A friend of mine bought a calendar and asked about this month’s photo (not the one above) which had the arrow (as shown above). I have no idea as to its purpose, and nor does anyone else I’ve asked.
If you’d like to see it for yourself, climb to the top of the parking deck at Moore Square Station. It has appeared in a previous blog post, but I didn’t really mention it.

My best guess as to its purpose is that it is for emergency helicopter landings–It points to an open field as seen from Google Maps. However, Moore Square seems to be better suited as it has a larger green area.