---
name: "blockmedian"
section: "1"
platform: "linux"
package: "gmt-manpages"
version: "3.4-1"
release: "Debian 3.0 (Woody)"
platforms: ["linux"]
url: "https://new.carta.tech/blockmedian"
---

# blockmedian(1)

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

## NAME

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

## SYNOPSIS

**blockmedian** \[ *xyz\[w\]file(s)* \] **-I***x\_inc*\[**m\|c**\]\[/*y\_inc*\[**m\|c**\]\] **-R***west/east/south/north*\[**r**\] \[ **-C** \] \[ **-F** \] \[ **-H**\[*nrec*\] \] \[ **-L** \] \[ **-Q** \] \[ **-V** \] \[ **-W**\[**io**\] \] \[ **-:** \] \[ **-bi**\[**s**\]\[*n*\] \] \[ **-bo**\[**s**\] \]

## 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 .gmtdefaults 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. Append **m** to indicate minutes or **c** to indicate seconds.

- **-R**

  *west, east, south,* and *north* specify the Region of interest. To specify boundaries in degrees and minutes \[and seconds\], use the dd:mm\[:ss\] format. Append **r** if lower left and upper right map coordinates are given instead of wesn.

## OPTIONS

- **-C**

  Use the center of the block as the output location \[Default uses the median location (but see **-Q**)\]. **-C** overrides **-Q**.

- **-F**

  Block centers have pixel registration. \[Default: grid registration.\] (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). Number of header records can be changed by editing your .gmtdefaults file. If used, **GMT** default is 1 header record. Not used with binary data.

- **-L**

  Indicates that the x column contains longitudes, which may differ from the region in **-R** by \[multiples of\] 360 degrees \[Default assumes no periodicity\].

- **-Q**

  (Quicker) Finds median z and (x, y) at that z \[Default finds median x, median y, median z\].

- **-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 median 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 input only, **-Wo** for weighted output only. \[Default uses unweighted i/o\]

- **-:**

  Toggles between (longitude,latitude) and (latitude,longitude) input/output. \[Default is (longitude,latitude)\]. Applies to geographic coordinates only.

- **-bi**

  Selects binary input. Append **s** for single precision \[Default is double\]. Append *n* for the number of columns in the binary file(s). \[Default is 3 (or 4 if **-W** is set) columns\].

- **-bo**

  Selects binary output. Append **s** for single precision \[Default is double\].

## EXAMPLES

To find 5 by 5 minute block medians from the double precision binary data in hawaii\_b.xyg and output an ASCII table, try

blockmedian hawaii\_b.xyg **-R**198/208/18/25 **-I**5**m -bi**3 \> hawaii\_5x5.xyg

## SEE ALSO

*blockmean*(l), *blockmode*(l), *gmt*(l), *gmtdefaults*(l), *nearneighbor*(l), *surface*(l), *triangulate*(l)