That could be my whole blog post. Just a PSA. Google Maps in Seoul is like Apple Maps was when they launched — dangerously inaccurate. *I don’t know what is helpful on iOS. I traveled last year with Android only, and my searches so far on iOS are coming up short.*
So what should you use? Anything OSM-based isn’t too bad. I really like OSMAnd. I haven’t done any deep analyses in this space, but OSMAnd has served me well. Also, you can record tracks, so if you see something wrong or out of date, OSMAnd will help you fix it in OpenStreetMap.
I adore Seoul’s subway system. It’s considered one of the largest in the world, ranks among the best, cleanest, etc.. Many stations are like 5 story malls that happen to have trains at the bottom; it’s really surreal. Oh, and for an English speaker it is not hard at all to navigate. Almost everything is in romanized characters / English + Korean, and the trains play nice sounding music as they approach.
It doesn’t hurt to have a good app, however. Subway Korea, though a little strange in interface is absolutely amazing once you use it. I say the interface is weird — it’s just transit graphic at a single static scale (it doesn’t change appearance as you zoom). But that graphic allows you to route between locations calculating train changes as necessary, let’s you optimize for time vs. number of train changes, and allows you to do routes by way of particular stops you may want to take on the way. It is great in large part because it’s designed with a deep understanding of how transit works and the kinds of questions people who don’t know the system need answers to. That’s a tall task. I can recall my first time navigating public transit in Boston, Cleveland, New York, DC, San Francisco, Portland, and Denver. Each of the above (even Portland!) was a little more difficult than Subway Korea and Seoul’s amazing wayfinding.