Yes you need to know the altitude at a defined point. Perhaps using the "Get Altitude" app or a similar app that looks up the altitude on the internet. You can then correct the current altitude in Run.GPS: Configuration >> GPS Settings >> manual altitude correction.
2 days ago I was running in Poland and everything worked correct. Yesterday I have travelled to London and made sightseeing running trip. Please check my workout log, because something is wrong:
http://www.gps-sport.net/trainings/London-Sightseeing_London_1600548
I use galaxy s4 and suppose that barometer require calibration. But I wonder how? There is no calibration settings in Android 4.4.2. Can you recomend some third-party app. Is possible to autoalibrate basing on internet and GPS data (I didn't turn on Internet since leaving Poland, perhaps this is reason of wrong altitude data?).