Project North, True North and Shadows

[milliharpreet]

I am very new to Revit Architecture 2009, in crawling stages. I have confusion regarding project north and Project North. I have read many threads, but they seem to be for user at certain level in revit. my question is as below:

1. when I specify my geographic location in revit, where does its north point out?

2. what is the relation between project north and north of geographic location we specify? do they point out in the same direction that is top of screen? does they maintain that relationship, if we rotate project north?

3. what is the relation between true north and north of geographic location? when we rotate true north, how revit calculate correct shadows.

this concept is bit confusing. could you please help?

[Alfredo Medina]

1. when I specify my geographic location in revit, where does its north point out?

When you specify a geographic location, by providing information such as street address, city, state, zip code, and country, or by providing Latitude and Longitude, you are locating a point in the world, and North always points up.

2. what is the relation between project north and north of geographic location we specify?

Project North is an orientation we choose to model our project, in the most comfortable way. Usually is the orientation where straight angles can be made with verticals and horizontal lines on the screen. It might be just the orientation that best fits in our title blocks, or both. North of geographic location is always the same North pointing up. Therefore, most likely there is some angle of deviation between that orientation you choose for your work, and the True North of the world.

...do they point out in the same direction that is top of screen?

If you don't specify an angle of deviation from that orientation you chose and the True North of the world, yes, both Project North and True North point up to the top of the screen.

...do they maintain that relationship, if we rotate Project North?

If you use the Rotate Project North tool, the True North will remain the same, but your project will rotate on the screen. However, I advise you to stay away from this tool. Some objects refuse to rotate along with the rest, with disastrous results. If you establish your floor plan view orientation and the True North correctly, you don't need this tool; it just adds to the confusion.

3. what is the relation between true north and north of geographic location?

True North and north of geographic location are the same thing. What varies is the angle of deviation of your project on the site in relation to the True North.

...when we rotate true north, how revit calculate correct shadows.

Revit calculates correct shadows with software adaptations of the theory of shadows in descriptive geometry, a theory developed by mathematicians about 200 years ago. The shadows are regions whose edges are determined by projecting on surfaces the points that result from the intersection of edges of model elements (such as roofs, columns, overhangs, etc) and a source of light from the Sun, which is determined by a vector created by two angles: the Azimuth (angle on the ground) and the Altitude (angle from the ground).

In order to use the correct azimuth and altitude, you must provide the following 3 pieces of information:

1) A geographical location of your project in the world. (town, latitude, longitude, etc.)
2) A true North: an angle of deviation between your project on the site and the True North.
3) A specific date and time.

See illustration.

In figure 1 the default Project North and True North are at their default positions. No geographical location has been provided. Orientation parameter does not have any effect. Shadows are not accurate.

In figure 2, True North was rotated 20 degrees to the right (for Revit that reads 20 degrees - West), since that was the deviation between the project and the True North.

In figure 3, True North remains True, and the orientation parameter of the floor plan view was set to Project North.

In figure 2 and 3, given that the 3 points mentioned above were provided, shadows in elevations will be accurate, regardless of the orientation of the floor plan.