#109 SAGA Wetness Index calculation fails

[Anonymous]

I'm using SAGA 2.0.5. The SAGA Wetness Index calculation for the attached ASCII grid (elev.asc) fails, the values are all -1.#IND00. However, the calculations of the intermediate values (catchment area, catchment slope, modified catchment slope) seem to succeed, i.e. the resulting grids appear reasonable. I am using a 't' parameter value of 10. No errors are reported. The output is:

[2010-11-23/14:18:56] Executing module: SAGA Wetness Index
Parameters
Grid system: 0.000185; 133x 122y; 76.521935x 11.588407y
Elevation: elev
Catchment area: Catchment area
Catchment slope: Catchment slope
Modified catchment area: Modified catchment area
Wetness index: Wetness index
t: 10.000000

[2010-11-23/14:18:56] Module execution succeeded

[Anonymous]

ESRI Arc/Info Grid (elevation)

[Volker Wichmann]

I can confirm this with version 2.0.6.

It seems to be related to the extremely small cell size (0.00018518518666664 m) of your grid. In case I simply hack the header to assume a cellsize of 1m, everything works ok.

Volker

[Volker Wichmann]

Under further investigation it turned out, that this is not a module bug but a problem with your input data. The elevation differences between the cells are too high in comparison to the cell size of your grid. This results in slope values near to 90 degree.

The wetness index is calculated by

index = log(modifiedCatchment / tan(a + slope))

where a = 0.0174532

Thus, with slope values very near to 90 degree, (a + slope) results in values above 90 degree (actually all computations are performed in radians, but I use degree to make the explanation more obvious). The tan then gets negative and you can not take the log of a negative number.

Seems like your data is not applicable to be used in 2.5D. I'm closing the bug, but keep the item open for posting.

Volker

[Johan Van de Wauw]

Just an assumption: are you using data in degrees? In that case you should reproject to meters prior to doing terrain analysis.