Point of Beginning

Serious "calibration" question (long).

November 26, 2008

Posted By Loyal Olson on 11/25/2008 at 1:28 PM

Oky Doky...this is actually a serious question for those who actually UNDERSTAND what a “calibration” ACTUALLY does internally (which I readily admit I'm a little [or a lot] vague on).

Let's say you go out to a new project. You job is to retrace, subdivide, record a Record of Survey of [lets say] Section 17.

You have in your hot little fist, a coordinate listing of the four(4) Corners of said Section (where you got it is irrelevant to this discussion).

Your modus operandi in such endeavors is to do a “site calibration,” and HOLD the “record” coordinates (fit the record so to speak).

The coordinates are as follows:

NE Corner 12,639.72N 12639.72E
SE Corner 7360.28N 12369.72E
SW Corner 7360.28N 7360.28E
NW Corner 12639.72N 7360.28E

Yeah, yeah...I know, but I'm trying to keep this as simple as possible.

You setup your RTK base-station somewhere convenient, lets say that there is a nice low hill right in the middle of the Section, which “sees” the whole project very well (including the four corners in question.

You shoot the four subject corners, enter the above coordinates and push the "calibrate button."

Okay, so here's the rest of the story, AND my actual question(s).

What you don't (and usually can't) know, is that Dufus & Goofus generated these coordinates, and did the following:

Dufus set his total station on that very same hill.

Being a flat-lander knucklehead, he had the ppm correction set @ ZERO

He also had the prism constant set at -30mm in the gun.

Goofus [of course] had the prism screwed on the 0-offset side of the bracket.

The environmental conditions were:

The barometric pressure was 20.4 inches of Hg.

The temperature was 35 degrees Fahrenheit.

And:

The Total Station had a 930nm EDM.

As Goofus marched around the section with his bipod carefully placing it on each stone/cap, Dufus wrapped multiple angles from his “natural” backsight on yonder cellphone tower.

When A. Shakey Rounder PLS gets the field notes that night and computes everything up, he rotates the coordinates to fit NORTH along the East line of the section.

The rest is obvious.

Now my question(s).

What kind of residuals would one expect to get out of the black-box calibration? (assuming “perfect” angles, and no transcription errors in the distances)

And,

How does this jive with Section 249?

249. In order to prevent any misunderstanding relative to the modus operandi indicated by the terms "retracement" and "resurvey," the following definitions of the same are here presented:

The retracement of a township boundary, or ANY LINE OF SURVEY, consists in the determination of the TRUE BEARINGS and DISTANCES between the successive corners along the entire length of such line; and the data thus obtained will be embodied in the field notes together with the detailed particulars of the methods employed. (emphasis added)

Resurvey paragraph ommitted.

I apologize in advance if I got the ppm corrections assbackwards, it's been awhile.

Loyal

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