Unfortunately the amount of free storage space on iOS is complicated. Apple reports different numbers in the Settings app than it does to applications. The number Apple shows in settings does not include files that iOS considers to be deletable. Unfortunately there is no way for apps to directly trigger iOS to delete those files.
If you are trying to download multiple map squares, and Goat Maps says you don't have enough free space, but Settings says you do, try downloading them one at time. As storage space gets low iOS will delete temporary files, clearing up more space for applications to use.