Grid & Ground Coordinates in TBC

If by grid coordinates you mean a projected system such as a state coordinate system or maybe UTM then you need to add a scale factor to get to ground if that's what you need.

You have probably set-up the TBC job as either a state plane or UTM system. Under project/project settings/coordinate system/local site you can add a scale factor to "bring up" the coordinates to surface values. Care should be taken just how this is accomplished and everything done should be documented.

Unless you are doing a very large project or are at high altitude you'll likely never notice the difference. Remember SPC systems are designed so that the difference between grid and ground will not exceed 1 part in 10000 at seal level.

And I seriously doubt you are interested in that kind of precision for top.

What is your scale factor at the site?

It depends on where this is. I work in at least one area where its almost 6' in 10000'-at sea level. Not that there is any ground at sea level there.....

Thanks Guys,

So as long as I am using a SPC system there really is no reason to worry about grid/ground? Reason I am asking is regarding this project the difference is .007' on the longest 5300' base line (not a problem, but I figured I got lucky on this one)but what your saying is I will likely see these results for nearly all my projects?

Does this remain true when setting up a project in the GPS collector as no datum / no projection?

If you are seeing .007' in 5300', then I wouldn't adjust anything. That is so small a correction that all you accomplish is to chase it forever.

I would not do-no projection/no datum.

I would also set-up each job before leaving the office.

If you are using new data collectors and programs you should be able to work with a total station and GPS seamlessly. However, from prism offsets to projections I would test everything out and make sure myself.

Scale factor depends where U are in relation to central

meridian.....With Liscad can export to TS by inserting scale factor and having grid coordinates exported as plane coordinates .

RADU

> It depends on where this is. I work in at least one area where its almost 6' in 10000'-at sea level. Not that there is any ground at sea level there.....

???....what is your grid scale factor, to get 6' in 10000. hmmm...I guess it would be around 0.9994 (or 1.0006). I have never seen a grid scale factor < 0.9999. I have only seen elevation scale factors to get such a difference. You say at seal level..so is there a projection to sea level from the ellipsoid that makes a big difference?

Sorry, I have only worked in one state. I am not disputing your statement. Of course I primarily work in State Plane and the Lambert conformal Conic as well.

Ground Scale factor = 1.0001577774 (this was calc'd by TBC using the "local site settings")project location

what I do not understand is why my inverse is different when I manually run the factor?

grid distance = 5334.394'
ground distance = 5334.400'

but if you calc the factor you end up with 5334.394'x 1.0001577774 = 5335.2356'

what do I not understand here?

You need to look at your combined scale factor. Which is the grid and sea level factor. Check that and the calcs should work.

Yeah, I didn't beleive it the first time I saw it either. But try about 46-30N and 106-30W in Montana. Then look at it at sea level and you'll see what I mean. Also, UTM can get large (or small depending how you look at it) factors.

There are two main factors....grid scale factor which is a factor to go from a "plane" to the curved surface at the ellipsoid. and the "elevation (or height) scale factor" which is a factor that is the proportion from the ellipsoid distance to the elevation above the ellipsoid you are at.

The smaller difference might be what you would get if you just applied the "grid scale factor".

Most, or many, people combine the two factors by multiplying them and they call it the 'combined scale factor'. and instead of taking two steps from the grid distance, they do it in one. I am guessing that your "Ground Scale factor" is the combined scale factor inverted (1/X). To me it is better to just divide by the combined factor instead of yet manipulating that number one more layer.

The even more purists, would divide your grid distance, by the gsf, then divide that by the elevation factor. It keeps things striaght.

My guess: 5334.394/gsf (grid scale factor) = 5334.4; and 5334.4/esf = 5335.2356 shortcut: 5334.394 X 1/csf)

I pulled an NGS datasheet for a point near the lat/lon he provided and he is correct. The grid scale factor is much larger that I have ever seen here in California. Certainly not in compliance with the 1:10000 accuracy requirement used to lay out the Lambert zones here:

PID - RU0587

Elev Factor x Scale Factor = Combined Factor
RU0587!SPC MT - 0.99985347 x 0.99943464 = 0.99928820

Mighty is talking about NAD83 Montana State Plane Coordinates.

When NAD83 was adopted, Montana elected to create a “State-Wide” Lambert Projection for their NEW NAD83 State Plane Projection.

The net effect of this, is projection induced scale factors in excess of 600ppm (0.999 400) in some areas. In fact, along the Central parallel, we are talking about 0.999 393 (3.2 ft. per mile), and that's NOT including the height factor.

Texas had a NAD27 Lambert projection that could return even larger distortions (the Shackelford Projection), and still has a NAD83 Lambert that can return a scale factor of .997 872 (2,132 ppm) or 11.3 Ft. per mile along its' central parallel (Texas Centric Mapping System/Lambert Conformal). There is an "equal area" version as well.

EDIT: I should make it clear here, that the above Texas projections are NOT Texas State Plane Projections.

There is MUCH confusion about the basic geometry of Mapping Projections in general, and State Plane Coordinate Systems in particular (which are simply mapping projections).

The “scale factor” may be better understood, as an expression of the projection induced distortion BETWEEN the 'developed surface' of the projection (the GRID), and the ellipsoidal surface of the datum (e.g. NAD83) at any given point. The GRID isn't really a “plane” until the developed surface (essentially a CONE [Lambert] or “cylinder” [Mercator]) is “laid out” on the table (so to speak). We do this “laying out” automatically (in our heads and CAD programs), but all of the math happens between the ellipsoidal surface and the 3d cone/cylinder.

The “elevation factor,” ISN'T really an “elevation” factor at all, but an “ellipsoid height factor.” Now back in the NAD27 days, the term “elevation factor” was reasonably accurate, but under NAD83, we should be using ELLIPSOID HEIGHT and NOT “elevation” (NAVD88) to compute and describe this FACTOR.

State Plane Coordinates (and UTM Coordinates) are wonderful tools when properly used and UNDERSTOOD. They are a nightmare when misused, misapplied, or misunderstood.

Loyal

If the latitude is shifted to the state line at 45 degrees north the scale factor approaches 1.000. Then the angle deflection from "true" north becomes the annoying factor while using MSP.

I try to avoid it, but at times a company or state agency wants or is required to work in it.

At that point you just tilt your head to the left so the sections lines look correct.

Right on Mighty.

The Standard Parallels for the Montana Lambert are 45° and 49°

Obviously the theta/gamma angles can get pretty large in a State with Montana's east-west extent (and Latiude).

Not a good system for cadastral work IMO.

Loyal

Thanks Loyal. Good explanation. I felt funny about calling it an elevation factor, because I knew it wasn't based on the height above the geoid.

I don't think I'll ever have the general understanding you and Mighty Moe have. It's great to have some of the resources we do on this web page.