# How to create a solar intensity map for a garden plot?

I'm interested in creating a raster surface that depicts the average solar intensity per unit area for a small garden plot. I imagine the inputs into the model would be the following:

• Plot area--for simplicity sake, let's assume the garden plot is circular.
• Average tree height of surrounding forest edge
• Solar intensity at various sun angles
• The average sun angle in relation to the garden plot over time.

I ultimately want to show the best location for plant growth within garden plots that are surrounded by trees. Could anyone point me toward a solution for this analysis? I'm sure someone has done this already, although I cannot find any references.

Edit:

For example purposes, the attached image shows a 1 acre garden plot surrounded by an even-aged stand of conifers. To further simplify this analysis, let's assume we're interested in only the solar zenith and intensity during the gardening season from June to October. Felling trees is not an option!

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If your plot is anything like mine--surrounded by trees of various sizes and species, as well as buildings, and on a slight slope--then these data won't give you anything realistic. Having closely observed the sunlight for a year, I have noted that it depends crucially on the specific positions of a few trees, the shapes of their crowns, the dates at which they leaf out, and the dates at which their leaves fall. All this in addition to the specific course of the sun through the sky and how it changes throughout the seasons... Averages just won't do. – whuber Jan 18 '13 at 21:58
For most purposes, local weather will make a lot of difference to plant growth (e.g. solar intensity isn't as important as say days above 15 degrees or 30 degrees or whatever, depending on plant type). – BradHards Jan 18 '13 at 22:04
@Brad That's true. The stated problem is to optimize planting within a local area. Possibly the biggest cause of local variations within a small area is variation in sunlight. Other sources of variation, like slope, drainage, soil type, and so on, are important, but are also more under the direct control of the gardener or are readily observable (like proximity to shelter). So there's value in knowing the patterns of sunlight during a growing season (it really makes a difference). My comment was aimed at pointing out the level of detail one would need to carry out this calculation effectively. – whuber Jan 18 '13 at 22:08
destructive comment possibly: Can the trees be felled? not that I want too, we have conifers... so the type of trees is important too. – Mapperz Jan 18 '13 at 22:18
For close to solstice, it'll be close to overhead, and you don't want to plant close to the trees for other garden reasons (like the tree roots will extract all the nutrients and water from your soil)... I do like the SketchUp idea (which is what a friend used for his solar panels) and SAGA GIS ideas though. – BradHards Jan 19 '13 at 0:29

I am not sure if you are comfortable using SketchUp, but there are tools that take all this into consideration. SketchUp will allow you to bring in GoogleMaps as imagery, so you can "heads-up" digitize your garden and surrounding features. Using a plugin called LightUp allows you to make solar maps. Here is a youtube video of the plugin in action.