This is a follow-up post to Tuesday’s Garden Placement Using Publicly Available LiDAR, which relied on ArcGIS’ Spatial Analyst extension. Huge hat tip to the LAStools Linkedin conversation, and particularly Daniel Grohmann, for mentioning the freely available SAGA GIS tool, Potential Incoming Solar Radiation. I should note that this is truly my first attempt at using SAGA, so apologies in advance if I’m leading you astray in some way.
To recap, we had created a digital elevation model (DEM) from publicly available LiDAR from the City of Prince George, and were ready to calculate solar radiation values for a city block in order to optimally locate a garden plot.
- Download, install, and open SAGA GIS (it’s free)
- Import your DEM into SAGA. I used the GDAL: Import Raster module (Modules -> File -> GDAL/OGR -> GDAL: Import Raster). Double-click the layer in the Data workspace window to view the raster and ensure it imported correctly.
- Open the Potential Incoming Solar Radiation tool (Modules -> Terrain Analysis -> Lighting -> Potential Incoming Solar Radiation). Here is how I set up my run:
- After changing the colour ramp to the default (blue -> red), we are left with a very similar, if not identical, solar radiation map comapred to that produced by Spatial Analyst:
Here’s a quick method for finding the sunniest spot on your property in Prince George, BC (and elsewhere if you can find the data). Unfortunately, this method is not free from start to finish (it requires ArcGIS Spatial Analyst), but there are a fair number of free tools and data used.
- Get the data. Specifically, you need the raw LiDAR (.las file) for your area, which are not available for download from the City of PG’s Open Data Catalogue. While you’re at it, also ask for the orthophotos (aerial photos) and cadastral data (parcel boundaries, road files, etc.). Get in touch with the City of Prince George’s GIS department to make your data request: email@example.com
- Load your orthophoto into your GIS program (I’m using ArcGIS, but you can use QGIS for the time being [it’s free]).
- Create a new polygon feature class and draw a polygon to narrow down your area of interest.
- Download LAStools (it’s free for the tools you need). Bring the LAStools toolbox into your GIS (there are ArcGIS and QGIS versions).
- Run the lasclip tool to ignore the millions of LiDAR points that you’re not interested in.
- Run las2dem to create an elevation raster of your LiDAR data.
- Consult your favourite source, like the Farmer’s Almanac, to determine the timing for your growing season, .
- Here is where I used a Spatial Analyst tool called Area Solar Radiation, which is not free (in fact, it’s darn expensive). Run the tool, using your latitude (PG is about 53.914), and frost free start and end dates (for PG, June 4 to Sept. 3). You should end up with something like this, which my legend tells me ranges from blue (353 watt hours per square metre) to red (883061 WH/m^2):
- Now, it’s a matter of overlaying parcel boundaries, finding your property, and seeing what kind of sunlight you can expect. I’ve circled a few good candidates on this block with lots of sun in the backyard, and (surprise!) some of them are existing gardens.
Bonus: another cool thing you can do (for free) is load your monochrome elevation model in Blender, extrude the terrain heights, drape the aerial imagery overtop, and create a 3D animation like this one.