如何从rasterbrick中提取时间？

Question

这可能是一个简单的问题，但我无法从栅格砖中提取日期，如下所示：

    > ndviStack
    class       : RasterBrick 
    dimensions  : 315, 317, 99855, 221  (nrow, ncol, ncell, nlayers)
    resolution  : 30, 30  (x, y)
    extent      : 232611.4, 242121.4, 2298929, 2308379  (xmin, xmax, 
                  ymin, ymax)
    coord. ref. : +proj=utm +zone=16 +datum=WGS84 +units=m +no_defs 
                  +ellps=WGS84 +towgs84=0,0,0 
    data source : /Users/vaughnsmith/Desktop/CR_Y_Data/Yucatan/area1/data/ndvi_stack_00_15.grd 
    names       : LE70200462000007, LE70200462000055, LE70200462000119, 
                  LE70200462000183, LE70200462000279, LE70200462000295, 
                  LE70200462000311, LE70200462001041, LE70200462001073, 
                  LE70200462001105, LE70200462001169, LE70200462001185, 
                  LE70200462001201, LE70200462001249, LE70200462001281, ... 

    min values  :             2077,             -758,            -1310,              664,             4413,             2121,             1294,             2312,             1138,             -639,             2433,             3138,              991,              150,             1384, ... 
    max values  :            10000,             8087,            20000,             8729,             8989,            20000,             9277,            20000,             7806,             8489,             8839,            20000,             9345,             9281,             9700, ... 
    time        : 2000-01-07, 2015-12-26 (min, max)

最后一行，时间，是我需要的。我认为它可能像名称（ndviStack）函数一样容易返回名称，但事实并非如此。有人可以帮忙吗？谢谢。

Answer 1

我们可以使用names(ndviStack)获取的图层名称来实现此目的 首先是一些名称，以获得可重现的例子：

nams <- c("LE70200462000007", "LE70200462000055", "LE70200462000119", 
"LE70200462000183", "LE70200462000279", "LE70200462000295", 
"LE70200462000311", "LE70200462001041", "LE70200462001073", 
"LE70200462001105", "LE70200462001169", "LE70200462001185")

使用as.POSIXct

nams <- substr(nams, nchar(nams)-6, nchar(nams))
datesfromnames <- as.POSIXct(nams, format = "%Y%j", tz = "UTC")
# "2000-01-07 UTC" "2000-02-24 UTC" "2000-04-28 UTC" "2000-07-01 UTC" "2000-10-05 UTC" 
# "2000-10-21 UTC" "2000-11-06 UTC" "2001-02-10 UTC" "2001-03-14 UTC" "2001-04-15 UTC" 
# "2001-06-18 UTC" "2001-07-04 UTC"

使用as.Date

首先，我们将名称分为年和日。

nams <- substr(nams, nchar(nams)-6, nchar(nams))
years <- substr(nams, 1, 4)
days <- substr(nams, 5, 7)

然后，我们创建一个data.frame

dates <- data.frame(years, days)
dates
#    years days
# 1   2000  007
# 2   2000  055
# 3   2000  119
# 4   2000  183
# 5   2000  279
# 6   2000  295
# 7   2000  311
# 8   2001  041
# 9   2001  073
# 10  2001  105
# 11  2001  169
# 12  2001  185

他们现在掌握了一年中的年份和数量。这些变量可用于生成Date个对象。

dates$dates <- apply(dates, 1, 
                     function(x){as.Date(as.numeric(x[2]), origin = paste0(x[1], "-01-01"))})
as.Date(dates$dates-1, origin = '1970-01-01')

#    years days      dates
# 1   2000  007 2000-01-07
# 2   2000  055 2000-02-24
# 3   2000  119 2000-04-28
# 4   2000  183 2000-07-01
# 5   2000  279 2000-10-05
# 6   2000  295 2000-10-21
# 7   2000  311 2000-11-06
# 8   2001  041 2001-02-10
# 9   2001  073 2001-03-14
# 10  2001  105 2001-04-15
# 11  2001  169 2001-06-18
# 12  2001  185 2001-07-04