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Forums >> Revit Building >> Technical Support >> Do you really trust Revit shadows?
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Joined: Tue, Aug 16, 2005
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Have you ever check if the sun studies you perform in Revit really match the real shadow at your place?
I did and they do not match.
What could be wrong with my experiment?
See the attached file e try to do the same.
Tel me about your results.
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Have you taken elevation into account? Are you showing your Revit model at true elevation? Solar time is affected by elevation due to the fact it is true bending of sunlight over the horizon.
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Even after changing the elevation, using the real local value (745 m) the shadows didn't match. They weren't affected by the change. The shadow is the same in both cases (elevation 0 m and 745m). In Revit, when we set up the geographic cordinates, there is no option to configure the local elevetion, is there?
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Have you set True North? If your building is drawn with the top of the monitor as north, it may not be exactly True North. This will make a big differance. Also, check to make sure that the building is in the right location. Latitude and Longitude is more accurate than just picking the nearest big city. I have included the help file for True North, as it iisn't rreally simple to figure out on your onw. Location is easy, and shouldn't be hard to learn.
"To orient a project to True North:
In the Project Browser, open a 2D plan view of the project. You must be in a 2D plan view to set the view orientation to True North.
Click View menu > View Properties.
TIP:You can also right-click in the drawing area, and click View Properties to access the Element Properties dialog.
In the Element Properties dialog, under Graphics, select True North for Orientation, and click OK.
NOTE:You must change the view orientation to True North before you can rotate the project to True North in the view.
Click Tools menu > Project Position/Orientation > Rotate True North.
On the Options Bar, for Angle from Project to True North, enter a value to set the angle of rotation. For example, if the difference between Project North and True North is 45 degrees, enter 45 in the Angle from Project to True North field. The project rotates in the view to the specified angle.
NOTE:Settings for project location and orientation, including the angle from True North, are saved with the project, not the view."
Hope this helps
ArchWestCY
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Thanks for the tutorial. Really to undestand the process of rotating true north is not that easy.
But I had already done that. So, the model is correctly oriented. The True North is realy the top of the monitor.
And now, what do I do?
Have you ever check the shadows of your projects? They really match with the real ones?
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Maybe its the difference between summer and wintertime clock settings.
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ZENO-architects
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So, if you already have True North set, I'm guessing that you have the location set as well. I saw that you have set the elevation to match your location. Wow, that's a melon scratcher. Are they significantly off? Could it be the Daylights Savings setting, like Zenoza suggested? Is your camera veiw set to True North. It is possable to have True North set, but your new camera view is still set to Project North. You typed that True North is the top of the monitor, does that mean that True North and Project North are equal? If so, that is pretty handy. I'm running out of ideas.
Wow. I should have looked at you .pdf first. That looks like a really cool experament you have going. It does seem as though the daylight savings time setting could play a part, becuase the differances are not incredably significant. I hope you find a concrete answer. Keep me posted. Are you taking into account the declination for your area. True North vs. Magnetic North?
ArchWestCY
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Oh...Declination beetween True and Magnetic North...Finally something I haven't tried yet...This can cause a visible difference. The angle between the shadow edges is about 19ยบ. I'll try that and I return the result to you...Thank for everybody's help...For a while....
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I'd also suggest filing this with support. They will be able to explain how the revit shadows are calculated. I'm sure the factory would like to know if they are incorrect.
File a support Request. It seems highly unlikely that a tool like this would be implemented that results in incorrect results.
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In the mean while, please answer: DID SOMEONE ALREADY HAVE THE CURIOSITY OF CHECKING THE SHADOWS? OR EVERYBODY TRUST BLINDLY ON THE MACHINE?
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Trust Blindly. Thank You.
ArchWestCY
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I have replied to this post in the Autodesk Discussion groups where you have posted it.
I agree that your problem is probably the differnece between True and Magnetic north.
I have tested the shadows and they are accurate if you use true north. ( have not psyically checked them as you did, but the altitude and azimuth numbers correctly compare the those we have for manually calculating the shadows, and I have compared a manual shadow to a revit one and that match to within a degree or two.
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ALELUIAH! I'VE GOT IT. WITH YOUR HELP FOR SURE.
THE ANSWER IS THE MAGNETIC DECLINATION. TRUE NORTH SHALL BE CHANGED BY A VALUE, IN MY CASE, OF ABOUT 19 DEGREES. THEN, THE SHADOWS MATCH!
SO, WE CAN BLINDLY TRUST IN REVIT AGAIN.
THANK YOU ALL FELLOWS!
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this is reassuring.
i allready had visions of hungry lawyers knocking on my door because of neighbours
getting more shadow that shown in townplanning permits.
(most councils in australia require a shadow diagramme as part of a townplanning application)
as this is so critical why can't revit put a warning on the dialog box with a reference to a tutorial?
if the programme was that intuative, more people would know about this.
the more they can support mere human beings like us the more chance there would be that we could actually start to like the clever chaps.
next thing:
how accurate is the shadow on a topo surface with a steep slope (incline)
i wonder if ivanvasc could test it for us as he allready got a testing rig?
thanks for bringing this to our notice
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glad to help Ivan 
Post edited on 2006-08-11 02:24:41
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Integrator Architectural Engineering Construction KarelCAD |
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Check_Revit_Shadow.pdf