emis_merge

[ibarraespinosa]

Emissions are estimated for each type of vehicle, street different hour. Also, emis_post generates
three type of files. At the end there are a lot of files. Hence, an emis_merge function would be useful. This function will not be related with the base::merge functions, but, it will load all emissions according to an specific format and mergte,rbind,cbind them (see #36).

It is important to preserve any spatial attribute, so this function would work better with sf objects (see #5 )