# earthtide

Compute grids or time-series of solid Earth tides

## Synopsis

**gmt earthtide**
**-T**[*min/max*/]*inc*[**+i**|**n**] |**-T***file*|*list*
**-G***outgrid*
[ **-C***x|e,y|n,z|v* ]
[ **-I***increment* ]
[ **-L***lon/lat* ]
[ **-R***region* ]
[ **-S** ]
[ **-V**[*level*] ]
[ **-r**reg ]
[ **--PAR**=*value* ]

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

## Description

Compute the three components of solid Earth tides as time-series or grids. Optionally compute also Sun and Moon position in (*lon, lat*).
The output can be either in the form of a grid or as a table printed to standard output. The format of the table data is:
*time north east vertical* in units of meters.

## Required Arguments

Either **-G**, **-S** or **-L** must be provided.

**-G***outgrid*[=*ID*][**+d***divisor*][**+n***invalid*][**+o***offset*|**a**][**+s***scale*|**a**][:*driver*[*dataType*][**+c***options*]]

Optionally, append =

IDfor writing a specific file format. The following modifiers are supported:

+d- Divide data values by givendivisor[Default is 1].

+n- Replace data values matchinginvalidwith a NaN.

+o- Offset data values by the givenoffset, or appendafor automatic range offset to preserve precision for integer grids [Default is 0].

+s- Scale data values by the givenscale, or appendafor automatic scaling to preserve precision for integer grids [Default is 1].

Note: Any offset is added before any scaling.+saalso sets+oa(unless overridden). To write specific formats via GDAL, use =gdand supplydriver(and optionallydataType) and/or one or more concatenated GDAL-cooptions using+c. See the “Writing grids and images” cookbook section for more details.

**-S**Output position of Sun and Moon in geographical coordinates plus distance in meters. Output is a Mx7 matrix where M is the number of times (set by -T) and columns are time, sun_lon, sun_lat, sun_dist moon_lon, moon_lat, moon_dist

**-L***lon/lat*Geographical coordinate of the location where to compute a time-series. Coordinates are geodetic (ellipsoidal) latitude and longitude. GRS80 ellipsoid is used. (Which can be considered equivalent to the WGS84 ellipsoid at the sub-millimeter level.)

## Optional Arguments

**-C****x**|**e**,**y**|**n**,**z**|**v**Select which component to write to individual grids. Requires

**-G**. Append comma-separated codes for available components:**x**or**e**for the east component;**y**or**n**for the north component; and**z**or**v**for the vertical component. For example,**-Ce**,**v**, will write 2 grids. One with east and other with the vertical components. If**-G**is set but not**-C**then the default is to write the vertical component.

**-I***x_inc*[**+e**|**n**][/*y_inc*[**+e**|**n**]]Set the grid spacing as

*x_inc*[and optionally*y_inc*].**Geographical (degrees) coordinates**: Optionally, append an increment unit. Choose among:**d**- Indicate arc degrees**m**- Indicate arc minutes**s**- Indicate arc seconds

If one of

**e**(meter),**f**(foot),**k**(km),**M**(mile),**n**(nautical mile) or**u**(US survey foot), the the increment 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 not given or given but set to 0 it will be reset equal to*x_inc*; otherwise it will be converted to degrees latitude.**All coordinates**: The following modifiers are supported:**+e**- Slightly adjust the max*x*(*east*) or*y*(*north*) to fit exactly the given increment if needed [Default is to slightly adjust the increment to fit the given domain].**+n**- Define the*number of nodes*rather than the increment, in which case the increment is recalculated from the number of nodes, the*registration*(see GMT File Formats), and the domain.**Note**: If**-R***grdfile*is used then the grid spacing and the registration have already been initialized; use**-I**and**-R**to override these values.

**-R***xmin*/*xmax*/*ymin*/*ymax*[**+r**][**+u***unit*]Specify the region of interest. (See full description) (See technical reference).

**-T**[*min/max*/]*inc*[**+i**|**n**] |**-T***file*|*list*Make evenly spaced time-steps from

*min*to*max*by*inc*. Append**+i**to indicate the reciprocal increment was given, or append**+n**to indicate*inc*is the number of*t*-values to produce over the range instead. Append a valid time unit (**d**|**h**|**m**|**s**) to the increment. If only*min*is given then we use that date and time for the calculations. If no**-T**is provided get current time in UTC from the computer clock. If no**-G**or**-S**are provided then**-T**is interpreted to mean compute a time-series at the location specified by**-L**, thus then**-L**becomes mandatory. When**-G**and**-T**, only first time T series is considered. Finally, dates may range from 1901 through 2099. For details on array creation, see Generate 1-D Array.

