[ferret_users] float to double conversion, axis

[Brockmann Patrick <Patrick.Brockmann@xxxxxx>]

Hi all,

I have detected a problem when you have a regular spaced axis
defined as float then saved with ferret. The axis becomes irregular spaced
because convertion to double brings numerical noice.

Note that this not detected by ferret, which still considers the saved 
axis as regular
but by other software such as paraview 3.6 that I am currently testing 
(netCDF reader in particular).

A simple ncdump shows the problem too.

Initial axis
#########################################
lat = 90, 87.46479, 84.92958, 82.39436, 79.85915, 77.32394, 74.78873,
   72.25352, 69.71831, 67.1831, 64.64789, 62.11267, 59.57747, 57.04226,
   54.50704, 51.97183, 49.43662, 46.90141, 44.3662, 41.83099, 39.29578,
   36.76056, 34.22535, 31.69014, 29.15493, 26.61972, 24.08451, 21.5493,
   19.01408, 16.47887, 13.94366, 11.40845, 8.87324, 6.338028, 3.802817,
   1.267606, -1.267606, -3.802817, -6.338028, -8.87324, -11.40845,
   -13.94366, -16.47887, -19.01408, -21.5493, -24.08451, -26.61972,
   -29.15493, -31.69014, -34.22535, -36.76056, -39.29578, -41.83099,
   -44.3662, -46.90141, -49.43662, -51.97183, -54.50704, -57.04226,
   -59.57747, -62.11267, -64.64789, -67.1831, -69.71831, -72.25352,
   -74.78873, -77.32394, -79.85915, -82.39436, -84.92958, -87.46479, -90 ;

Saved axis:
#########################################
lat = -90, -87.4647887323944, -84.9295774647887, -82.3943661971831,
   -79.8591549295775, -77.3239436619718, -74.7887323943662,
   -72.2535211267606, -69.7183098591549, -67.1830985915493,
   -64.6478873239437, -62.112676056338, -59.5774647887324,
   -57.0422535211268, -54.5070422535211, -51.9718309859155,
   -49.4366197183099, -46.9014084507042, -44.3661971830986,
   -41.830985915493, -39.2957746478873, -36.7605633802817, 
-34.225352112676,
   -31.6901408450704, -29.1549295774648, -26.6197183098592,
   -24.0845070422535, -21.5492957746479, -19.0140845070422,
   -16.4788732394366, -13.943661971831, -11.4084507042253,
   -8.87323943661971, -6.33802816901408, -3.80281690140844,
   -1.26760563380281, 1.26760563380282, 3.80281690140846, 6.33802816901409,
   8.87323943661973, 11.4084507042254, 13.943661971831, 16.4788732394366,
   19.0140845070423, 21.5492957746479, 24.0845070422535, 26.6197183098592,
   29.1549295774648, 31.6901408450704, 34.2253521126761, 36.7605633802817,
   39.2957746478873, 41.830985915493, 44.3661971830986, 46.9014084507042,
   49.4366197183099, 51.9718309859155, 54.5070422535211, 57.0422535211268,
   59.5774647887324, 62.112676056338, 64.6478873239437, 67.1830985915493,
   69.7183098591549, 72.2535211267606, 74.7887323943662, 77.3239436619718,
   79.8591549295775, 82.3943661971831, 84.9295774647887, 
87.4647887323944, 90 ;

The problem does not occur if axis is initially defined with double 
precision.

Let me know if this problem can be confirmed, referenced and corrected 
in a next release.

bug reference available at:
http://dods.ipsl.jussieu.fr/brocksce/ferret_bugs/err6.401_axis_float_doudle.jnl
http://dods.ipsl.jussieu.fr/brocksce/ferret_bugs/axis_lat.nc

Patrick