I have done some research but was not able to find a solution to my task!

What do I want:

Pretty simple - I have a building and I want to know how its shadow is influencing a certain site at different days of a year.

Questions asked:

--> How long (hours) is the building shading the site on a specific day?

--> When starts/ends (daytime) the impact by the buildings shadow on the site?

Whats the best way to do this? Can it be done using QGIS/GRASS? Or is ESRIs City Engine the only feasible solution?

  • AutoCAD or any major CAD software can handle this. I don't know if City Engine can do this. You can find one way to do it here: gis.stackexchange.com/questions/17155/… – Delonix R. Feb 21 '17 at 16:11
  • 1
    Welcome to GIS SE! As per the tour there should only be one question per question, but as it is now you are asking multiple questions for multiple GIS packages. Please edit your question to focus on one specific problem for one GIS system. If you want to gather answers for other questions or other software please ask them separately. – Midavalo Feb 21 '17 at 18:40
  • Although the OP has asked the question relative to a variety of potential tools, which does make it rather broad, the original question is straight forward. If @Midavalo or another moderator can remove the hold I will answer the top line question. – katahdin Feb 21 '17 at 21:08
  • @katahdin I have reopened the question, however it will need to be edited by the asker to narrow the focus to just that one question – Midavalo Feb 21 '17 at 21:11

The top-line question about how to find the shadow that a building casts at a particular day, and time, is not generally one that GIS systems answer. Geographic information systems tend to focus on the earth, and not so much on the sun. However, with a 3D modelling system we can find the answer with some work.

First, we need to know were the sun is. The best reference for this that I could find is here, and the algorithm is also implemented in the Python package Pysolar.

Next, we need a way to render the building, and evaluate how the sun at the given position would cause it to cast a shadow. For this a modelling system like Blender or POV-ray could work well. The code implementation below uses POV-ray and the excellent vapory wrapper.

from pysolar import solar
import datetime
from vapory import Camera, LightSource, Sphere, Texture, Pigment, Scene, Plane, Box, Finish, Normal
from math import sin, cos, radians

def sun_location(altitude, azimuth, from_point, dist_to_sun=100):
    """given an altitude and azimuth to the sun, an assumed
    distance to the sun of 100, and a point of origin 
    find the XYZ location of the sun
    Note: the reference system from pysolar is:
    The azimuth to the sun is realtive to South, so subtract 270
    to get Easterly-based directions.

    x_from, y_from, z_from = from_point
    x_sun = dist_to_sun * sin(radians(azimuth-270))
    y_sun = dist_to_sun * sin(radians(altitude))
    z_sun = dist_to_sun * cos(radians(azimuth-270))

    return x_sun, y_sun, z_sun

# calculate the sun's position in the sky on a given day
d = datetime.datetime(2016, 2, 21, 12, 0, 0)
altitude = solar.get_altitude(44.800682, -68.770021, d)
azimuth = solar.get_azimuth(44.800682, -68.770021, d)
sun_loc = sun_location(altitude, azimuth, [0,0,0], dist_to_sun=100)

# create a light source for the scene with the sun's location
light = LightSource( sun_loc, 'color', [1,1,1] )

# set up some screne elements
camera = Camera( 'location', [0,2,-3], 'look_at', [0,0,2] )

plane = Plane([0, 1, 0], 0, Texture( Pigment( 'color', [1,1,1] )))

building = Box([0,0,0], [0.5, 0.5, 0.5],
                         Texture( Pigment( 'color', [0.5,0.5,0.5])),
                        'rotate', [0, 40, 0])

# set up the scene in POV-Ray with the given elements
scene = Scene( camera = camera ,
           objects= [light, plane, building],
           included = ["colors.inc"]) # headers that POV-Ray may need

    width = 600, height=400,
    antialiasing = 0.01,
    quality=1) # quality=1 => no shadow/reflection, quality=10 is 'normal'

It's not beautiful, but here is what the rendered image for the box-like building looks like:

enter image description here

There are a variety of issues to work through, such as:

  • properly locating the building in a geographic coordinate system
  • the effects of local terrain variation on the shadow
  • occlusion of the sun by other objects
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