Answer:
Let's discuss the meaning of these modifiers first.
A misplaced modifier, as the name suggests, is a word that modifies the wrong word, thus changing the meaning of the sentence (An old child's shirt was used to stop the bleeding - this would suggest that a shirt belonged to an old child, which is highly unlikely. Correct way to say this is A child's old shirt was used...)
A dangling modifier is a modifier that can not be logically connected to the word it modifies, most often because the word it modifies is left out of the sentence (When five years old, my mom remarried - it would be absurd if someone married, let alone remarried at the age of five. This modifier lacks the word it modifies: when I was five years old, my mom remarried).
We can conclude that the correct way to revise these sentences is to change a modifier's place (if misplaced) or add its modifying word (if dangling).
In our example, we see that the map was useless because someone (possibly the reader) was confused by its symbols. However, the confused reader is left out of this sentence, so it might seem as if the map was confused, which is highly illogical.
So, this is an example of a <em>dangling modifier</em> and the best way to revise this sentence is to add a missing modifying word.
"Since Jack was confused by the symbols, the map was useless" could be one of the correct revisions.
