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Extract values from a Raster*TS object for the spatial locations which can be specified by spatial points, lines, polygons or an Extent (rectangle) object or raster cell number(s).
An rts object.
extract(x, y, time)
Arguments
x is a raster time series (Raster*TS) object created by rts
y is a SpatialPoints*, SpatialPolygons*, SpatialLines, Extent object, or a vector (representing cell numbers)
time is Optional; the time index for which the values in raster should be extracted. It can be numeric, timeBased or ISO-8601 style (see details)
This function uses the raster and xts packages to extract the values in space and subset them in time by specifying ISO-8601 compatible range strings. This allows for natural range-based time queries without requiring prior knowledge of the underlying time object used in construction.
When a raw character vector is used for the time, it is processed as if it was ISO-8601 compliant. This means that it is parsed from left to right, according to the following specification:
CCYYMMDD HH:MM:SS.ss+
A full description will be expanded from a left-specified truncated one.
Additionally, one may specify range-based queries by simply supplying two time descriptions seperated by a forward slash:
CCYYMMDD HH:MM:SS.ss+/CCYYMMDD HH:MM:SS.ss
The algorithm to parse the above is .parseISO8601 from
the xts package.
file <- system.file("external/ndvi", package="rts")
ndvi <- rts(file) # read the ndvi time series from the specified file
n1 <- extract(ndvi,125)# extract the time series values at cell number 125 for all times
n1
plot(n1)
n2 <- extract(ndvi,125,"/20090101") # extract the time series values at cell number 125
# for all times after 2009-01-01
n2
plot(n2)
n3 <- extract(ndvi,125,"200901/") # extract the time series values at cell number 125 for
# all times before 2009-01
n4 <- extract(ndvi,10:20,"2008") # extract the values at cell numbers of 10:20 in
# the year of 2008
n4
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