon(s) in A, else 0
INV 1 1 1 / A
ISFINITE 1 1 1 if A is finite, else 0
ISNAN 1 1 1 if A == NaN, else 0
J0 1 1 Bessel function of A (1st kind, order 0)
J1 1 1 Bessel function of A (1st kind, order 1)
JN 2 1 Bessel function of A (1st kind, order B)
K0 1 1 Modified Kelvin function of A (2nd kind, order 0)
K1 1 1 Modified Bessel function of A (2nd kind, order 1)
KEI 1 1 kei (A)
KER 1 1 ker (A)
KM2DEG 1 1 Converts Kilometers to Spherical Degrees
KN 2 1 Modified Bessel function of A (2nd kind, order B)
KURT 1 1 Kurtosis of A
LCDF 1 1 Laplace cumulative distribution function for z = A
LCRIT 1 1 Laplace distribution critical value for alpha = A
LDIST 1 1 Compute minimum distance (in km if -fg) from lines in multi-segment ASCII file A
LDIST2 2 1 As LDIST, from lines in ASCII file B but only to nodes where A != 0
LDISTG 0 1 As LDIST, but operates on the GSHHG dataset (see -A, -D for options).
LE 2 1 1 if A <= B, else 0
LOG 1 1 log (A) (natural log)
LOG10 1 1 log10 (A) (base 10)
LOG1P 1 1 log (1+A) (accurate for small A)
LOG2 1 1 log2 (A) (base 2)
LMSSCL 1 1 LMS (Least Median of Squares) scale estimate (LMS STD) of A
LMSSCLW 2 1 Weighted LMS (Least Median of Squares) scale estimate (LMS STD) of A for weights in B
LOWER 1 1 The lowest (minimum) value of A
LPDF 1 1 Laplace probability density function for z = A
LRAND 2 1 Laplace random noise with mean A and std. deviation B
LT 2 1 1 if A < B, else 0
MAD 1 1 Median Absolute Deviation (L1 STD) of A
MAX 2 1 Maximum of A and B
MEAN 1 1 Mean value of A
MEANW 2 1 Weighted mean value of A for weights in B
MEDIAN 1 1 Median value of A
MEDIANW 2 1 Weighted median value of A for weights in B
MIN 2 1 Minimum of A and B
MOD 2 1 A mod B (remainder after floored division)
MODE 1 1 Mode value (Least Median of Squares) of A
MODEW 2 1 Weighted mode value (Least Median of Squares) of A for weights in B
MUL 2 1 A * B
NAN 2 1 NaN if A == B, else A
NEG 1 1 -A
NEQ 2 1 1 if A != B, else 0
NORM 1 1 Normalize (A) so max(A)-min(A) = 1
NOT 1 1 NaN if A == NaN, 1 if A == 0, else 0
NRAND 2 1 Normal, random values with mean A and std. deviation B
OR 2 1 NaN if B == NaN, else A
PCDF 2 1 Poisson cumulative distribution function for x = A and lambda = B
PDIST 1 1 Compute minimum distance (in km if -fg) from points in ASCII file A
PDIST2 2 1 As PDIST, from points in ASCII file B but only to nodes where A != 0
PERM 2 1 Permutations n_P_r, with n = A and r = B
PLM 3 1 Associated Legendre polynomial P(A) degree B order C
PLMg 3 1 Normalized associated Legendre polynomial P(A) degree B order C (geophysical convention)
POINT 1 2 Compute mean x and y from ASCII file A and place them on the stack
POP 1 0 Delete top element from the stack
POW 2 1 A ^ B
PPDF 2 1 Poisson distribution P(x,lambda), with x = A and lambda = B
PQUANT 2 1 The B’th Quantile (0-100%) of A
PQUANTW 3 1 The C’th weighted quantile (0-100%) of A for weights in B
PSI 1 1 Psi (or Digamma) of A
PV 3 1 Legendre function Pv(A) of degree v = real(B) + imag(C)
QV 3 1 Legendre function Qv(A) of degree v = real(B) + imag(C)
R2 2 1 R2 = A^2 + B^2
R2D 1 1 Convert Radians to Degrees
RAND 2 1 Uniform random values between A and B
RCDF 1 1 Rayleigh cumulative distribution function for z = A
RCRIT 1 1 Rayleigh distribution critical value for alpha = A
RINT 1 1 rint (A) (round to integral value nearest to A)
RMS 1 1 Root-mean-square of A
RMSW 1 1 Root-mean-square of A for weights in B
RPDF 1 1 Rayleigh probability density function for z = A
ROLL 2 0 Cyclicly shifts the top A stack items by an amount B
ROTX 2 1 Rotate A by the (constant) shift B in x-direction
ROTY 2 1 Rotate A by the (constant) shift B in y-direction
SDIST 2 1 Spherical (Great circle|geodesic) distance (in km) between nodes and stack (A, B) Example
X

