iLand Tech blog

Vegetation snapshots and Javascript

Tuesday 29 of November, 2011

During the last couple of weeks our efforts are concentrated on the evaluation of the currently developed fire module (see also the last blog post). Besides, we - and especially Rupert - are working hard on getting manuscripts ready for publication.
But back to the fire module: a part of this exercise is to compare simulated with historic fire sizes. This requires, of course, simulated "historic" vegetation states, something we are working on anyways. However, the hard thing would be to run iLand for hours just to reach a point in time after, say, 400 years of simulation.
To make things more tractable, we invented the so-dubbed vegetation snapshots. Essentially, a snapshot is a SQLite database containing the full dynamic content of the vegetation, i.e. all non-dynamic variables of all trees, of all trees in the regeneration layer, of all standing dead trees, and of all soilpools.
Thanks to the very small list of such variables, this endeavor turned out to be pleasantly straightforward. The full C++ code to create the database, save and restore the data is not more than 350 lines of code.

The second ingredient to an efficient fire-evaluation environment, is making both fire events and the control of vegetation snapshots scriptable, and thus automatable. Not a big deal, thanks to Qt and Javascript. This allows the modeler to explicitly (and reproducible!) load a "historic" vegetation snapshot, then to ignite repeated wildfires with certain properties at a certain location, and last but not least, to save fire dispersal patterns as grids. A "fire simulation" mode was also introduced to allow fires spread without actually modifying the vegetation state.
Adding some automation magic on the R side allowed to set up and run a quite sophisticated simulation experiment almost in auto-pilot.

And what if one wants to reuse the analysis scripts when doing a fully dynamic simulation (including dynamic fire) - well, than adding a javascript "event handler" that executes user-defined javascript code after each fire is just a couple of lines away...
And just to show how such a javascript function could look like, a small example:

// event handler. Set the fire.onAfterFire to "afterFireProcessing()"
function afterFireProcessing() {
   var praefix = Fire.id;
   // save the form of the fire in ESRI raster format
   // save a file for each fire
   Fire.gridToFile('spread', 'output/fire/spread' + praefix + '.txt');

So what have we learned?

  • if the simulated entities are simple with regard to state variables, manipulating them is also simple
  • if (service-) functionality is exposed to be scriptable, somebody may some day starts to combine those pieces to create something new - well, that is exactly the point of scripting...
  • the underlying technical components of the iLand project, i.e. Qt with Javascript bindings, and the SQLite database, are great tools, besides being real open source (yes, Qt also!)