Coordinate System and Scale Factor

[shajahmadi]

When I change the coordinate system, it doesn't affect the scale factor, which it should. 

When I want to work on a drawing for a long road and implement it by GPS, I need it to be on the same coordinate system as my GPS. 

[SoaresASBCAD]

Hi @shajahmadi 

No, you can follow that everythig is fine

[shajahmadi]

So why it's like this?

[SoaresASBCAD]

Need to adjust the coordinate system, but it doesn't affect the scale

[shajahmadi]

So what's the point of changing the coordinates system, other than having an aerial image?

Even when I change the scale factor on the transformation tab it doesn't affect anything. 

[ChicagoLooper]

@shajahmadi wrote

<<..... I need it to be on the same coordinate system as my GPS....>>

Just curious, what's the coordinate system of your GPS? Do you know and have control over that coordinate system? Or someone else decide the CS for you?

[shajahmadi]

I'm using MGA50 GDA94 

[Pointdump]

Hi @shajahmadi,
You need to type in a point number or click the little arrow in the upper left of the Geodetic Calculator to get it to populate with information.
Dave

[shajahmadi]

Thanks for this. 

So this value for the scale factor means geodetic distance divided by grid distance? Or opposite?

[Pointdump]

@shajahmadi,
Scale Factor is Grid Scale Factor, the relationship of the flat projection "cutting" the Ellipsoid. Grid Scale Factor is different for every point on your map.
Dave

[shajahmadi]

I understand the calculations and for MGA50 GDA94 the scale factor is 0.9996 which I calculated on the field as well but in this calculator, it says 1.000579583678

[Pointdump]

@shajahmadi,
Grid Scale Factor is only 0.9996 along the Central Meridian, 117° Longitude.
Dave

[shajahmadi]

Thanks so much Dave

[rl_jackson]

To add to @Pointdump advise, perhaps an explanation in order. You data collector is inputting the 0.9996 which is the scale factor to get the Ground point location to the plane. The 1.0005..... is the inverse of that, which scales the point to ground based on the elevation which is the Combined scale factor.

[shajahmadi]

So you are saying that the scale factor that Civil 3D gives us is the scale factor to get the Plane point location to the Ground?

[rl_jackson]

That's correct. If you import the points with the scale fact at the 1.0005 then that pushes the data to ground

[shajahmadi]

Then it's opposite of this definition: 

[Pointdump · Accepted solution]

@shajahmadi,
You ready for a deep dive? Attached, please find the very best explanation on this subject that I've found.
Dave

[shajahmadi · Accepted solution]

Fantastic, thanks Dave

[45eptop]

Good evening!

After reading all the answers, and especially the pdf from Dave Stoll I have some questions on how civil 3d works when we design corridors. I have a corridor Midsection and layout. I have noticed that the length of the midsection is 1000meters and the same length is on the layout, but the layout is on a grid with coordinates. If I use those coordinates to stake out, the stakeout distance will be greater than 1000meters because of the scale factor, and that's a obvious problem. So I'm trying to understand a solution to this problem, and I think the solution might be in the way how the corridor was designed in civil 3d.  So is there a specific way how a corridor is designed on a grid in Civil 3d? Is the midsection always refereed to ground distance?