## To compute all distances to north pole:

gmt grdmath -Rg -I1 0 90 SDIST = dist_to_NP.nc

SDIST2 2 1 As SDIST but only to nodes that are != 0
SAZ 2 1 Spherical azimuth from grid nodes to stack lon, lat (i.e., A, B)
SBAZ 2 1 Spherical back-azimuth from grid nodes to stack lon, lat (i.e., A, B)
SEC 1 1 sec (A) (A in radians)
SECD 1 1 sec (A) (A in degrees)
SIGN 1 1 sign (+1 or -1) of A
SIN 1 1 sin (A) (A in radians)
SINC 1 1 sinc (A) (sin (pi*A)/(pi*A))
SIND 1 1 sin (A) (A in degrees)
SINH 1 1 sinh (A)
SKEW 1 1 Skewness of A
SQR 1 1 A^2
SQRT 1 1 sqrt (A)
STD 1 1 Standard deviation of A
STDW 2 1 Weighted standard deviation of A for weights in B
STEP 1 1 Heaviside step function: H(A)
STEPX 1 1 Heaviside step function in x: H(x-A)
STEPY 1 1 Heaviside step function in y: H(y-A)
SUB 2 1 A - B
SUM 1 1 Sum of all values in A
TAN 1 1 tan (A) (A in radians)
TAND 1 1 tan (A) (A in degrees)
TANH 1 1 tanh (A)
TAPER 2 1 Unit weights cosine-tapered to zero within A and B of x and y grid margins
TCDF 2 1 Student’s t cumulative distribution function for t = A, and nu = B
TCRIT 2 1 Student’s t distribution critical value for alpha = A and nu = B
TN 2 1 Chebyshev polynomial Tn(-1<t<+1,n), with t = A, and n = B
TPDF 2 1 Student’s t probability density function for t = A, and nu = B
TRIM 3 1 Alpha-trim C to NaN if values fall in tails A and B (in percentage)
UPPER 1 1 The highest (maximum) value of A
VAR 1 1 Variance of A
VARW 2 1 Weighted variance of A for weights in B
WCDF 3 1 Weibull cumulative distribution function for x = A, scale = B, and shape = C
WCRIT 3 1 Weibull distribution critical value for alpha = A, scale = B, and shape = C
WPDF 3 1 Weibull density distribution P(x,scale,shape), with x = A, scale = B, and shape = C
WRAP 1 1 wrap A in radians onto [-pi,pi]
XOR 2 1 0 if A == NaN and B == NaN, NaN if B == NaN, else A
Y0 1 1 Bessel function of A (2nd kind, order 0)
Y1 1 1 Bessel function of A (2nd kind, order 1)
YLM 2 2 Re and Im orthonormalized spherical harmonics degree A order B
YLMg 2 2 Cos and Sin normalized spherical harmonics degree A order B (geophysical convention)
YN 2 1 Bessel function of A (2nd kind, order B)
ZCDF 1 1 Normal cumulative distribution function for z = A
ZPDF 1 1 Normal probability density function for z = A
ZCRIT 1 1 Normal distribution critical value for alpha = A

