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RTK Offset Shots  

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Field Dog
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The office frowns upon RTK offset shots. I had to use that particular feature today. I had 2 rebar, about a 0.10' apart, next to a 6' chain link fence with barbed wire on top. I picked a nice round number, 1.000', for an offset distance. The bearing I chose was S 10°00'00" W, which I got from a compass app. I tried to hold my phone steady. The displayed azimuth varied between 188° and 190°, but mostly 190°. Being particular like I am, I calc'd what difference a few degrees of error would make at 1.000'. It's only 0.017'/ft. x 2 = 0.034'. I could live with that.

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Williwaw
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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.

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I've done the same. I usually try to do a third at 45° 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. 

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Field Dog
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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!

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(@jbmo74)
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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.

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Field Dog
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Posted by: JBMO74

Check your data collector, some of them have a compass function on-board.

I was told mine does. I have to learn how to use it.

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Trimbleman
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I've done a handful of offsets with gps. For topo locating trees or items with more error tolerance than a property corner a bearing distance offset is probably sufficient, especially if your talking small distance offsets. you start talking 50' plus offsets the bearing, distance may not be your pick of offsetting.

Property corners on the other hand I have found a two point distance, distance offset has given the best result when i'm trying to obtain max results.

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(@samlucy3874)
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2 good points and double tape all 3 sides of the triangle.

I really like what the professional surveyor said about field books.

No substitute for a good field sketch, especially of screwy stuff. If only to jog the old noodle of what the situation was.

O L D,  happens, licensed since February 1991, i think.

 

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Nate The Surveyor
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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

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Field Dog
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Posted by: Nate The Surveyor

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

Good methodology!

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Field Dog
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Posted by: Nate The Surveyor

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.

On second thought. If you were to strike an arc at x distance from a GPS pole in the top of a 6" to 8" diameter wood fence post, you'd need a pretty good eyeball to get it directly above the monument. Just sayin'.

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A Harris
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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.

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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.

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Mark Mayer
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No need to set hubs.  I just plumb up the rod and tape the distance to the point. I do not try to set it at a specific distance.  I measure whatever the distance might be.   I get more accurate results that way.  And I always get three points, preferably surrounding the points with 120° 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.  I enter the data into StarNet to get an adjusted position.  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.  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.       

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