== The program i3d ==

=== Configuration file ===

The options for the execution of ''i3d'' are set in the configuration file '''i3d.inp''':

{{{
1)  y            ! north_hemisphere (y/n)
2)  30., 90.     ! lat_min, lat_max
3)  1            ! 0=nearest, 1=weighted, 2=both 
4)  2	     ! number of tags
    CH4 
    O3
5)  n                                          ! use one init-file
6)  /private/CNx_ECN_3d_24_1.5                 ! input directory (init-files)
    CNx                                        ! prefix of init-files (3 character)
7)  data/mipas_isen_noonpos_030103_030131.nc   ! input file (aircraft/satellite data)
8)  outdir                                     ! output directory
9)  testout                                    ! output prefix
10) y                                          ! write warnings  (y/n) 
}}}


{{{
1)  y            ! north_hemisphere (y/n)
2)  30., 90.     ! lat_min, lat_max
3)  2            ! 0=nearest, 1=weighted, 2=both
4)  2            ! number of tags
    CH4 
    O3
5)  y                            ! use one init-file
6)  test1/init_BN0_02100112.nc   ! init-file
7)  test1/pos_A_0_01100112.nc    ! input file (aircraft/satellite data)
8)  test1                        ! output directory
9)  testout                      ! output prefix
10) n                            ! write warnings  (y/n)
}}}

Remarks:

 1. Start triangle for the walking triangle algorithm:
 y/n -- north/south hemisphere (does not work. Now, the $n/2$ ($n-$number of all triangles) is chosen as a start triangle).
 1. Latitude range of the (external) grid where interpolations have to be determined.
 1. [[i3d/InterpolationType | Interpolation type]]
 1. Number of species to add to input file (line 7) and their names. If Variable ''TRACER'' should be added to input file, it should be specified at the end of the list.<<BR>> 
 If the number of species is "0", all species from CLaMS file (line 6) listed on Variable ''SPECIES'' and Variable ''TRACER'' are added to input file. 
 1. use one CLaMS file for all dates in input file (y/n)
 1. [[i3d/ClamsInputFiles | CLaMS input files]]:<<BR>>  
 line5='y': Name of CLaMS file <<BR>> 
 line5='n': directory with CLaMS output and prefix of these files 
 1. [[i3d/InputFiles | Input file with the external grid]]
 1. Output directory
 1. Name of the [[i3d/OutputFile | NetCDF output file]] 
 1. Write warnings if triangle is not found

=== The walking triangle algorithm ===

The walking triangle algorithm is described in [1]. It describes how in 2D (and 3D) a Delaunay triangle can be found that contains a given point ''X''. The walking triangle uses the following criterion to find such a triangle: 

''X'' is inside of a triangle ''ABC'' if the given point ''X'' and 
the triangle point that does not define the considered triangle side are on the same side of the considered triangle side
for each of the 3 possible sides of the triangle.

E.g. for the side ''AB'', the following relation has to be valid:

  $[{\bf n}\cdot({\bf c}-{\bf a})][{\bf n}\cdot({\bf x}-{\bf a})] > 0$

with ${\bf n}={\bf a}\times {\bf b}$. This means that ''X'' and ''C'' are on
the same side of ''AB'' if both scalar products have the same sign.

The walking triangle algorithm checks if ''X'' is inside of the
the start triangle by applying criterion for each triangle side. 
If for one triangle side this criterion is not valid, 
one has to walk to the corresponding neighboring
triangle.
In this way one walks in this triangle world directly to 
the triangle containing ''X''.

=== References ===

[1] M. Sambridge, J. Braun, and H. !McQueen. Geophysical parameterization and interpolation of irregular data using natural neighbors.
''Geophys. J. Int.'', 122:837--857, 1995.