[GRASS-user] r.hants -how to determine frequency

[Veronica Andreo]

Hi Rajat,

> Dear All,
>
> I'm trying to reconstruct a cloud free NDVI time series using r.hants
> module in GRASS.
> I have a total of 318 files covering a period of 14 years (Feb 2000 - Dec
> 2013).
> I would like to know if there is any quantitative test to set the
frequency
> for this data range.

Unfortunately there's no test to determine "nf" parameter in hants, but
there's however a rule of thumb that works just fine... To consider at
least 3 more frequencies than what you expect for your variable. For
example, ndvi in a temperate area has 1 cycle per year, then you should use
at least 4 nf.

Quoting Markus Metz here "you need 4-6 cycles per year to accurately
approximate NDVI with one peak per year. The additional cycles are
needed to approximate seasonal differences in the increase or decrease
of NDVI."
The base period is by default the number of input maps. If you feed the
algorithm with one year of data, you don't need to specify this parameter.
If you, however, feed in the whole series of maps, you should use the
number of maps you have in one year as base_period.

> I used frequency values 4, with base period as 14, and frequencies of 28,

> 42 and 56 without any value for base period. Other parameters set were,
> high and low suppression flag, invalid data rejection range (-0.2 and
0.9),
> Fit error tolerance of 0.05, and degree of over determinedness
> as 155.

AFAIK, range option is only for input data, it won't limit values in the
output.

The degree of overdetermination is how many extra maps you want to use to
fit the curve. The algorithm per se uses 2*NF-1 valid data points to fit
the curve. With dod you are specifying how many extra data points it should
use (take into account you have a lot of missing values).

For FET, I just tried different values ranging from 0.01 to 10, letting all
the rest fixed.

> However, none of the curves obtained match the expected distribution. (in

> the study area there is cloud cover for 4 continuous months ).

I worked with chlorophyll data in southern hemisphere and had the same
problem... Maybe it will also happen to you that in those periods hants
will overshoot and predict values outside the valid range of NDVI. You'll
have to decide what to do with them. I fixed them with r.mapcalc (actually
with t.rast.mapcalc).

HTH :)

Cheers,

Vero
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