"The lines are fallen unto me in pleasant places; yea, I have a goodly heritage" (Psalm 16:6, emphasis mine)
In this verse, David is referring to his heritage. The Hebrew word translated "lines" also translates elsewhere as "lot". That which you have "inherited" (another translation of the word). The word in question is chebel. Incidentally, the river Chebar is where "the heavens were opened, and [Ezekiel] saw visions of God and angels. (Ezekiel 1:1) This might sound like a stretch when this inference is drawn from these two points. But it's wholly true when you realize that God can meet you anywhere. Those things that are ingrained in your life. The thought patterns and processes that you "inherited", from your family, your church, your culture. The things that color your day, your week, your life. God can open the heavens if need be and show you the most otherworldly and amazing visions wherever you find yourself, with whatever tools are at hand. "Pleasant places" indeed.
David in the above verse was expressing his gratitude for things that had been given to him by God. Gifts in their raw state. Things like talent, ability, confidence (and charisma) and appearance. Whenever someone tries to flatter me regarding some aspect of my physical appearance, I always tell 'em that "it's not my fault". Personally, I think this is what he's referring to. Over time, it would seem we become blind, or at least desensitized to the things that God has gifted us with. Gratitude is always the order of the day, because hey, let's face it: God didn't have to create us. And staying within those borders—those lines—is where the blessing of God can reach us.
It's the way God teaches us. "Line upon line". "What man (or woman) is he (she) that feareth the Lord? Him (and her) shall He teach in the way that He shall choose." (Psalm 25:12, emphasis mine) A goodly heritage, you say? But what about all of my dreams and hopes and aspirations? When do I get those? When is God to "increase my greatness and comfort me on every side" (Psalm 71:21)? Now. Isaiah 28, verse 10 says "For precept must be upon precept, precept upon precept; line upon line, line upon line; here a little and there a little". (Isaiah 28:10, emphasis mine.) It's happening slowly. As we speak.
This is how God takes us from where we start to where He ultimately wants us to be. Imagine a piece of paper, on which are drawn a dozen or so lines, one on top of another. Without the bottom line, you can't have the knowledge that the top line represents. Line upon line...
Check out Psalm 65:9. "Thou visitest the earth, and waterest it: Thou greatly enrichest it with the river of God, which is full of water: Thou preparest them corn, when Thou hast so provided for it. Thou waterest the ridges thereof abundantly: Thou settlest the furrows thereof: Thou makest it soft with showers: Thou blessest the springing thereof."
It says that God settlest the furrows. The lines. What does this mean? I do know that a furrow is a tilled line in the soil, ready for planting. God's word is that seed that we plant in the lines of our heart.
Going back to the top. If we fill the "lines"—as David called them in Psalm 16—with God's word, then God can take us any way He chooses as it says in Psalm 25:12.
"And make straight paths for your feet..." (Hebrews 12:13)