Creating a shade layer (not shaded relief)

Question

Ok, I've tied searching for several days and not come up with a solution.  I am trying to create a raster that depicts the area upon the ground on which shade is being produced the tree canopy.  Ultimately a series of these layers could be added together to determine how much a point on the ground is shaded during the day.  Unfortunatly the standard hillshade model doesn't project the shade onto the surface of the ground but rather onto the surface of the input file.  For example, consider a tall tree surrounded by shorter trees.  The shorter trees and the tall tree itself would naturally shade the area directly under the tall tree.  However, using the standard hillshade model every canopy-shade layer would show the top of the tall tree being illuminated.  When all of the canopy-shade layers are summed to produce the final shade layer, the location under the tall tree would show little, if any shadowing because the shadowing is being applied to the tree canopy surface and not onto the ground. This is illustrated in the attached JPG.  The area where the star is located is not being modeled as being shaded because it is directly underneath the tall tree. 
[ATTACH=CONFIG]22953[/ATTACH]

Any suggestions would be helpful.

Thank you

shading Illustration.jpg

doronwen_0-1755288989172.png

MarkBeaty · Answer

I am trying to model where shadows are being cast onto the ground from a tree canopy/elevation model.  The ESRI hillshade modeler when the "model shadows" is not checked identifies shaded area based on the slope and asprct of each pixel.  When the "Model Shadows" option within the ESRI hillshade model is enabled it produces shadows that appear to be cast upon the landscape except where the shadow is interrupted by an illuminated pixel.  I have not been able to find a description of how the hillshade model models shadows when the option has been enabled.

curtvprice · Answer

Good news: 
I found the page describing how a hillshade is calculated. Here

I'm pretty sure I can construct a model to output the ground level shade from this reference.  And here's the "But"

But this hillshade algorithm does not show how to account for the length of the shadow when the "Model Shadows" option in the standard hillshade model is toggled on.

What are you trying model with respect to shadow length?   ArcGIS is not a true 3-d modeling environment though sometimes we can do simple approximations using the basic raster tools.

MarkBeaty · Answer

Good news: 
I found the page describing how a hillshade is calculated. Here

I'm pretty sure I can construct a model to output the ground level shade from this reference.  And here's the "But"

But this hillshade algorithm does not show how to account for the length of the shadow when the "Model Shadows" option in the standard hillshade model is toggled on.

Any references to how this is done is appreciated