Arbor Shadow - Brief Program Overview

New Developments: This program calculates and plots tree shadows on to a site plan image. The relevance of tree shadows in relation to new or existing development is important to ensure that proposed recreational areas such as gardens and windows (on building elevations) will not be excessively shaded by adjacent trees. If these areas do become excessively shaded, the tree may become a target for pruning or even removal. By understanding the position and motion of tree shadows at an early stage problems can be avoided (by for example, adjusting the proposed site layout). It is the job of the contracting arboriculturist to keep his/her client informed of these issues and to supply them with data from which they can modify their plans. The assessment of a trees shading capability should be included where necessary within an Arboricultural Implication Assessment (AIA) which considers the relationship between existing trees and a proposed development.

Local Authorities: Tree officers working for Local Authorities are obliged to ensure that important trees are retained where possible in relation to new development. They must consider a number of factors, including the issue of sunlight. Councils receive a large number of requests to prune or remove protected trees due to sunlight interference. They can of course refuse the requests, but this may be overturned on appeal to the Secretary of State, who may, for example, judge that the light loss is unreasonable and the tree should be removed. To avoid such a set of circumstances arising, local authorities must ensure at an early planning stage that the issue of light interruption, both in the present and in the future has been considered. They should request information from the developers demonstrating that a reasonable level of sunlight will be available to gardens, recreational areas, windows etc. 

Arbor-Shadow - Is a tool which calculates and plots tree shadows onto a digitalbitmap plan.

Canopy Spread Projection - A simple tree canopy is based upon 4 dimensions being measured from a tree centre in the direction of north, south, east and west. Assuming the tree is standing on level ground and the plane of the canopy lies parallel to the ground then each point around the perimeter of the tree canopy will be of the same height but at a different distance from the tree centre. Since light travels in straight lines and the rays of sunlight which strike the earth are considered to be parallel to each other (due to the size of the sun), it follows that an exact 'copy' or shadow of the canopy shape will appear on the ground (see following image).

Imagine this projected disc as being the base of a trees crown. If another disc was defined at a taller height, so it represented another 'slice' through the trees crown then this can also have its shadow projected onto the ground, but would be slightly further away from the tree (and probably be of a smaller diameter in relation to a narrowing tree crown). If the process were continued right to the top of the tree then I would have an irregular representation of the trees shadow on the ground.

 If the distance of each 'slice' step was small then I would see an accurate representation of the trees shadow.  

However, as you can see, a number of assumptions are made. First the trees are assumed to be straight and vertical. The ground is assumed to be flat and at right angles to the tree. The crown outline won’t be an unbroken oval as depicted here.

Although some of these restraints can be overcome such as allowing for a leaning tree and a sloping crown, other factors such as a true crown shape would be impossible to cater for within a program. This is due to the infinite possibilities in actual shape and branch layout, even between trees of the same species and habit.

Tree Dimensions - The necessary dimensions required to calculate a shadow projection is tree height, clear stem height, lower crown dimensions (as north, south, east & west) and upper crown dimensions (although these could be automatically calculated as a percentage of the lower crown). 

Day Shadow Trace - Arbor-Shadow can plot a shadow projection to represent the area that would be 'touched' by a trees shadow during the course of a day. Think of this as the above shadow projection being calculated non-stop from 5am in the morning until 10.00pm at night. All the projected shadow images would merge and form a continuous band of varying width and shape as the sun transits across the sky from east to west.

Shadow Display - Arbor-Shadow can display a tree shadow for a single moment of time or can show its duration for a whole day as a 'day trace'. They are both shown graphically in 2D. You can imagine that are looking vertically down over the tree and viewing the projected shadow as a shaded area. The screen image will always be orientated with its top towards North.

The following picture shows a tree with its projected shadow laid over a 1 meter grid.

The shadows are created by projecting horizontal 'slices' of the tree canopy. The following image shows the upper and lower canopy outlines projected onto the ground.

The upper and lower canopy projections can be seen within Arbor-Shadow by selecting the 'Show Shadow Detail' selector from a drop down menu. The following image shows these projections outlined in blue.

In this mode the detail also shows two shadow length dimensions. The 'Maximum length' is calculated using the presence of tree canopy spread (as these can increase distance) while the 'Simple Shadow Length' is calculated using just the tree height alone. The 'Width Markers' show the maximum width of the shadow when measured at right angles from the shadow centre line.

 The following image shows the difference in the two shadow length calculations. Dimension A shows the 'Simple' calculation based upon tree height alone while B shows the true 'Maximum' shadow length. The influence of canopy spread can increase some broad tree canopy shadow lengths by a significant number of meters. 

The Tree Shadow Trace - The tree Shadow Trace shows the area which the normal tree shadow would transit during a day. As the tree shadow moves from west to east its 'imprint' is recorded on the screen to leave a large shaded area. The smaller true shadow can be displayed on top if required (see below). Note the flat ends to the trace shadow on the eastern and western extents. This is a 100m cut off point (measured from the tree centre) which had to be included for memory conservation reasons. The grid below is spaced at 10m and the sun shows a position early in the day.