问题描述
我需要计算一组点和一条特定线之间的最短距离，并确定所有点到这条线的方向。
从视觉上看，它是这样的：
蓝线表示具体线路。黑点代表 data.table 中的城市。 我需要找到每个点相对于蓝线的方向：是在南还是北？
地图可以来自gadm36_CHN_shp。
china_map_city <- rgdal::readOGR(".../gadm36_CHN_shp/gadm36_CHN_0.shp",use_iconv = TRUE,encoding = "UTF-8") %>%
rmapshaper::ms_simplify()
好的...现在，考虑一个简单的案例，有两个数据集
# data 1: the specific line
line_QH <- structure(
c(120.85553,119.097961,118.881958,118.720215,118.071899,107.718445,107.263489,107.038696,106.805664,106.228394,106.076172,105.298462,105.045898,104.656494,104.313293,104.356018,104.190125,103.901672,103.536499,103.391479,103.178772,103.053833,102.727783,102.369141,102.292175,102.242859,101.742188,101.603149,101.261902,101.37738,101.110962,100.573059,99.984131,99.522827,99.161377,98.848755,98.713623,98.30249,97.725647,97.432373,97.513123,97.425354,97.414124,97.194336,96.803162,96.653564,96.239746,96.013123,95.752869,95.117554,94.685913,94.127991,93.586121,91.132019,90.49884,89.984924,89.81665,89.533264,89.566956,89.457825,89.172485,87.809143,86.641785,85.723511,85.306396,84.101685,83.470276,82.929993,82.289795,82.058105,81.575012,81.050537,80.276123,80.106506,79.853943,79.58313,79.032227,33.002075,32.450989,32.957642,32.817261,33.146362,32.322021,32.474304,32.495605,32.696472,32.679138,32.759399,32.639954,32.641296,32.67688,32.833984,33.184204,33.517761,33.681458,33.677002,33.744812,33.815857,34.175781,34.21228,34.031433,33.713318,33.349243,33.250244,33.531799,33.671753,33.154724,32.917358,32.587158,32.861694,32.728821,33.053528,33.217224,33.530518,33.856995,34.006836,33.743225,33.145508,32.545105,32.315918,32.099182,31.912415,31.736206,31.93042,31.718384,31.761292,32.268372,32.626282,32.441528,32.472107,33.202515,33.251099,33.71991,34.500061,35.282593,35.846008,36.153809,36.226196,36.364563,36.212952,35.726562,35.770752,35.360474,35.399841,35.576965,35.650635,35.438782,35.245422,35.386414,35.212402,35.049377,34.855347,34.686584,34.333313),.Dim = c(77L,2L),.Dimnames = list(NULL,c("long","lat")))
# data 2: a simplified data.table that contains the centoird for each city.
LatLon_ad4 <- structure(list(ad4 = c("1404","1405","1406","1407","1408","4203","4205","4206","4207","4208"),latW = c(36.203361,35.491546,39.330639,37.686307,35.027401,32.63122,30.694427,32.011398,30.395263,31.037631),lonW = c(113.116422,112.845429,112.426381,112.746207,111.001265,110.793338,111.282676,112.115983,114.886585,112.193188)),row.names = c(NA,-10L),class = c("data.table","data.frame") )
我的思路如下：
第一步，我计算了每个城市到这条线的最短距离
pts <- LatLon_ad4[,.(lonW,latW)] %>% as.matrix()
LatLon_ad4$dist <- dist2Line(pts,line_QH)[,1]/10^3
第 2 步，找出每个点到线的方向。但是怎么……？？？我不知道如何继续。
我的问题是，如何找出每个城市（LatLon_ad4$ad4）相对于线（line_QH）的方向：哪个城市在line_QH，即在线下方（上方）。
并生成一个将方向标识为 LatLon_ad4 的新变量，如果城市在南方，则取值为 1，否则取值为 -1。
谢谢！
解决方法
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