Can someone explain to me why there is a rotation in this process? It would appear to me if you were to convert grid to ground it would just be a matter of "holding" a point and scaling the rest of the SPCs by the CF...therefore your orientation would be grid north. Any help would be appreciated....Just trying to get my thinking straight.

I think someone mentioned in a message a few weeks ago that this provides the ability to have your "ground" local system registered to a local north or true north if you used the grid mapping angle backwards. Both seem like useful options. I haven't checked myself to see if they can perform these operations though....

- jlw

I guess what got my curiosity up was that when I tried to convert grid to ground with 2.4 I chose the "grid" option expecting the "new" coordinate values to be pretty close to the original since the CF was something like 1.000004658. I was surprised to see the coordinates so much different until I figured the software was rotating them in addition to scaling. I was just wondering why it does this. I wanted grid north. Is my thinking off track?

I think that if you want ground distances & grid north, you put a rotation of "0" in the dialogue box. Otherwise, it rotates to geodetic north at the point of your choosing.
terry

Under orientation in the dialogue box I chose grid...the angle option was "grayed out". You are right, I wanted ground distances & grid north. It doesn't matter what I do, the coordinates get rotated unless I'm just missing something. It looks like to me that if I choose my orientation to be "grid" then the software should not rotate the coordinates. I also tried the angle option set to 0 to see what would happen...You guessed it...rotated coordinates. Thank you guys for your input. Could some of you try this and see what happens?

Edward,

I have been there, done that. Now, what I found to be the case, was that 2.4, in the grid to ground function, setting the rotation angle to 00-00-00 will take the convergence from state plane to geodetic at the selected point and will in fact rotate the project to geodetic at the given point.
Try this. Resolve your network in SPC, and under the "report" function in the file menu, go ahead and print out your site positions in SPC. I think along with the site positions it also gives you the convergence at each point.
Then in the grid to ground conversion, apply the convergence in the opposite direction at your selected origin site. If the angle dialog box gives you fits like others have stated (me included) then just use the complimentary angle (if you want say (-) 00-09-56 then try 359-50-04) and see what happens.
But don't hang me if it goes wrong. You need to perform a celestial observation, try the NGS program "inverse" (free download of the geodetic toolkit at the NGS website) and apply the convergence; whatever to suit yourself that the program is indeed performing what you want.
Give that a shot and let us know what you find out.
Search this message board back to 8/06/01 and check out the thread "solutions 2.4, tip #1" that I posted and JD followed up on.
Modified By Dave Huff on 8/27/2001 at 12:21 PM

It sounds to me like you are making the wrong selection. If you select grid, then your job is grid all the way. This is why the selections are greyed out.

What you want instead is to select under coordinate system in project settings, the GROUND option, maybe local coordinates would do it to, but haven't got to it yet....

Anyway then you either pick a ground sytem you have already defined -OR- it will be set to NEW and you hit the little ... button next to it where it says NEW. This gives you a dialog box to setup your ground system. Defining a point in your job to local coordinates and what rotation to apply, etc.

It looks like the local grid is really custom projection and a different cat, but could maybe be used to accomplish the same thing but with much more effort.

- jlw

It is important to recognize that when using a local grid, perhaps rotated to geodetic north at the center of your project, you must relate and rotate all solar observations to the "central meridian" of your grid. Its about 1 minute per mile in easting.

Your local grid has a central meridian just like SPC, with its own convergence. The only reason you like the local grid is that it is true to ground measure of distances.

The convergence of your local grid will be the same as the convergence of the SPC system in your area, but you look only at the east-west distance from the SPC central meridian to calculate it.

Also, when filing a map of the survey, the meridian of the map, if it is GPS geodetic north, or solar astronomic north, must be accompanied by information about the actual position of the solar, or central meridian. Otherwise, it cant be rotated to other surveys. The proper basis of bearings will read "Astronomic North, as determined by solar observations AT THE CENTER OF SECTION 5" or at some known point.

If this isnt obvious I will be glad to expand on it later.


Modified By Nearly Normal PLS on 8/28/2001 at 3:12 AM

I spoke with a Solutions engineer regarding whether or not Solutions will rotate the Ground system so that the Azimuths are True North when a 0 degree Angle is chosen in the Ground System Definition box. This is a situation whereby a NEW Ground system is being created.

The azimuths in the Ground system are still related to Grid North in a Stereographic Projection when a zero degree angle is entered. The Meridian Convergence is zero at the origin of this Stereographic Projection.

Best regards,

Richard