Thou compassest my path - Margin, "winnowest." The Hebrew word - זרה zârâh - means properly "to scatter," to cast loosely about - as the wind does dust; and then, to winnow - to wit, by throwing grain, when it is thrashed, up to the wind: Isaiah 30:24; Jeremiah 4:11; Ruth 3:2. Then it means "to winnow out;" that is, to winnow out all the chaff, and to leave all the grain - to save all that is valuable. So here it means that God, as it were, "sifted" him. Compare Isaiah 30:28; Amos 9:9; Luke 22:31. He scattered all that was chaff, or all that was valueless, and saw what there was that was real and substantial. When it is said that he did this in his "path and his lying down," it is meant that he did it in every way; altogether; entirely.
And art acquainted with all my ways - All the paths that I tread; the whole course of my life. All that I do, in all places and at all times, is fully known to thee.

Thou compassest my path - זרית zeritha thou dost winnow, ventilate, or sift my path; and my lying down, רבעי ribi, my lair, my bed.
And art acquainted - Thou treasurest up. This is the import of סכן sachan. Thou hast the whole number of my ways, and the steps I took in them.

Thou (b) compassest my path and my lying down, and art acquainted [with] all my ways.
(b) So that they are evidently known to you.

Thou compassest my path and my lying down,.... The Targum adds,
"to study in the law.''
His walk in the daytime, and every step he took, and his lying down at night. It denotes his perfect knowledge of all his actions, day and night; he surrounds every path of man, that they cannot escape his knowledge. Or, "thou winnowest", as some render the word (c); he distinguishes actions; he discerns and separates the good from the bad, or the goodness of an action from the evil and imperfection of it, as in winnowing the wheat is separated from the chaff. Or, "thou measurest my squaring" (d); all his dimensions, his length and breadth, as he lay down in his bed;
and art acquainted with all my ways; the whole of his life and conversation, all his works and doings: God knows all the evil ways and works of his people; he takes notice of them, and chastises for them; and all their good works, and approves and accepts of them; he knows from what principles of faith and love they spring, in what manner they are performed, and with what views, aims, and ends; see Revelation 2:2, Psalm 1:6.
(c) "ventilasti", Pagninus, Montanus; so Tigurine version and Ainsworth. (d) "quadraturam meam spithama mensurasti", Gussetius, p. 775. "spithama metiris", Cocceius.

Compassest - Thou discernest every step I take. It is a metaphor from soldiers besieging their enemies, and setting watches round about them.