The discussion of single precision and double precision in Flood Modeller is frequently visited; which is most appropriate and how these are run from the user interface, particularly within the batch runner.
The first question our support team is usually asked is why double precision (or what?). Single precision solves the numerical solution to approximately 7 significant figures. Double precision gives you 15. But surely 7 is more than enough? And the answer is usually, yes, it is. However, there are certain circumstances where significant information can get lost beyond the default precision. Typical examples include:
Direct rainfall modelling (particularly linked to Flood Modeller 2D)
Reservoir modelling, especially those with large surface areas and/or high elevations above datum (e.g. >100m) and/or small timesteps
When you convert rainfall to SI units of m/s (or even to m/timestep, which is typically of the same order) you can probably guess why, when compared to the other "numbers" in the numerical model.
Also, you may have even seen such reservoir model results as mentioned above - those which show a nonzero net inflow, but the water elevation doesn't change (or vice versa). This suggests: "Use the double precision version".
The other question revolves around how to pick up the double precision version of Flood Modeller. There are different executable files for single precision (the default, isisf32.exe) and double precision (isisf32_DoubleP.exe).
To set the precision for an individual simulation, double-click on your simulation in the Project Panel to open the Simulation Run window. Then select the "Double precision version" radio button from the Backward compatibility tab.
Note: You may need to select “View” and check the box to the left of the tab you want displayed as shown in the figure below. By default, Flood Modeller does not display all these tabs automatically.
If running your simulation through the Batch Runner tool, the precision will be determined by the simulation files. As in the above section, select the desired precision for each of your simulation (.ief) files. The user interface will then read this and know which version to use to run the simulation.
If you wish to run batch simulations of double precision Flood Modeller outside of the user interface, e.g. using a .bat file, the syntax (followed by an example batch file) is as follows: isisf32_doubleP -sd "myieffile.ief". You will need to explicitly select the double precision version of the simulation engine as described, since there is no user interface to read and interpret the simulation file.