BLOCKMEDIAN(1)

NAME
blockmedian - filter to block average (x,y,z) data by L1 norm.

SYNOPSIS
blockmedian [ xyz[w]file(s) ] -Ixinc[unit][=|+][/yinc[unit][=|+]] -Rxmin/xmax/ymin/ymax[r] [ -C ] [ -E[b] ] [ -F ] [ -H[i][nrec] ] [ -Q ] [ -Tquantile ] [ -V ] [ -W[io] ] [ -:[i|o] ] [ -b[i|o][s|S|d|D[ncol]|c[var1/...]] ] [ -f[i|o]colinfo ]

DESCRIPTION
blockmedian reads arbitrarily located (x,y,z) triples [or optionally weighted quadruples (x,y,z,w)] from standard input [or xyz[w]file(s)] and writes to standard output a median position and value for every non-empty block in a grid region defined by the -R and -I arguments. Either blockmean, blockmedian, or blockmode should be used as a pre-processor before running surface to avoid aliasing short wavelengths. These routines are also generally useful for decimating or averaging (x,y,z) data. You can modify the precision of the output format by editing the D_FORMAT parameter in your .gmtdefaults4 file, or you may choose binary input and/or output using single or double precision storage.

xyz[w]file(s)
    3 [or 4] column ASCII file(s) [or binary, see -b] holding (x,y,z[,w]) data values. [w] is an optional weight for the data. If no file is specified, blockmedian will read from standard input.

-I  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 c to indicate arc seconds. If one of the units e, k, i, or n is appended instead, the increment is assumed to be given in meter, km, miles, or nautical miles, respectively, and will be converted to the equivalent degrees longitude at the middle latitude of the region (the conversion depends on 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 = 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 + 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 Appendix B for details. Note: if -Rgrdfile is used then grid spacing has already been initialized; use -I to override the values.

-R  xmin, xmax, ymin, and ymax specify the Region of interest. For geographic regions, these limits correspond to west, east, south, and north and you may specify them in decimal degrees or in [+-]dd:mm[:ss.xxx][W|E|S|N] format. Append r if lower left and upper right map coordinates are given instead of w/e/s/n. The two shorthands -Rg and -Rd stand for global domain (0/360 and -180/+180 in longitude respectively, with -90/+90 in latitude). Alternatively, specify the name of an existing grid file and the -R settings (and grid spacing, if applicable) are copied from the grid. For calendar time coordinates you may either give (a) relative time (relative to the selected TIME_EPOCH and in the selected TIME_UNIT; append t to -JX|x), or (b) absolute time of the form [date]T[clock] (append T to -JX|x). At least one of date and clock must be present; the T is always required. The date string must be of the form [-]yyyy[-mm[-dd]] (Gregorian calendar) or yyyy[-Www[-d]] (ISO week calendar), while the clock string must be of the form hh:mm:ss[.xxx]. The use of delimiters and their type and positions must be exactly as indicated (however, input, output and plot formats are customizable; see gmtdefaults).

OPTIONS

-C  Use the center of the block as the output location [Default uses the median x and median y as location (but see -Q)]..

-E  Provide Extended report which includes s (the L1 scale of the median), l, the lowest value, and h, the high value for each block. Output order becomes x,y,z,s,l,h[,w]. [Default outputs x,y,z[,w]. For box-and-whisker calculation, use -Eb which will output x,y,z,l,q25,q75,h[,w], where q25 and q75 are the 25% and 75% quantiles, respectively. See -W for w output.

-F  Force pixel node registration [Default is gridline registration]. (Node registrations are defined in GMT Cookbook Appendix B on grid file formats.) Each block is the locus of points nearest the grid value location. For example, with -R 10/15/10/15 and and -I 1: with the -F option 10 <= (x,y) < 11 is one of 25 blocks; without it 9.5 <= (x,y) < 10.5 is one of 36 blocks.

-H  Input file(s) has header record(s). If used, the default number of header records is N_HEADER_RECS. Use -Hi if only input data should have header records [Default will write out header records if the input data have them]. Blank lines and lines starting with # are always skipped. Not used with binary data.

-Q  (Quicker) Finds median z and (x, y) at that the median z [Default finds median x, median y independent of z]. Also see -C.

-T  Sets the quantile of the distribution to be returned [Default is 0.5 which returns the median z]. Here, 0 < quantile < 1.

-V  Selects verbose mode, which will send progress reports to stderr [Default runs "silently"].

-W  Weighted modifier[s]. Unweighted input and output has 3 columns x,y,z; Weighted i/o has 4 columns x,y,z,w. Weights can be used in input to construct weighted mean values in blocks. Weight sums can be reported in output for later combining several runs, etc. Use -W for weighted i/o, -Wi for weighted inputonly, -Wo for weighted output only. [Default uses unweighted i/o].

-:  Toggles between (longitude,latitude) and (latitude,longitude) input and/or output. [Default is (longitude,latitude)]. Append i to select input only or o to select output only. [Default affects both].

-bi Selects binary input. Append s for single precision [Default is d (double)]. Uppercase S or D will force byte-swapping. Optionally, append ncol, the number of columns in your binary input file if it exceeds the columns needed by the program. Or append c if the input file is netCDF. Optionally, append var1/var2/... to specify the variables to be read. [Default is 3 (or 4 if -Wi is set)].

-bo Selects binary output. Append s for single precision [Default is d (double)]. Uppercase S or D will force byte-swapping. Optionally, append ncol, the number of desired columns in your binary output file. [Default is 3 (or 4 if -Wo is set)]. -E adds 3 additional columns.

-f  Special formatting of input and/or output columns (time or geographical data). Specify i or o to make this apply only to input or output [Default applies to both]. Give one or more columns (or column ranges) separated by commas. Append T (absolute calendar time), t (relative time in chosen TIME_UNIT since TIME_EPOCH), x (longitude), y (latitude), or f (floating point) to each column or column range item. Shorthand -f[i|o]g means -f[i|o]0x,1y (geographic coordinates).

ASCII FORMAT PRECISION
The ASCII output formats of numerical data are controlled by parameters in your .gmtdefaults4 file. Longitude and latitude are formatted according to OUTPUT_DEGREE_FORMAT, whereas other values are formatted according to D_FORMAT. Be aware that the format in effect can lead to loss of precision in the output, which can lead to various problems downstream. If you find the output is not written with enough precision, consider switching to binary output (-bo if available) or specify more decimals using the D_FORMAT setting.

EXAMPLES
To find 5 by 5 minute block medians from the double precision binary data in hawaii_b.xyg and output an ASCII table, run

blockmedian hawaii_b.xyg -R 198/208/18/25 -I 5m -bi 3 > hawaii_5x5.xyg

To compute the shape of a data distribution per bin via a box-and-whisker diagram we need the 0%, 25%, 50%, 75%, and 100% quantiles. To do so on a global 5 by 5 degree basis from the ASCII table depths.xyz and send output to an ASCII table, run

blockmedian depths.xyz -Rg -I 5 -F -Eb > depths_5x5.txt

SEE ALSO
blockmean(1), blockmode(1), GMT(1), gmtdefaults(1), nearneighbor(1), surface(1), triangulate(1)