I assume you want to cut the post in half lengthwise. If so,find a cloth measuring tape such as used for sewing. If the post is round at one end or both ends, you'll have to find the circumference and divide it by 2. With the post laying on a flat level surface with it chocked so it can't roll, set a square on the floor at one end and measuring from one edge of square blade to each side of post, find the center and make a mark on post. Do this at the other end without moving post. Now you'll need to put the cloth measuring tape end on the mark and measure around the post half the circumference and mark a mark there. I don't know how ornate this post is, but you'll have to have a straight edge long enough for the lenght of post to scribe a straight line from one end to the other. Again I'm assuming this is a metal post. If so, you'll next need a sawzall with metal cutting blades to make this cut from one end to the other. I don't know how long this is, so good luck. Hope my instructions are clear enough.
Mike A

That is a good way to mark the post. I would use a 1/8 cutoff blade in a 4 1/2 grinder. It may take a few blades but it will cut faster. At work we have an air operated cutoff grinder which takes 8" blades. You might find a blade for a 7 1/4 wood saw. Good luck