Add SRTM Relief doc (#135)

* Add SRTM Relief doc

* Optimised images with calibre/image-actions

---------

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: Esteban82

docs/_static/GMT_earth_dist.jpg

@@ -106,6 +106,12 @@ Information about the GMT data server and mirrors can be found on the
         :img-bottom: /_static/GMT_earth_daynight_thumbnail.jpg
         :text-align: center
 
+    .. grid-item-card:: NASA SRTM Relief
+        :link: srtm-relief
+        :link-type: doc
+        :img-bottom: /_static/GMT_srtm_relief_thumbnail.jpg
+        :text-align: center
+
     .. grid-item-card:: WDMAM World Digital Magnetic Anomaly Map
         :link: earth-wdmam
         :link-type: doc

docs/_static/GMT_earth_dist_thumbnail.jpg

@@ -0,0 +1,54 @@
+NASA SRTM Relief
+-----------------
+
+.. figure:: /_static/nasa-logo-web-rgb.png
+   :align: right
+   :scale: 20 %
+
+.. figure:: /_static/GMT_srtm_relief.jpg
+   :height: 888 px
+   :width: 1774 px
+   :align: center
+   :scale: 40 %
+
+Usage
+~~~~~
+
+You access a SRTM relief grid by specifying the special name
+
+   @srtm_relief[_\ *rru*\]
+
+The SRTM (version 3) 1 and 3 arc-sec tiles are only available over land between 60 degrees south and north latitude.
+This dataset contains a digital elevation model (DEM) of Earth using radar interferometry. 
+The following codes for the optional *rr*\ *u* are supported:
+
+.. _tbl-srtm_relief:
+
+  ==== ================= === =======  ==================================================
+  Code Dimensions        Reg Size     Description
+  ==== ================= === =======  ==================================================
+  03s    432001 x 216001 g    6.8 GB  3 arc second global relief (SRTM3S original)
+  01s   1296001 x 432001 g     41 GB  1 arc second global relief (SRTM1S original)
+  ==== ================= === =======  ==================================================
+
+Note that both resolutions are tiled.
+See :gmt-docs:`Remote Datasets <datasets/remote-data.html>` for more information. 
+
+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 srtm_relief files being placed in an ``earth/earth_relief`` sub-directory. If you
+do not specify a CPT then the default CPT for this dataset (the GMT master *geo* CPT) will be used.
+
+Technical Information
+~~~~~~~~~~~~~~~~~~~~~
+
+The 3 and 1 arc second data are the SRTM 1x1 degree tiles from NASA. The data is stored as highly compressed JPEG2000 tiles on the GMT server. 
+These are individually downloaded as requested, converted to netCDF grids and stored in subdirectories srtm1 and srtm3 under the server directory, 
+and assembled into a seamless grid using grdblend.
+**Note**: This dataset is utilized by other relief grids (e.g., earth_relief) when resolutions of 03s or 01s are requested. In these cases, the other grid is automatically up-sampled to fill in missing values over the oceans and in high-latitude regions (above 60°).
+
+Data References
+~~~~~~~~~~~~~~~
+
+#. SRTMGL3 tiles: [https://lpdaac.usgs.gov/products/srtmgl3v003].
+#. SRTMGL1 tiles: [https://lpdaac.usgs.gov/products/srtmgl1v003].

docs/_static/GMT_srtm_relief.jpg

@@ -0,0 +1,12 @@
+gmt begin GMT_srtm_relief jpg
+	gmt set GMT_THEME cookbook GMT_DATA_SERVER candidate
+	gmt grdclip @earth_relief_10m -Gearth_relief.nc -Sb0/NaN -R-180/180/-60/60
+	gmt basemap -Rd -JQ0/15c -B+g128
+	gmt grdimage earth_relief.nc -I -Q
+	gmt coast -Da -W1/0.2p 
+    gmt psconvert -A -P -E150 -FGMT_srtm_relief_thumbnail -Tj
+gmt end
+
+rm -f gmt.conf gmt.history earth_relief.nc
+mv GMT_srtm_relief.jpg ../docs/_static/
+mv GMT_srtm_relief_thumbnail.jpg ../docs/_static/