Funny Basis of Bearing

What more could you possibly want than that:-P

> BASIS OF BEARINGS BASES
> ON GPS OBSERVATIONS:
> DATE: XX/XX/XXXX
> BEARINGS: TRUE
> DISTANCES: SURFACE

I would presume that to mean that the distances shown on the drawing are all along the ground surface (on slopes up and down bumps and hills); and, of course, each bearing is true along the geoid in relation to true north, and not adjusted to a state plane grid rotation.
🙂

Translation: They setup an RTK Base somewhere, hit 'here' and went to work.

In which case the bearings are grid unless tinkered with.

Wouldn't "here" produce geodetic bearings?

Alternative interpretation: the distances are horizontal distances at the surface as opposed to on the ellipsoid.

They probably have a projection or a "calibration" of some kind, producing a grid system with geodetic north and grid north being equal at the set-up point.

Now they may well have ground distances, but some of the data collectors will put everything on the ellipsoid unless you set it up to get it to the surface.

In my part of the world that's easily checked so I don't worry to much about it. Figuring out the real basis of bearings takes a bit of detective work.;-)

> They probably have a projection or a "calibration" of some kind, producing a grid system with geodetic north and grid north being equal at the set-up point.
>
> Now they may well have ground distances, but some of the data collectors will put everything on the ellipsoid unless you set it up to get it to the surface.
>
> In my part of the world that's easily checked so I don't worry to much about it. Figuring out the real basis of bearings takes a bit of detective work.

My favorite variation on the "here" grid was one I saw one time on a large ranch in hilly country where the use of RTK would almost certainly have required multiple base setups. As nearly as I could tell, at each one of those setups, a new grid-du-jour was created around the previously surveyed coordinates of the control point occupied by the base. The result was each of several different parts of the project had its own bearing basis with significant breakage when different parts joined.

> Wouldn't "here" produce geodetic bearings?

Yes and no. It's not impossible, but impractical to survey in a LLH environment, so typically one would calibrate that "here" position to say 5000,5000, or something else. At THAT point, you will generate geodetic bearings. However, since you've taken LLH to a grid (not THE GRID), then they further you travel from your "here" position, you generate a theta/gamma/angle of convergence, except this AOC isn't modeled and exceedingly difficult to generate meta data on. It IS reproducible, but only if the users follow the same procedures from the original user, within reason (i.e. setting a base close to that base and rolling on).

For something small, say 20 acres, you won't see more than a few seconds of arc drift from one side to the other, so no biggie. In the oil patch were we measure it in 700-acre units, hell yes it's a problem, and a damn big one.

As long as they were to provide a Basis of Coordinates to go along with that Basis of Bearing, vis a vis a GP that's within a few feet of the true position of their base, I just don't see the big deal. As long as they get me close enough to find the corners on the ground ...

What Kent wrote falls into a whole different category of 'cluster struck'.

Kent

That's disgusting. And, I see it here too. (No pun intended!)

N

My favorite variation on the "here" grid was one I saw one time on a large ranch in hilly country where the use of RTK would almost certainly have required multiple base setups. As nearly as I could tell, at each one of those setups, a new grid-du-jour was created around the previously surveyed coordinates of the control point occupied by the base

I can see how that can happen. When I first was introduced to GPS a lot of time was spent training me how to do "calibrations". They went so far to do one each day.

It quickly dawned on me that instead of doing "calibrations", wouldn't it be much simpler to work in a projection all the time and not mess with it-much more stable and accurate.

If they were going out each day and preforming a "calibration" before starting, it could easily degrade into a new single point calibration each day with a new "true north".

Of course, they would have to completely misunderstand the point of a calibration, but I've seen that a lot of people do.

Trimble is a billionaire cause of calibrations. It made it possible to sell GPS to a bunch of surveyors that didn't understand geodetic surveying and allow them about 20 years to figure it out. A calibrations sets up a projection, the operator just didn't know it.

It wasn't very long after I started using GPS that I seen the problem with calibrations. At first I went to SPC's then found the better way with LDP's (thanks to Loyal Olsen showing me the light).

I have about 15 years data now all tied to the CORS (within a few cm) that I can bring into any file and be very close. I don't think I've used the HERE KEY more than a dozen times in the last ten years and that only lasts until I could do the OPUS on the base point. One time I had a problem using the HERE KEY cause I couldn't remember how it worked. Took me a few minutes.

GPS vectors are stored in ECEF dX, dY, dZ. So if you get the correct place for the beginning of the vector the end will be in the right place. So just get the base point right and process all the vectors from it and then you can view it in any projection you choose.

So multiple base points using the HERE KEY and a calibration can be easily sorted out if you can get solid coordinates for all the base station points and then reprocess all your GPS vectors in the same file projecting it any way you choose. But if you don't know or understand the system what you got isn't anything more than a pile of crap.

If it is "no projection/no datum" then the bearings are very close to geodetic bearings.

If a grid projection is selected then it would be grid north.

They, of course, set up the calibration system to merge the geodetic with the grid systems that were already on the ground.

There had to be some way to do it, so the calibration/localization systems were set-up with some align and fit programs in the data collectors to do it.

It was also set-up that way to control the z value. Compare bench mark elevations to ellipsoid heights and slope a figure in to match it up. The problem with that is it creates a plane and not a geoid shape which you do create when you run levels. If you happen to be in a area with a lot of geoid contours then it didn't work very well at all-even for small areas you could see problems quickly arise.

But now, I see adjusted control run with static and levels, checked to the newest geoid models, then crews going to the field and doing a "calibration" to the control. WTH are they doing?

Imagine; you have static control, adjusted, a geoid model applied, which checks to level runs within say 0.02' and a crew goes to the field in the morning, sets up a base takes some RTK shots on control and then applies a calibration-you've got to be kidding me. What a waste of time and what a dangerous thing to do.

And some people do it every day-so I've been told!!!:'(

I know what you mean.

At my former employer, for large surveys, we were required to perform static sessions on intervisible control points surrounding the site, then run a total station traverse thru the control points. Yep, you guessed it, then calibrate the adjusted static data to the total station derived coordinates for the site control.

For some reason the owner (engineer) believed the GPS for the lats/lons and only trusted the total station for the N, E, Z values.