GEBCO Global Earth Relief
-------------------------

.. figure:: /_static/igpp.png
   :align: right
   :scale: 20 %

.. figure:: /_static/GMT_gebcosi.jpg
   :height: 888 px
   :width: 1774 px
   :align: center
   :scale: 40 %

Usage - Global Earth Relief Grids
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You access the GEBCO global relief grid by specifying the special name

   @earth_gebco_\ [*rr*\ *u*\ [_\ *reg*\ ]]

This grid only contains observed relief and inferred relief via altimetric gravity.
A second grid that gives sub-ice (si) elevations is also available and can be accessed via

   @earth_gebcosi_\ [*rr*\ *u*\ [_\ *reg*\ ]]

and is the one displayed above.  The following codes for *rr*\ *u* and the optional *reg* are supported (dimensions are listed
for pixel-registered grids; gridline-registered grids increment dimensions by one):

.. _tbl-earth_gebco:

==== ================= === =======  ================================================
Code Dimensions        Reg Size     Description
==== ================= === =======  ================================================
01d       360 x    180 g,p  114 KB  1 arc degree global relief (GEBCO 2021 @ 111 km)
30m       720 x    360 g,p  395 KB  30 arc minute global relief (GEBCO 2021 @ 55 km)
20m      1080 x    540 g,p  832 KB  20 arc minute global relief (GEBCO 2021 @ 37 km)
15m      1440 x    720 g,p  1.4 MB  15 arc minute global relief (GEBCO 2021 @ 28 km)
10m      2160 x   1080 g,p  3.0 MB  10 arc minute global relief (GEBCO 2021 @ 18 km)
06m      3600 x   1800 g,p  8.0 MB  6 arc minute global relief (GEBCO 2021 @ 10 km)
05m      4320 x   2160 g,p   11 MB  5 arc minute global relief (GEBCO 2021 @ 9 km)
04m      5400 x   2700 g,p   17 MB  4 arc minute global relief (GEBCO 2021 @ 7.5 km)
03m      7200 x   3600 g,p   30 MB  3 arc minute global relief (GEBCO 2021 @ 5.6 km)
02m     10800 x   5400 g,p   65 MB  2 arc minute global relief (GEBCO 2021 @ 3.7 km)
01m     21600 x  10800 g,p  237 MB  1 arc minute global relief (GEBCO 2021 @ 1.9 km)
30s     43200 x  21600 g,p  864 MB  30 arc second global relief (GEBCO 2021 @ 1.0 km)
15s     86400 x  43200 p    2.9 GB  15 arc second global relief (GEBCO 2021)
03s    432000 x 216000 g    6.8 GB  3 arc second global relief (SRTM3S)
01s   1296000 x 432000 g     41 GB  1 arc second global relief (SRTM1S)
==== ================= === =======  ================================================

See :gmt-docs:`GMT remote dataset usage <datasets/remote-data.html#usage>` for when resolution codes are optional or required.
All of these data will, when downloaded, be placed in your ~/.gmt/server directory, with
the earth_relief files being placed in an ``earth/earth_relief`` sub-directory.  If you
do not specify a CPT then this dataset default to the GMT master *geo*.

Technical Information - Global Earth Relief Grids
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

As you see, the 30s and lower resolutions are all derivatives of GEBCO's 2021 15 arc second grids
(GEBCO_Compilation_Group, 2020).  We have downsampled it via Cartesian Gaussian filtering to prevent
aliasing while preserving the latitude-dependent resolution in the original 15 arc sec grid.
The full (6 sigma) filter-widths are indicated in parenthesis. The 3 and 1 arc second data
are the SRTM 1x1 degree tiles from NASA.  **Note**: The 3 and 1 arc second grids only extend
to latitudes ±60˚ and are only available over land.  When these grids are accessed as
@earth_gebco_01s or @earth_gebco_03s we will automatically up-sample the relevant @earth_gebco_15s
tiles to fill in the missing ocean values. If you just want the original land-only SRTM tiles
you may use the special names @srtm_relief_03s or @srtm_relief_01s instead. Almost all grids
are available in both gridline- and pixel-registered formats except the original pixel-registered
GEBCO 2021 (here called @earth_gebco_15s) and the gridline-registered SRTM tiles.

Data References - Global Earth Relief Grids
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

#. GEBCO 2021: [https://www.gebco.net/data_and_products/gridded_bathymetry_data/].
#. SRTMGL3 tiles: [https://lpdaac.usgs.gov/products/srtmgl3v003].
#. SRTMGL1 tiles: [https://lpdaac.usgs.gov/products/srtmgl1v003].