## Symbols¶

The following symbols have special meaning:

 PI 3.1415926… E 2.7182818… EULER 0.5772156… PHI 1.6180339… (golden ratio) EPS_F 1.192092896e-07 (single precision epsilon XMIN Minimum x value XMAX Maximum x value XRANGE Range of x values XINC x increment NX The number of x nodes YMIN Minimum y value YMAX Maximum y value YRANGE Range of y values YINC y increment NY The number of y nodes X Grid with x-coordinates Y Grid with y-coordinates XNORM Grid with normalized [-1 to +1] x-coordinates YNORM Grid with normalized [-1 to +1] y-coordinates XCOL Grid with column numbers 0, 1, …, NX-1 YROW Grid with row numbers 0, 1, …, NY-1 NODE Grid with node numbers 0, 1, …, (NX*NY)-1 NODEP Grid with node numbers in presence of pad

## Notes On Operators¶

1. For Cartesian grids the operators MEAN, MEDIAN, MODE, LMSSCL, MAD, PQUANT, RMS, STD, and VAR return the expected value from the given matrix. However, for geographic grids we perform a spherically weighted calculation where each node value is weighted by the geographic area represented by that node.

2. The operator SDIST calculates spherical distances in km between the (lon, lat) point on the stack and all node positions in the grid. The grid domain and the (lon, lat) point are expected to be in degrees. Similarly, the SAZ and SBAZ operators calculate spherical azimuth and back-azimuths in degrees, respectively. The operators LDIST and PDIST compute spherical distances in km if -fg is set or implied, else they return Cartesian distances. Note: If the current PROJ_ELLIPSOID is ellipsoidal then geodesics are used in calculations of distances, which can be slow. You can trade speed with accuracy by changing the algorithm used to compute the geodesic (see PROJ_GEODESIC).

The operator LDISTG is a version of LDIST that operates on the GSHHG data. Instead of reading an ASCII file, it directly accesses one of the GSHHG data sets as determined by the -D and -A options.

3. The operator POINT reads a ASCII table, computes the mean x and mean y values and places these on the stack. If geographic data then we use the mean 3-D vector to determine the mean location.

4. The operator PLM calculates the associated Legendre polynomial of degree L and order M (0 <= M <= L), and its argument is the sine of the latitude. PLM is not normalized and includes the Condon-Shortley phase (-1)^M. PLMg is normalized in the way that is most commonly used in geophysics. The C-S phase can be added by using -M as argument. PLM will overflow at higher degrees, whereas PLMg is stable until ultra high degrees (at least 3000).

5. The operators YLM and YLMg calculate normalized spherical harmonics for degree L and order M (0 <= M <= L) for all positions in the grid, which is assumed to be in degrees. YLM and YLMg return two grids, the real (cosine) and imaginary (sine) component of the complex spherical harmonic. Use the POP operator (and EXCH) to get rid of one of them, or save both by giving two consecutive = file.nc calls.

The orthonormalized complex harmonics YLM are most commonly used in physics and seismology. The square of YLM integrates to 1 over a sphere. In geophysics, YLMg is normalized to produce unit power when averaging the cosine and sine terms (separately!) over a sphere (i.e., their squares each integrate to 4 pi). The Condon-Shortley phase (-1)^M is not included in YLM or YLMg, but it can be added by using -M as argument.

6. All the derivatives are based on central finite differences, with natural boundary conditions, and are Cartesian derivatives.

7. Files that have the same names as some operators, e.g., ADD, SIGN, =, etc. should be identified by prepending the current directory (i.e., ./LOG).

8. Piping of files is not allowed.

9. The stack depth limit is hard-wired to 100.

10. All functions expecting a positive radius (e.g., LOG, KEI, etc.) are passed the absolute value of their argument. (9) The bitwise operators (BITAND, BITLEFT, BITNOT, BITOR, BITRIGHT, BITTEST, and BITXOR) convert a grid’s single precision values to unsigned 32-bit ints to perform the bitwise operations. Consequently, the largest whole integer value that can be stored in a float grid is 2^24 or 16,777,216. Any higher result will be masked to fit in the lower 24 bits. Thus, bit operations are effectively limited to 24 bit. All bitwise operators return NaN if given NaN arguments or bit-settings <= 0.

