[GRASS-dev] Semantic of r.cross' -z flag

[Nikos Alexandris]

I need help to understand r.cross' `-z` flag and the semantic(s) of the
zero-data values.

The following:

r.category geology_Test

217     CZfg
405     CZbg
766     CZg

r.category basins_1_2

1
2

r.cross input=geology_Test,basins_1_2 out=cross_Test --o
r.category cross_Test

0
1       CZfg; category 2
2       CZbg; category 1
3       CZbg; category 2
4       CZg; category 1

# Why is there a '0' above?

r.cross input=geology_Test,basins_1_2 out=cross_Test --o -z
r.category cross_Test

0
1       CZfg; category 2
2       CZbg; category 1
3       CZbg; category 2
4       CZg; category 1


# and then...

r.category basins_0_12

0       0
12      Twelve

r.cross input=geology_Test,basins_0_12 out=cross_Test --o
r.category cross_Test

0
1       CZfg; Twelve
2       CZbg; 0
3       CZbg; Twelve
4       CZg; 0

r.cross input=geology_Test,basins_0_12 out=cross_Test --o -z
r.category cross_Test

0
1       CZfg; Twelve
2       CZbg; 0
3       CZbg; Twelve
4       CZg; 0


Nikos
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