Jamieson, Fausset and Brown on Revelation 21:15

15. had a golden reed--so Coptic. But A, B, Vulgate, and Syriac read, "had (as) a measure, a golden reed." In Re 11:2 the non-measuring of the outer courts of the temple implied its being given up to secular and heathen desecration. So here, on the contrary, the city being measured implies the entire consecration of every part, all things being brought up to the most exact standard of God's holy requirements, and also God's accurate guardianship henceforth of even the most minute parts of His holy city from all evil.