11. When OpenMP support is compiled in, a few operators will take advantage of the ability to spread the load onto several cores. At present, the list of such operators is: LDIST, LDIST2, PDIST, PDIST2, SAZ, SBAZ, SDIST, YLM, and grd_YLMg.

12. Operators DEG2KM and KM2DEG are only exact when a spherical Earth is selected with PROJ_ELLIPSOID.

## Grid Values Precision¶

Regardless of the precision of the input data, GMT programs that create grid files will internally hold the grids in 4-byte floating point arrays. This is done to conserve memory and furthermore most if not all real data can be stored using 4-byte floating point values. Data with higher precision (i.e., double precision values) will lose that precision once GMT operates on the grid or writes out new grids. To limit loss of precision when processing data you should always consider normalizing the data prior to processing.

## 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 …)

## Geographical And Time Coordinates¶

When the output grid type is netCDF, the coordinates will be labeled “longitude”, “latitude”, or “time” based on the attributes of the input data or grid (if any) or on the -f or -R options. For example, both -f0x -f1t and -R90w/90e/0t/3t will result in a longitude/time grid. When the x, y, or z coordinate is time, it will be stored in the grid as relative time since epoch as specified by TIME_UNIT and TIME_EPOCH in the gmt.conf file or on the command line. In addition, the unit attribute of the time variable will indicate both this unit and epoch.

## STORE, RECALL and CLEAR¶

You may store intermediate calculations to a named variable that you may recall and place on the stack at a later time. This is useful if you need access to a computed quantity many times in your expression as it will shorten the overall expression and improve readability. To save a result you use the special operator STO@label, where label is the name you choose to give the quantity. To recall the stored result to the stack at a later time, use [RCL]@label, i.e., RCL is optional. To clear memory you may use CLR@label. Note that STO and CLR leave the stack unchanged.

## GSHHG Information¶

The coastline database is GSHHG (formerly GSHHS) which is compiled from three sources: World Vector Shorelines (WVS, not including Antarctica), CIA World Data Bank II (WDBII), and Atlas of the Cryosphere (AC, for Antarctica only). Apart from Antarctica, all level-1 polygons (ocean-land boundary) are derived from the more accurate WVS while all higher level polygons (level 2-4, representing land/lake, lake/island-in-lake, and island-in-lake/lake-in-island-in-lake boundaries) are taken from WDBII. The Antarctica coastlines come in two flavors: ice-front or grounding line, selectable via the -A option. Much processing has taken place to convert WVS, WDBII, and AC data into usable form for GMT: assembling closed polygons from line segments, checking for duplicates, and correcting for crossings between polygons. The area of each polygon has been determined so that the user may choose not to draw features smaller than a minimum area (see -A); one may also limit the highest hierarchical level of polygons to be included (4 is the maximum). The 4 lower-resolution databases were derived from the full resolution database using the Douglas-Peucker line-simplification algorithm. The classification of rivers and borders follow that of the WDBII. See The Global Self-consistent, Hierarchical, High-resolution Geography Database (GSHHG) for further details.

## Inside/outside Status¶

To determine if a point is inside, outside, or exactly on the boundary of a polygon we need to balance the complexity (and execution time) of the algorithm with the type of data and shape of the polygons. For any Cartesian data we use a non-zero winding algorithm, which is quite fast. For geographic data we will also use this algorithm as long as (1) the polygons do not include a geographic pole, and (2) the longitude extent of the polygons is less than 360. If this is the situation we also carefully adjust the test point longitude for any 360 degree offsets, if appropriate. Otherwise, we employ a full spherical ray-shooting method to determine a points status.

