.. _example_14:

(14) Gridding of data and trend surfaces
----------------------------------------

This example shows how one goes from randomly spaced data points to an
evenly sampled surface. First we plot the distribution and values of our
raw data set (same as in Example :ref:`example_12`). We choose an equidistant grid and run
:doc:`blockmean </blockmean>` which preprocesses the
data to avoid aliasing. The dashed lines indicate the logical blocks
used by :doc:`blockmean </blockmean>`; all points inside
a given bin will be averaged. The logical blocks are drawn from a
temporary file we make on the fly within the shell script. The processed
data is then gridded with the :doc:`surface </surface>`
program and contoured every 25 units. A most important point here is
that :doc:`blockmean </blockmean>`, :doc:`blockmedian </blockmedian>`, or
:doc:`blockmode </blockmode>` should always be run prior
to running :doc:`surface </surface>`, and both of these
steps must use the same grid interval. We use
:doc:`grdtrend </grdtrend>` to fit a bicubic trend
surface to the gridded data, contour it as well, and sample both grid
files along a diagonal transect using
:doc:`grdtrack </grdtrack>`. The bottom panel compares
the gridded (solid line) and bicubic trend (dashed line) along the
transect using :doc:`psxy </psxy>`

.. literalinclude:: /_verbatim/example_14.txt
   :language: bash

.. figure:: /_images/example_14.*
   :width: 500 px
   :align: center

   Gridding of data and trend surfaces.