[R-sig-Geo] NDVI time-series filter

[Tomislav Hengl]

Dear Jonathan,

Interesting topic. Please send us also your code example and if possible part of your dataset (I assume it is a SpatialGridDataFrame?).

Operations on lists can be speed up by using lapply and by implementing your own functions, consider also running Rprof to check which operation is using the most time (read more in "R inferno" or e.g. here: http://manuals.bioinformatics.ucr.edu/home/programming-in-r#Progr_noloops), eventually if the operation is so complex, you simply cannot anticipate to have the results within few minutes (except if you try using super or grid computing; see http://cran.r-project.org/web/views/HighPerformanceComputing.html).

HTH,

T. Hengl
http://home.medewerker.uva.nl/t.hengl/ 


> -----Original Message-----
> From: r-sig-geo-bounces at stat.math.ethz.ch [mailto:r-sig-geo-
> bounces at stat.math.ethz.ch] On Behalf Of Jonathan Thayn
> Sent: Friday, January 29, 2010 5:02 PM
> To: r-sig-geo at stat.math.ethz.ch
> Subject: [R-sig-Geo] NDVI time-series filter
> 
> I need to smooth a collection of annual NDVI time-series data for a fairly large
> image. Right now I am using an interruptive method that compares the difference
> between each point and the mean of its two neighbors. This process repeats until
> a threshold is reached. It works well, but it takes a long time. When I process
> the image without smoothing the data it takes about 2 hours – with the smoothing
> it looks like it will take up to 7 days.
> 
> The method needs to elevate troughs in the data but retain the peaks – I assume
> that any troughs are really cloud contamination and that the peaks are accurate
> NDVI values. Does anyone know of such a smoother that isn't so time consuming?
> 
> 
> Jonathan B. Thayn, Ph.D.
> Illinois State University
> Department of Geography - Geology
> 200A Felmley Hall
> Normal, Illinois 61790-4400
> 
> (309) 438-8112
> jthayn at ilstu.edu
> my.ilstu.edu/~jthayn
> 
> 
> 
> 
> 
> 
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