## Macros¶

Users may save their favorite operator combinations as macros via the file grdmath.macros in their current or user directory. The file may contain any number of macros (one per record); comment lines starting with # are skipped. The format for the macros is name = arg1 arg2 … arg2 : comment where name is how the macro will be used. When this operator appears on the command line we simply replace it with the listed argument list. No macro may call another macro. As an example, the following macro expects three arguments (radius x0 y0) and sets the modes that are inside the given circle to 1 and those outside to 0:

INCIRCLE = CDIST EXCH DIV 1 LE : usage: r x y INCIRCLE to return 1 inside circle

Note: Because geographic or time constants may be present in a macro, it is required that the optional comment flag (:) must be followed by a space.

## Examples¶

Note: Below are some examples of valid syntax for this module. The examples that use remote files (file names starting with @) can be cut and pasted into your terminal for testing. Other commands requiring input files are just dummy examples of the types of uses that are common but cannot be run verbatim as written.

To compute all distances to north pole:

gmt grdmath -Rg -I1 0 90 SDIST = dist_to_NP.nc


To take log10 of the average of 2 files, use

gmt grdmath file1.nc file2.nc ADD 0.5 MUL LOG10 = file3.nc


Given the file ages.nc, which holds seafloor ages in m.y., use the relation depth(in m) = 2500 + 350 * sqrt (age) to estimate normal seafloor depths:

gmt grdmath ages.nc SQRT 350 MUL 2500 ADD = depths.nc


To find the angle a (in degrees) of the largest principal stress from the stress tensor given by the three files s_xx.nc s_yy.nc, and s_xy.nc from the relation tan (2*a) = 2 * s_xy / (s_xx - s_yy), use

gmt grdmath 2 s_xy.nc MUL s_xx.nc s_yy.nc SUB DIV ATAN 2 DIV = direction.nc


To calculate the fully normalized spherical harmonic of degree 8 and order 4 on a 1 by 1 degree world map, using the real amplitude 0.4 and the imaginary amplitude 1.1:

gmt grdmath -R0/360/-90/90 -I1 8 4 YLM 1.1 MUL EXCH 0.4 MUL ADD = harm.nc


To extract the locations of local maxima that exceed 100 mGal in the file faa.nc:

gmt grdmath faa.nc DUP EXTREMA 2 EQ MUL DUP 100 GT MUL 0 NAN = z.nc
gmt grd2xyz z.nc -s > max.xyz


To demonstrate the use of named variables, consider this radial wave where we store and recall the normalized radial arguments in radians:

gmt grdmath -R0/10/0/10 -I0.25 5 5 CDIST 2 MUL PI MUL 5 DIV STO@r COS @r SIN MUL = wave.nc


To create a dumb file saved as a 32 bits float GeoTiff using GDAL, run

gmt grdmath -Rd -I10 X Y MUL = lixo.tiff=gd:GTiff


To compute distances in km from the line trace.txt for the area represented by the geographic grid data.grd, run

gmt grdmath -Rdata.grd trace.txt LDIST = dist_from_line.grd


To demonstrate the stack-reducing effect of -S, we compute the standard deviation per node of all the grids matching the name model_*.grd using

gmt grdmath model_*.grd -S STD = std_of_models.grd


To create a geotiff with resolution 0.5x0.5 degrees with distances in km from the coast line, use

grdmath -RNO,IS -Dc -I.5 LDISTG = distance.tif=gd:GTIFF


## References¶

Abramowitz, M., and I. A. Stegun, 1964, Handbook of Mathematical Functions, Applied Mathematics Series, vol. 55, Dover, New York.

Holmes, S. A., and W. E. Featherstone, 2002, A unified approach to the Clenshaw summation and the recursive computation of very high degree and order normalized associated Legendre functions. Journal of Geodesy, 76, 279-299.

Press, W. H., S. A. Teukolsky, W. T. Vetterling, and B. P. Flannery, 1992, Numerical Recipes, 2nd edition, Cambridge Univ., New York.

Spanier, J., and K. B. Oldman, 1987, An Atlas of Functions, Hemisphere Publishing Corp.