John had already seen (and described) the walls and gates of the city in the previous scene (Revelation 21:12–14). That the angel now measures the city is his way of demonstrating his Master’s ownership of the city, and in so doing showing up the glory of the Owner. See further Psalm 48:12–14; Zechariah 1:16–17; Zechariah 2:1–2 (and David’s misguided efforts to number the people, 2 Samuel 24:1–25).
15 And the one who spoke with me had a measuring rod of gold to measure the city and its gates and walls.