Sometimes can't be helped, particularly if the point is underneath ornamental shrubbery you can't just hack into. I always do two shots as close to 90 as I can get, setting a couple nails from which I can tape the offset distances and then do a dist-dist intersect, playing close attention to and noting in the field book both the distance and bearing so as to eliminate picking the wrong solution. It's either that or break out the totalstation, and sometimes that is the right call. One shot on a compass bearing might cut it for a utility pole but not a corner. Compass can get wonky if there's power or something nearby tugging on it, even the hammer in your vest.

Something you can do with monuments under fence corners.

Set up pole near cor. Say 0.56' from Mon, via box tape, down near ground.

Then, go up to top of post, and go from gps pole BACK 0.56', and set a nail in the top of post, directly above the Mon.

You can even do it 2x, from 2 directions. 2x check that way. Then, take shot on nail, on top of post.

Old man Nate

I usually set two straddler hubs at some equal distance and locate the straddler hubs and compute the location of the obstructed monument.

Some times they form a triangle or they on a straight line.

Other times I will measure some obscure distance away from the obstructed monument and locate a temp hum that is aligned with another located point.

I use a lath. A lath is straight and has a fixed distance.

I can usually poke the lath through the ground so it doesn't move and I center the tip of the lath over the center of the monument.

Then it's two shots. The first is for distance at the end of the lath. The second is for direction as close to the monument along the lath as I can get.

With Leica, I use the hidden point feature. With SurveyPro and Trimble Access, I just cogo the point using the two points I shot.

I can check my work by moving the lath and repeating the process.

Other crews hover the gps head eyeball style over the fence where they think the monument is. The office questions my method because they don't know what the other crews are doing.

I like the two distance offset rather than distance-bearing, store the two reference points and calculate in the field then check in the office. I have never seen a problem with that method. Although the bearing-distance is good for tie in trees because all you need is one reference point. Check your data collector, some of them have a compass function on-board.

No need to set hubs.?ÿ I just plumb up the rod and tape the distance to the point. I do not try to set it at a specific distance.?ÿ I measure whatever the distance might be.?ÿ ?ÿI get more accurate results that way.?ÿ And I always get three points, preferably surrounding the points with 120?ø between them, but usually all on one side. 3 points gives redundancy. Sure, it takes an extra couple of minutes but I'm only doing this once a month or so.?ÿ I enter the data into StarNet to get an adjusted position.?ÿ The positions I get are darn close to as good as a direct observation.

If you can get the tip of the rod on the point but just can't plumb up there is an alternate method.?ÿ I have tried it and am satisfied that it is accurate. Set the rod on the point with the rod tilted. Brace it in position with the bipod, and the rod all the way down. Record a position. With moving the rod extend it fully and record a second position. In CAD draw a 3d line between those two points, and then extend that line by your lower rod height. The end of said line is the tip of the rod and your monument position. The math can also be done with a spreadsheet. This is more office work but has the advantage of getting an accurate elevation on the final point.

I wish that my field software had functions for these calcs. VIVA had a routine for it. Survey Pro, as far as I know, has not. StarNet will do the calcs but requires a fair amount of manual input.?ÿ ?ÿ ?ÿ ?ÿ

All good methods.?ÿ Much better than the single offset I've gotten back before, with no bearing or any other reference, just that the monument is a 2' offset from shot.?ÿ

I would think the line-in with two offsets would extrapolate any small positional errors GNSS offers, where offsets to a line, either side of the object, would produce a fraction of the random positional error. ?ÿ

I've done the same. I usually try to do a third at 45?ø as well when it is a monument or a more important shot than just a topo shot. It has proven to be quite accurate. As long as you apply the proper procedures that is.?ÿ

SurvCE has a couple of different options, I prefer the bearing distance intersection. I carry a 6-ft folding ruler in my pocket. I lay it on the ground with the tip on the center of the corner (if possible). I take two shots along the ruler inputting the distance from the corner on each shot. Obviously the ruler provides the offset distance and the two shots provide the bearing.?ÿ

Timble use to use a function where you could cast a rod shadow over the point then add the o/s distance.?ÿ I always thought that was a cool function for topo pick up.?ÿ Mark, for your three point check I would always offset on a line (drop plumb bob) or at near right angles on random distances measuring the distance directly between the two offset points for a inverse check prior to accepting the tie.?ÿ Jp

Posted by: Williwaw

Compass can get wonky if there's power or something nearby tugging on it, even the hammer in your vest.

Good point!

I was at a conference yesterday in Belgium where there was a demo of the new Leica GS18 T.

Did some tests and the results were great. Who needs offsets anymore 😉

?ÿ

Posted by: Jp7191

Timble use to use a function where you could cast a rod shadow over the point then add the o/s distance.

Interesting. I would love to see the code for that!