WHAT IS THIS FILE?
This file contains random ramblings that I jotted down (usually late
at night) while coding this program. General comments, stuff for
programmers reading my code, wish list of features, etc.

GENERAL DESCRIPTION OF MEDIAN POLISH
For the median polish, you have some sites and a vector grid.  The first
step is to overlay the sites on the grid and associate each site with
the closest node.  A node can have zero, one, or more sites.
Then, you subtract medians from rows and columns until convergence
(see the man page).  Effectively, it removes the large-scale variation
(trend) from the data.  Then, the residuals can be used for kriging.

DOCUMENTATION
See Cressie (1991) and the doc directory (sites/s.medp/doc).  There's
some results from a book that I've reproduced using this software.
Further documentation regarding median polish, its uses, etc, may 
come later.

COMMENTS ABOUT CODE
The code may be a little messy in some parts, but it has plenty
of comments. Doing the median polish would have been difficult
using existing structs, so I rolled my own.  There are some places
where I should have probably stuck to grass structs, but...

ADDITIONS THAT NEVER MADE IT
It would be good if this could output three gnuplot surface plot data 
files: original, trend, residuals.  Maybe if there was a way to
have a "hidden" flag (something that wouldn't show up when you
use "help" as the sole arg)...

Cressie (1991) gives interpolation/extrapolation equations that could
be used to create raster map at the current resolution.  

  Foreach (cell in the output map)
    find corresponding x & y values,
    determine z from interpolation/extrapolation eqns.
    record value as new category
  end.
  Then, reclassify similar values into same category.

This would probably take about 2 or 3 coding sessions to complete
(add -rast=filename option).

REFERENCES
@Book{ cressie91,
  author =       "Noel A. C. Cressie",
  title =        "Statistics for Spatial Data",
  publisher =    "John Wiley \& Sons",
  year =         "1991",
  series =       "Wiley Series in Probability and Mathematical Statistics",
  address =      "New York, NY"
}

ROTATION
\documentstyle{article}
\begin{document}
% so I don't forget these
If $P$ has coordinates $(x,y)$, then after rotating, we get
$P'$ with coordinates $(x',y')$, where
\begin{displaymath}
x'=x \cos \phi - y \sin \phi \:\:\:\:\:\: \mbox{and} \:\:\:\:\:\: 
y'=x\sin\phi+y\cos\phi
\end{displaymath}
or
\begin{displaymath}
x=x'\cos \phi + y' \sin \phi \:\:\:\:\:\: \mbox{and}\:\:\:\:\:\:  
y=-x'\sin\phi+y'\cos\phi
\end{displaymath}
This rotates $P$ counterclockwise through an angle $\phi$ about
the origin or a Cartesian coordinate system.
\end{document}
--
Darrell McCauley (mccauley@ecn.purdue.edu)
<04 Jun 1992>