从不规则间距x和y的ncdf创建光栅砖

Question

我是将R用于GIS目的的新手。我有一个netcdf文件，其中包含多个具有多个维度（x、y、z、value和time）的变量。我正在尝试将其变成光栅砖。数据相当大，因此我需要从指定的时间窗口和z（深度）中提取数据。这不是问题，并使用下面的代码提取具有适当维度的数组。

library(ncdf4)
library(raster)
t <- ncvar_get(nc, "model_time")
t1<-ncvar_get(nc,"model_time_step")
tIdx<-t[t> 20120512 & t < 20120728]
tIdx2<-which(t> 20120512 & t < 20120728)
# Depths profiles < 6 meters
dIdx<-which(nc$dim$depthu$vals <6)
# ncdf dimension lengths
T3      <- nc$var[[7]]
varsize <- T3$varsize
# Define the data (depths,time,etc.) you wish to extract from the ncdf
start <- c(x = 1, y= 1,depthu=1, time_counter = min(tIdx2))
count <- c(x = max(varsize[1]), y = max(varsize[2]),depthu=1, time_counter = 
max(tIdx2)-min(tIdx2)+1)
# order of the dimensions
dim.order <- sapply(nc$var$votemper$dim, function(x) x$name)
temp<-ncvar_get(nc,"votemper",start=start[dim.order],count=count[dim.order])
nc$var$votemper

我的数据示例（删除深度/z和时间维度）

   temp<-structure(c(0,0,0,0,0,0,0,15.7088003158569,15.3642873764038,14.9720048904419,,15.9209365844727,14.9940872192383,15.0184164047241,15.0260219573975, 0,15.7754755020142, 15.424690246582, 15.6697931289673,15.6437339782715, 0,15.6151847839355, 15.5979156494141, 15.6487197875977,15.432520866394), .Dim = c(x = 5L, y = 5L))

从ncdf中提取的纬度和经度是不规则间隔的，每个都有两个维度（即每个单元的lat和lon间隔不规则）

lon<-structure(c(-71.2870483398438,-71.2038040161133,-71.1205596923828,-71.0373153686523, -70.9540710449219, -71.2887954711914, -71.2055587768555,-71.122314453125, -71.0390701293945,-70.9558258056641,-71.2905654907227,-71.2073211669922,-71.1240844726562,-71.0408401489258,-70.9576034545898,-71.292350769043,-71.209114074707, -71.1258773803711, -71.0426330566406,-70.9593963623047, -71.2941513061523, -71.2109222412109, -71.127685546875,-71.0444488525391, -70.9612045288086), .Dim = c(5L, 5L))

lat<-structure(c(38.5276718139648, 38.529125213623, 38.5305824279785,38.532039642334, 38.5334968566895, 38.5886116027832, 38.5900802612305,38.591552734375, 38.5930252075195, 38.5944976806641, 38.6494789123535,38.6509628295898, 38.6524467468262, 38.6539344787598, 38.6554222106934,38.7102699279785, 38.7117652893066, 38.713264465332, 38.7147674560547,38.7162704467773, 38.7709808349609, 38.7724952697754, 38.7740097045898,38.7755241394043, 38.777042388916), .Dim = c(5L, 5L))

通常，我会使用

Temp_brick <- brick(temp, xmn=min(lat), xmx=max(lat), ymn=min(lon), ymx=max(lon),transpose=T)
Temp_brick<-t(flip(Temp_brick,1))

这，然而并不能说明不规则的行间距和光栅单元格值位于错误的位置（lon， lat）.我已经搜索了整个堆栈溢出和其他gis帮助源，我找不到类似的问题与解决方案或我没有问正确的问题.我不是特别确定如何去做这件事.不确定是否应该在从netcdf提取数据时处理这件事或者是否应该在光栅砖已经创建后处理没有定义的范围.我已经试图找到一种方法来定义光栅的lon lats没有任何运气。尝试将lon，lat和value转换为3列数据帧，然后使用光栅::rasterFromXYZ函数。这对于我正在处理的数据大小来说不够快，实际上是197（x）*234（y）*2（z）*900（时间）*5（变量）*12（年（单独的netcdf文件）。

非常感谢您的帮助

Answer 1

一个带有akima的选项，首先将数据插入常规网格，然后将其转换为栅格：

    # define the regular lon lat or just pass the nx, ny param to interp functions
    lonlat_reg <- expand.grid(lon = seq(min(lon), max(lon), length.out = 5), 
        lat = seq(min(lat), max(lat), length.out = 5))

    # interp irregular data to a regular grid
    # both solution return the same results because 
    # i've define the regular grid as akima default
    test <- interp(x = as.vector(lon), y = as.vector(lat), z = as.vector(temp), 
        xo = unique(lonlat_reg[,"lon"]), yo = unique(lonlat_reg[,"lat"]), 
        duplicate = "error", linear = FALSE, extrap = FALSE)

    test <- interp(x = as.vector(lon), y = as.vector(lat), z = as.vector(temp), 
        nx = 5, ny = 5, linear = FALSE, extrap = FALSE)

    # turn into a raster
    test_ras <- raster(test)

检查函数的参数以选择执行的插值等，如果使用外推，请小心！

我也见过这种方法

干杯