Issue with setting out

[spud84]

I am on a site where I have three station points, however none of them can be seen from one another. I can set up in the middle of the site and see all three stations. My question is how do I set up and verify that the co-ordinates for these three points are still accurate as the information for them is over three years old. I will be using a Nikon 362 total station.

 

[underhill]

You can use a technique known as "free station" (or "modern resection"):

It is usually used to establish the position of your unknown "middle" station, via observations of angles and distances to the a few control points.

In your case, you'd make several (to get good averages) observations to the three control stations, do the free-station calculation, and examine the residuals. A residual represents how much an observed amount had to be adjusted to fit the math model and the assumed control values. Small residuals mean things are consistent (which is what you're trying to verify), while large residuals mean there's a discrepancy between your careful measurements and the assumed control point coordinates. Possibly, you'll have good residuals to two stations, say, and poor residuals to a third, suggesting one of your control stations is bad. (You'll never know for certain, however, as all survey calcs are based on some assumptions of correctness.)

Many total stations have free-station/resection software built-in (i don't know about the nikon). If not, you can use Copan, the free CoGo tool from UGL.

 

[underhill]

Another technique is to traverse between the known control points:

Traversing is a much more common technique, where you usually set up at one control point, back-sight on another control point (to get proper orientation), measure angles and distance to a new "traverse point", move to the new point, back-sight to the previous control point, measure angles and distance to another new point, and repeat the procedure until you've measured to, either another known control point, or back to your original point. The angular and coordinate misclosure will be an indication of the quality of your traverse observations. Finally, do a traverse adjustment.

In your case, because your control points are not intervisible, you'd need to start at the first control point and initially back-sight onto a distant, well-defined (but arbitrary) target. Then traverse around your site, visiting the other two control points, and back to the first. (The number of intermediate traverse points will depend on your site, but i'm guessing you'll need one or two between each control point.) When you calculate the traverse, you need to first use an arbitrary, assumed bearing for your distant (arbitrary) back-site to get arbitrarily-oriented coordinates for your traverse points, examine the misclosure, adjust the traverse, then rotate[/] your new arbitrarily-oriented coordinates into your original control point system! Discrepancies, if present, will then reveal themselves.

Again, some or all of that calculation ability may be available within your total station. If not, use land surveyor's software, such as Copan, it can do all i describe above.