Lines may need to be snapped with v.clean tool=snap,break,rmdupl.
Boundaries may need to be cleaned with v.clean tool=break,rmdupl,rmsa repeatedly until the rmsa tool (Remove small angles at nodes) no longer modifies any boundaries. If vector topology is still not clean, boundaries may also need to be snapped with v.clean tool=snap,break,rmdupl.
When using the -a flag, the user has to make sure that the features in the different maps added to the output map do not have overlapping category numbers, unless identical category numbers reflect identical attributes, otherwise the attributes of the added maps are lost. To avoid this, the user can use v.category option=sum to change category values of some of the maps before patching.
v.patch input=geology,streams out=geol_streams
g.copy vect=roads,transport v.patch -a input=railroads output=transport --overwrite
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