**-bo***record*[**+b**|**l**] (more …)Select native binary format for table output.

**-o***cols*[**+l**][**+d***divisor*][**+s***scale*|**d**|**k**][**+o***offset*][,*…*][,**t**[*word*]] (more …)Select output columns and transformations (0 is first column,

**t**is trailing text, append*word*to write one word only).

**-r**[**g**|**p**] (more …)Set node registration [gridline].

**-V**[*level*]Select verbosity level [

**w**]. (See full description) (See technical reference).

**-^**or just**-**Print a short message about the syntax of the command, then exit (

**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 exit.

**-?**or no argumentsPrint a complete usage (help) message, including the explanation of all options, then exit.

**--PAR**=*value*Temporarily override a GMT default setting; repeatable. See gmt.conf for parameters.

## Generate 1-D Array

We will demonstrate the use of options for creating 1-D arrays via math.
Make an evenly spaced coordinate array from *min* to *max* in steps of *inc*, e.g.:

```
gmt math -o0 -T3.1/4.2/0.1 T =
3.1
3.2
3.3
3.4
3.5
3.6
3.7
...
```

Append **+b** if we should take \(\log_2\) of *min* and *max*, get their nearest integers,
build an equidistant \(\log_2\)-array using *inc* integer increments in \(\log_2\), then undo
the \(\log_2\) conversion. E.g., **-T**3/20/1**+b** will produce this sequence:

```
gmt math -o0 -T3/20/1+b T =
4
8
16
```

Append **+l** if we should take \(\log_{10}\) of *min* and *max* and build an
array where *inc* can be 1 (every magnitude), 2, (1, 2, 5 times magnitude) or 3
(1-9 times magnitude). E.g., **-T**7/135/2**+l** will produce this sequence:

```
gmt math -o0 -T7/135/2+l T =
10
20
50
100
```

For output values less frequently than every magnitude, use a negative integer *inc*:

```
gmt math -o0 -T1e-4/1e4/-2+l T =
0.0001
0.01
1
100
10000
```

Append **+i** if *inc* is a fractional number and it is cleaner to give its reciprocal
value instead. To set up times for a 24-frames per second animation lasting 1 minute, run:

```
gmt math -o0 -T0/60/24+i T =
0
0.0416666666667
0.0833333333333
0.125
0.166666666667
...
```

Append **+n** if *inc* is meant to indicate the *number* of equidistant coordinates
instead. To have exactly 5 equidistant values from 3.44 and 7.82, run:

```
gmt math -o0 -T3.44/7.82/5+n T =
3.44
4.535
5.63
6.725
7.82
```

Alternatively, let *inc* be a *file* with output coordinates in the first column,
or provide a comma-separated *list* of specific coordinates, such as the first 6
Fibonacci numbers:

```
gmt math -o0 -T0,1,1,2,3,5 T =
0
1
1
2
3
5
```

**Notes**: (1) If you need to pass the list nodes via a dataset file yet be understood
as a list (i.e., no interpolation), then you must set the file header to contain the string “LIST”. (2) Should
you need to ensure that the coordinates are unique and sorted (in case the
*file* or *list* are not sorted or have duplicates) then supply the **+u** modifier.

If you only want a *single* value
then you must append a comma to distinguish the list from the setting of an increment.

If the module allows you to set up an absolute time series, append a valid time unit from the list
**y**ear, m**o**nth, **d**ay, **h**our, **m**inute, and **s**econd
to the given increment; add **+t** to ensure time column (or use **-f**). **Note**: The internal time
unit is still controlled independently by TIME_UNIT. The first 7 days of March 2020:

```
gmt math -o0 -T2020-03-01T/2020-03-07T/1d T =
2020-03-01T00:00:00
2020-03-02T00:00:00
2020-03-03T00:00:00
2020-03-04T00:00:00
2020-03-05T00:00:00
2020-03-06T00:00:00
2020-03-07T00:00:00
```

A few modules allow for **+a** which will paste the coordinate array to the output table.

Likewise, if the module allows you to set up a spatial distance series (with distances computed
from the first two data columns), specify a new increment as *inc* with a geospatial distance unit from the list
**d**egree (arc), **m**inute (arc), **s**econd (arc), m**e**ter, **f**oot, **k**ilometer,
**M**iles (statute), **n**autical miles, or s**u**rvey foot; see **-j** for calculation mode.
To interpolate Cartesian distances instead, you must use the special unit **c**.

Finally, if you are only providing an increment and will obtain *min* and *max* from the data, then it is
possible (*max* - *min*)/*inc* is not an integer, as required. If so, then *inc* will be adjusted to fit the range.
Alternatively, append **+e** to keep *inc* exact and adjust *max* instead (keeping *min* fixed).

## Examples

To compute a global grid of the vertical component with a grid step of 30m at noon of 18 Jun 2018,
(note: we are using the defaults for **-R** and **-I**) try

```
gmt earthtide -T2018-06-18T12:00:00 -Gsolid_tide_up.grd
```

To obtain a one day long time-series, starting at same date, at the -7 *W*, 37 *N* and 1 minute interval, try

```
gmt earthtide -T2018-06-18T/2018-06-19T/1m -L-7/37 > solid_tide.txt
```

The get the Sun and Moon position in geographical coordinates at the *now* time

```
gmt earthtide -S
```

## Notes

All times, both input and output, are in UTC.