While editing the maps, you may notice that some of the roads are not labeled properly. This means, the road type might be wrong (a highway is marked as a street) or that one road has a few road types (a highway is marked as Freeway, highway and primary at different segments). The importance of proper labeling is in two aspects:
Other sites. R 101: The Subset Function. The subset function is available in base R and can be used to return subsets of a vector, martix, or data frame which meet a particular condition. In my three years of using R, I have repeatedly used the subset() function and believe that it is the most useful tool for selecting elements of a data structure.
Subset of a Set Formula. Consider the subset of a set, X = set of every student in your school and Y = set of every student in your school. It is obvious that set of every student in your class is going to be in your school. So, every component of Y is also a component of X. We state that Y is a subset of X.