2018 GPS on Benchmarks - Status and Adapters

Posted by: Tyler Parsons

As of March 30, 2018, only 362 of the requested 5746 benchmarks have been submitted and approved. We?ÿ have until August to get them in.

I emailed NGS and asked which ephemeris was acceptable. ...?ÿ Of course, they would like precise - the response was that there is nowvery little difference between rapid and precise, will accept rapid and may possibly weight the results for inclusion in the interim datum.

My philosophy is "why go with less than the best" when the cost is only a couple weeks of delay getting it submitted.?ÿ I it can be any small amount better don't compromise.?ÿ Even though I find it is difficult to be patient.

What's your experience in finding their priority marks and their suitability??ÿ There are several around here that have last report as Not Found, quite a few that don't have decent sky, and at least one vertically mounted so you can't get a GPS on it (marked unsuitable in last update after I emailed).?ÿ

Selection of good marks at desired spacing is difficult.?ÿ Last fall I searched using Google Earth 50+ miles along one level line and came up with 3 disks suitable to do sessions on.?ÿ My submissions of those are probably what put them on the priority list.?ÿ I'm hoping DOT will do the second session on each of them.?ÿ It would make me feel better to have someone else confirm the measurements rather than do the second session myself.

I'm not an expert at this but I have some time as business is slow, I have 2 GPS, and I'm getting near retirement. I've submitted 17 marks, 13 of which have been approved, and 4 are pending. I had one do-over as my first submission was more than 6 cm different from another surveyor's elevation done 7 years earlier. I try to do ones that are near roads as they want the tripods sandbagged and I'm getting too old to carry those very far. As the days get longer, I figure I may be able to do 4 per day at 4 to 5 hours minimum if they aren't too far apart and are near roads. I "tend" them - not leaving them alone for more than about 2 hours. I take a book or my tablet with an ebook from the library along for company.

I use Google Earth with DSWorld to locate potential marks, then look at street view along roads that have it to see what the terrain and tree cover is like. Some they want are obviously not suitable because of heavy tree cover or along highways through mountains with not much sky visible. Some they want haven't been found for a long time and the distance from a small town Post Office may be bogus as the PO has been relocated.?ÿ

A couple of things I've learned; 1) get a small (5"x7" or so) whiteboard, mark with the BM Id and the PID and take your first (and maybe last) picture at the site of the whiteboard so you don't lose which mark is which, or record the picture numbers and info for each mark, and 2) take all your pictures in landscape orientation as the shared solutions pictures will be turned sideways if they are submitted in portrait orientation. Clicking on the mark or vicinity picture will bring it up in full size and the way submitted, but portrait submissions look funny on the share page itself.

Posted by: Tyler Parsons

I had one do-over as my first submission was more than 6 cm different from another surveyor's elevation done 7 years earlier.

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I debated with myself on one that disagreed with a DOT submission by 34 mm.?ÿ I finally looked at (MyGPS + DOT_GPS)/2 - NAVD88 and decided the average matchedbetter with other marks in the county than either session alone, so went ahead and submitted it.?ÿ And a purist would say it's cheating to cherry-pick your data so if you measure it you must submit it.

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I try to do ones that are near roads as they want the tripods sandbagged and I'm getting too old to carry those very far.

I haven't always been weighting the tripod.?ÿ I figure if the final HI matches the initial HI and it's still plumb that's good enough, and the only risk is wasting the session time if it does tip.?ÿ I weight when on concrete that a leg might slip on.?ÿ When ground is soft or frozen, weighting would tend to increase settling.?ÿ When it's?ÿ soft I use hubs with dimples in them to set the tripod legs in.

I have been using a tripod and tribrach instead of a fixed-height pole, so am really careful about measuring in feet and meters before and meters after.?ÿ I checked several marks against other peoples' data to be sure I was doing the measure-up correctly before I started submitting.

Pedantic question: If a rod has a dimple in it, is the mark the top of the rod or the bottom of the dimple??ÿ Not that it matters for GPS work, but nice to know.?ÿ I know disks were measured with a flat shoe on the rod, and I'm remiss in not adding a half mm for their dimple when I use the pointed 4-piece Trimble measuring rod.?ÿ But then again, I wouldn't trust that rod to one mm if it really mattered because I don't think the silk-screened graduations are placed any better than that.

Mark Armstrong gave us the PVC adaptor hack during OPUS Projects training a few years back. I've also heard of folks using a small section of rubber hose ( But that's been determined by the State of California to cause cancer).?ÿ NGS guidance for setting the dimple on a driven rod is to keep it less than .5mm deep, which is barely enough to hold the tip of a rod.