Decimal Values in Hazard Outputs

[tuflow support 0]

Question:

Why can I see decimal values in my hazard results? How should these values be categorised?

Answer:

The hazard results in TUFLOW will be output as integer values in the .xmdf and .dat files at the cell corners.  However, when exporting to grid (in either .flt, asc or .nc format), the raster output is north south aligned and a regular grid (but your TUFLOW model may be rotated, may have 1D triangles and may have multiple domains).  Therefore, the TUFLOW results are interpolated onto a north-south grid when directly writing grids from TUFLOW or when using the TUFLOW_to_GIS utility. 

Since TUFLOW build 2013-12-AD when directly writing a gridded output (e.g. .asc, .flt, .nc) the hazard outputs (except for z0, which is the velocity - depth product) are always output as an integer value, i.e. you only get a value 1,2,3 or 4 when directly writing hazard output to .asc . This is done in by rounding to the nearest integer, e.g. when interpolating a hazard value of 3.05 this is output as 3.0.

If using an older version of TUFLOW, or using TUFLOW_to_GIS to post process, this can result in decimal values in the .flt or.asc grids between the desired integer values for hazard classification.  To be consistent with how the latest TUFLOW Build is categorising floating point decimal values to integers when it directly outputs hazard grids (described above),  the approach would be to categorise to the nearest integer value, e.g. 1.500-2.499  as 2 etc.  If you did wish to reclassify  1.001-2.000 as 2 etc, this would provide a conservatively high hazard classification for decimal values, as you would be counting anything greater than the absolute integer in the next band.