Accounting for the real True North

[Anonymous]

In our Australian jurisdiction, surveyors provide site surveys to us in DWG format oriented to Map Grid of Australia (MGA) North. MGA North is not exactly True North: the difference is slight but does vary by a degree or two across the country. This difference will affect shadow studies, especially for tall buildings - where it is vital to get the information right.

My understanding is that the current Revit best practices is to use the MGA North as the orientation for project setup here in Oz. Is this correct? Does anyone use the real True North? Does Revit already take this into account?

If not, how to you take the MGA to True North difference into account when producing shadow studies? In the past we have linked the project file into a blank Revit file and rotated it by the desired angle to produce the shadow studies there. Is that a reasonable course of action?

Any thoughts or ideas would be much appreciated. Thanking you in advance.

(And please note that I am not talking about Magnetic North here. I am referring to the difference between surveyor north and the real true north.)

Anthony

[Corsten.Au]

This will clear your doubts...

http://qarcsystems.com.au/blog/true-north-vs-magnetic-north/

excerpt from above link..

Survey North = True (Solar) North. In this case you would rotate Revit True North to True North. You would be able to tag your property lines successfully and your shadow studies would be accurate.
Survey North = Magnetic North.

a) You don’t require accurate shadow studies.
Rotate Revit True North to Magnetic North. The property lines will tag correctly but shadow studies will be a bit out. The extent of the inaccuracy will depend on the deviation between True and Magnetic North at the site.

b) You require accurate shadow studies.
To achieve accurate shadow studies Revit True North must be rotated to True (Solar) North not Magnetic North.
You can determine True North by adjusting Magnetic North for the deviation appropriate to the site.

[Anonymous]

Thank you for your quick reply, Ganesh, but this is not the information I was seeking.

My post was not asking about Magnetic North. I am referring to the difference between surveyor (MGA) north and the real True North.

The real True North varies across the country and in Sydney is actually about 1° West of MGA North. This is referred to as the Grid Convergence. This is sometimes referred to as Solar North, or Geodetic North. On any project be aware that there is the Project North, MGA North and the real Solar True North. Magnetic North has no influence on our work.

Determine the Grid Convergence with this  http://www.ga.gov.au/nmd/geodesy/datums/redfearn_grid_to_geo.jsp  

Anthony

[Corsten.Au]

You are right, there are variations as per site, location, State to state and city to city within Australia..

Fix is

"you can determine True North by adjusting Magnetic North for the deviation appropriate to the site."

[Anonymous]

Thanks for your reply. I'm still not sure that is what I need.

I also posted to Revitforum and got a suggestion to create a new named Site. I've tested that and it is looking promising: yet to try it on a project though.

https://www.revitforum.org/architecture-general-revit-questions/39669-real-true-north.html 

[Anonymous]

In the Revit world I have never had to worry about magnetic north.  True North is always straight up and is basically what Civil will show as North...  As a former Civil designer, we also showed Magnetic North on our plans in certain jurisdictions.  Not everyone required it though.  I recall my plat maps had it, but we still oriented our drawings to North.  So like in Revit where we had North and Project North arrows we had North and Mag. North Arrows for Civil...but it was just a detail to the sheet, didn't affect the drawing at all.

[Anonymous]

Thanks for your input, but I was not talking about Magnetic North. I am discussing the difference between surveyor';s north and the real north pole of the earth. They are a different thing. I have found a Revit solution here:

https://www.revitforum.org/architecture-general-revit-questions/39669-real-true-north.html 

[Anonymous]

We called it Magnetic North....but yeah, technically it is called Magnetic Declination... here we are about 14.27 positive and I would show that number next to my Survey North arrow.  It changes a bit yes...as I'm 14.17 35 miles East..

With Revit I show whatever the Civil guys are using...If they took account for declination...I show it as it is.

[SteveKStafford]

I don't think it means the same thing, I don't see this site describing magnetic north at all for example.

[Corsten.Au]

On a lighter note...

does Revit takes Earth's Surface Curve into account while calculating shadow???

and Sun a point source of light or parrallel sunrays ??

cause even when north is correct, the shadows wont be correct to be precise..

I think that's where this post is going...

Idea is whats the precision one is looking for and for what purpose... and whats a satisfying level

of details council considers fine to valuate the project...

True north - to calculate shadow

Project north - to orient project ( on Revit sheet etc )

MGA - plays no role in Architectural drawings.

MGA - thats Surveyors scope ..

There's a life cycle of project.. the shadow which is casting on one particular day ( say 26 June 2018 )  will vary after 1000 years cause of fluctuation and variation of earths til, sun, moon, earth movement.... I really don't wanna go into it..... but i hope one needs to establish the purpose of the project and move on...