Thank you for the reply Veronica. I shall try again with number of maps in one year as the base period and with varying FET. Thank you again, Regards
Rajat Nayak On Mon, Feb 9, 2015 at 6:21 AM, Veronica Andreo <veroand...@gmail.com> wrote: > 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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