I am working on a DoD installation and came across a stainless rod monument set by NGS that had this cap/sleeve on top, I have never come across one of these before and was wondering if this is supposed to be left on the top of the rod, or removed to reach the true datum point? Slips right off... So far I have been measuring to the top of this cap, changes the height by about 0.01 feet and as you can see, the punch mark is slightly off center of the center hole in the cap, this was set as a Primary Airfield Control Station (PACS) and only has a GNSS derived vertical published to 0.01 meters, so 0.01 feet in height probably isn't critical, just trying to get educated about the cap.
When I did some of the observations for the HARN network, NGS supplied us with some of those that we returned. I think someone forgot the one you found. I don't remember the center hole being as large as the one you have.
As Jerry suggested, that's just a centering cap designed to keep the antenna pole point on the rounded top of the (typically) 9/16" stainless steel rod. They're pricey enough to take them with you when you leave the station, so it looks like someone just forgot.
When I took the OPUS projects class, Mark Armstrong told us he used PVC caps of appropriate diameter and drilled a small hole in the center and slid them over the ngs rods to hold poles on top the rods for extended occupation times. Cost- about $0.70 each.
Well dang, I guess I scored one for the toolbox, I can fix the verticals, going to have a bit of centering error, but for what I am doing I don't think it is the end of the world...
SHG
I have a knock off of one of these that I got from mark Armstrong back when he was with OBEC. They work great for keeping your rod on the top of those stainless steel rods used in vertical monuments.
My first occupation yesterday I was in a hurry (have to have an escort stading around watching me) and didn't investigate this very well, noticed it came off this morning, but carried on, wishing now I had of pulled that off yesterday, I guess I am no worse off than anybody else using that point, everybody has been off the mark dimple by a few mm. Well I will carry on tomorrow with that thing in my pocket, probably have two more days of using this point as my base station, like I said I can adjust my antenna heights to compensate, going to be more difficult to fix the Hz not that it probably matters for 1-2 mm...
SHG
> like I said I can adjust my antenna heights to compensate
They're designed to allow the point to contact the rod top, so in theory you shouldn't have to change the antenna height.
I think it depends on the taper of the rod tip...
Anyway, I have a tripod with tribrach set up on this point for my base station, when I measured the antenna height I was most definitely NOT touching the top of the rod, rather the top of this cap thingy, in addition no matter the setup type, you would not be exactly centered on the rod punch mark since it isn't in the center of the rod (makes a case for ALWAYS punching the exact center of any monument). Like I said, a couple millimeters isn't the end of the world, just gripes me I didn't investigate this a bit more to start with, I just thought it was a bigger diameter rod with a bigger center mark since I had never seen or used one of this rod sleeves before.
Leads one to wonder if this mark was observed originally with or without the sleeve, it would make a difference on where you should center after the fact ie: the center of the rod or the punch mark, even though a minor amount, I still detest NOT doing everything as perfect as I can.
SHG
I've always just used the dimple on the rod.