[GRASS-user] Extracting vegetation phenology from Landsat-based time series

[Nikos Alexandris]

Nikos Alexandris:

>>> Is the number of cycles per year in the Wiki as well?  Going through,
>>> last time, I think I didn't grasp that.  Can you pin-point?  It's
>>> exactly what we need at the moment.

Veronica Andreo:

>>Yes, maybe it is not well explained, but it is here:
>>https://grasswiki.osgeo.org/wiki/Temporal_data_processing#Filling_and_reconstructing_time_series_data_with_gaps_-_HANTS

>>and also in the manual under "NOTES"
>>https://grass.osgeo.org/grass70/manuals/addons/r.hants.html

>>The idea is, once you are happy with the result of hants, you ask for
>>amplitude outputs and with those amplitude maps (you'll have one map per
>>frequency) you run r.series method=max_raster. This will give you the most
>>important frequency (according to amplitude value of that frequency) in
>>each pixel. If the result is that the most important frequency is 0, then
>>you have one cycle per year (that, if you use a base period of one year of
>>course).

>>Hope it is clearer now :)

Nikos:

>Yes. It was even before -- just me read through quickly.
>
>r.hants is awesome!
>
>We have 24 Landsat 8 derived EVI2 maps (2 for each month) for one year.
>Clouds and Water surfaces removed. Another set of "relatively normalised
>images, using `i.histo.match` is also ready.  We really need to "fix"
>`i.histo.match` to crunch floats directly.  The resulting ranges worry
>me a bit -- I just followed the "old" way as discussed some years ago in
>a relevant thread (floats > integers > histo-matching > floats).
>
>Anyhow, I am testing the following:
>
>for NF in 4 5 6 7 8 ;do r.hants file=evi2_maps nf=$NF fet=0.05 dod=3 base_period=24 suffix=_hants_nf_$NF amplitude=amplitude_hants_nf_$NF phase=phase_hants_nf_$NF ;done
>
>(thinking loudly... it would be super-nice to have `t.rast.hants`)
>
>Not sure about `dod`. Perhaps it should also follow a patten like 3 4 5
>6 and 7 for the above?
>
>for NF in 5 6 7 8 ; do t.rast.series evi2_hants_nf_$NF method=max_raster output=dominant_frequencies_hants_nf_$NF --o

Just in case, the above *should* read:

for NF in 4 5 6 7 8 ;do r.series input=`g.list rast pattern=amplitude*nf_${NF}* separator=comma` method=max_raster output=dominant_frequencies_hants_nf_${NF} --o ;done


>"Dominant" frequencies 0, 1, 2 and 3 appear to be within the areas
>of our interest (agricultural surfaces).  Very good.  And convincing.
>Yet I am learning to interpret this correctly. And the "phase" as well.
>Reminds math studies, years ago.

It is a "good" thing to actually have similar dominant frequency "maps"
for all of the experimented Number of Frequencies.

A confirmation would be reassuring.

Thanks, Nikos

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