With the -z flag NULL values are not crossed. This means that if a NULL value occurs in any input data layer, this combination is ignored, even if other data layers contain non-NULL data. In the example given below, use of the -z option would cause 3 categories to be generated instead of 5.
If the -z flag is not specified, then map layer combinations in which some values are NULL will be assigned a unique category value in the resulting output map.
Category values in the new output map layer will be the cross-product of the category values from these existing input map layers.
map1 map2
___________
NULL 1
NULL 2
1 1
1 2
2 4
map1 map2 output
____________________
NULL 1 0
NULL 2 1
1 1 2
1 2 3
2 4 4
category category
value label
______________________________
0 layer1(0) layer2(1)
1 layer1(0) layer2(2)
2 layer1(1) layer2(1)
3 layer1(1) layer2(2)
4 layer1(2) layer2(4)
Available at: r.cross source code (history)
Latest change: Thursday May 08 20:58:05 2025 in commit: 4144ba81fa125e1549fc52e1cf5feb4fa004be8b
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