坐标中的多边形 [英] polygons from coordinates

问题描述

我有一个data.frame，其中lat s和lng s定义了矩形框的边界，就像这样

I've got a data.frame with lats and lngs that define the boundaries of rectangular boxes, like so

  geohash north_lat south_lat  east_lng  west_lng
1   gbsuv  48.69141  48.64746 -4.306641 -4.350586
2   gbsuy  48.69141  48.64746 -4.262695 -4.306641

将其转换为包含POLYGON s列的sf对象的最简单方法是什么?

What's the easiest way to convert this into an sf object that holds a column of POLYGONs?

推荐答案

创建多边形的关键是坐标必须按顺序形成一个封闭区域(即，最后一个点与第一个点相同) .

The key to creating polygons is that the coordinates have to be in sequence to form a closed area (i.e., the last point is the same as the first point).

因此，您的数据将需要一些操作才能创建坐标，并将其整理好.在我的示例中，我使用lapply

So your data will need a bit of manipulation to create the coordinates, and put them in order. In my example I've done this with an lapply

然后可以从 sf示例

lst <- lapply(1:nrow(df), function(x){
  ## create a matrix of coordinates that also 'close' the polygon
  res <- matrix(c(df[x, 'north_lat'], df[x, 'west_lng'],
           df[x, 'north_lat'], df[x, 'east_lng'],
           df[x, 'south_lat'], df[x, 'east_lng'],
           df[x, 'south_lat'], df[x, 'west_lng'],
           df[x, 'north_lat'], df[x, 'west_lng'])  ## need to close the polygon
         , ncol =2, byrow = T
  )
  ## create polygon objects
  st_polygon(list(res))

})

## st_sfc : creates simple features collection
## st_sf : creates simple feature object
sfdf <- st_sf(geohash = df[, 'geohash'], st_sfc(lst))

sfdf
# Simple feature collection with 2 features and 1 field
# geometry type:  POLYGON
# dimension:      XY
# bbox:           xmin: 48.64746 ymin: -4.350586 xmax: 48.69141 ymax: -4.262695
# epsg (SRID):    NA
# proj4string:    NA
# geohash                    st_sfc.lst.
# 1   gbsuv POLYGON((48.69141 -4.350586...
# 2   gbsuy POLYGON((48.69141 -4.306641...

plot(sfdf)

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