"Is there a map?"
No, only if you're declaring something's existence with います or あります would you use が. が is used to announce that the topic is doing something. If it's a specific things attribute you use は. This sentence is basically saying "is THE map here?" Which could mean "do you have the map?"
Tldr: it's a specific map whose attributes are being inquired of: use は.
To quote (https://www.learn-japanese-adventure.com/arimasu-imasu-existence.html), use は with あります/います when both the speaker and listener know about the SPECIFIC thing, you are asking where the specific thing is, or with negative answers/statements.
In this case, it is a question about a map specific to this area you are in (which we mean when we ask "Is there a map?"), not some random map that won't be any use to you. Think of it as "The map, is it there?" or "The map, do you have it?".
In negative answers it's still a specific thing but it's the whole concept of that thing. Or you could think of it as all of that thing.
This is more helpful because it creates continuity just like の always being possessive just more generally and abstractly (sometimes it's hard to see).
Otherwise very helpful and thank you for helping everyone understand this tricky concept.
I think this is a very good explanation: http://nihonshock.com/2010/02/particles-the-difference-between-wa-and-ga/ . But it took me a bit of studying until I actually felt like I understood it. Also, some of the uses are idiomatic and hard or impossible to guess a priori.
This is great. Also, some idiomatic uses become more clear when you understand the underlying grammar. For example, "x ga suki desu" is hard until you understand that suki is an adjective in japanese "x is likeable (to me)" so you use ga because there is always an invisible "watashi wa" before it.
Not necessarily. Ga makes the question more 'is there a map' rather than wa, which mahes it closer to 'could I see the map.' Languages seldom translate neatly. There are always loads of exceptions to perceived 'rules,' and one needs to remember to be flexible in learning.
は is referring to something specific here, whether it's location or a specific map. Duolingo just loosely translated it to "is there a map?" because it has the same impact of basically asking you to procure the map of the specific location in context or the specific map I'm refering to.