不规则多边形的栅格化坐标会改变原始形状答案

【问题标题】：Rasterizing coordinates for an irregular polygon changes original shape不规则多边形的栅格化坐标会改变原始形状
【发布时间】：2019-12-09 20:50:05
【问题描述】：

我正在尝试平滑 R 中不规则多边形的边缘，即将其尖角变成圆形边缘。我正在尝试使用smoothr::smooth 来执行此操作，但是此函数对来自包sf 或sp 的对象进行操作，而我所拥有的只是一组坐标。不知何故，将我的data.frame 变成SpatialPolygonsDataFrame 对象（来自包sp 的对象类）的结果是一个矩形，其极限是原始多边形的极限。有谁知道如何在保持原始多边形形状的同时将我的坐标集转换为与smoothr::smooth 兼容的类的对象？这是我所做的，部分遵循this 页面上的说明：

rm(list=ls()) # my compulsive habit of making sure R's memory is a clean slate

# Example dataset:
dd <- data.frame(
        Lon = c(18.95379, 18.82409, 18.58987, 18.80541, 18.92427, 19.00264),
        Lat = c(-32.42492, -32.32498, -31.89642, -31.73606, -32.16217, -32.37052)
        )

plot(0,0,
    xlim=c(18.5,19.1), ylim=c(-32.5,-31.6), 
    xlab="Longitude", ylab="Latitude"
    )
polygon(dd[,"Lon"],dd[,"Lat"], border="red")

# To make it smooth I plan on using
library(smoothr)
# But smoothr:: smooth works on objects from packages sf or sp so I need to  convert dd.
#convert to spatial points
library(sp)
coordinates(dd) = ~Lon + Lat
# convert to raster
library(raster)
rr <- raster::raster(dd)
#convert raster to polygons
sp = rasterToPolygons(rr, dissolve = T)
map(sp, add=T, col="green", fill=F)
# somehow my irregular polygon turned into a rectangle.
sps <- smooth(sp, method = "ksmooth", smoothness=5)
# this works, but of course is only rounding the corners of sp
map(sps, add=T, col="blue", fill=F)

红色是data.frame dd的原始多边形，绿色是对象sp，蓝色是sp、sps的平滑版本。函数smooth 完成了这项工作，问题在于将dd 转换为sp 兼容对象。我怀疑问题是由raster() 引起的，但我不确定为什么或如何解决它。非常感谢。

【问题讨论】：

标签： r gis r-raster smoothing rasterize

【解决方案1】：

这里我使用了sf，因为我个人觉得这更容易：

library(sf)
library(smoothr)

# Example dataset:
dd <- data.frame(
  Lon = c(18.95379, 18.82409, 18.58987, 18.80541, 18.92427, 19.00264),
  Lat = c(-32.42492, -32.32498, -31.89642, -31.73606, -32.16217, -32.37052)
)

# cast to polygon, use multipoint first though.   
polygon <- as.matrix(dd) %>%
  sf::st_multipoint() %>%
  sf::st_cast("POLYGON")

#  smooth polygon
polygon_smoothed <- smoothr::smooth(polygon, method = "ksmooth", smoothness = 0.5)

# plot to check
plot(polygon, col = "red")
plot(polygon_smoothed, col = "blue", add = T)

【讨论】：

【解决方案2】：

我在这里找到了另一个解决方案：https://rstudio-pubs-static.s3.amazonaws.com/202536_7a122ff56e9f4062b6b012d9921afd80.html

# Example dataset:
dd <- data.frame(
        Lon = c(18.95379, 18.82409, 18.58987, 18.80541, 18.92427, 19.00264),
        Lat = c(-32.42492, -32.32498, -31.89642, -31.73606, -32.16217, -32.37052)
        )       
plot(0,0,
    xlim=c(18.5,19.1), ylim=c(-32.5,-31.6), 
    xlab="Longitude", ylab="Latitude"
    )
polygon(dd[,"Lon"],dd[,"Lat"], border="red")

library(sp)
p = Polygon(dd)
p2 = Polygons(list(p),1) # I believe this aggregates polygons, so in this case it doesn't do anything.
sp = SpatialPolygons(list(p2))
sps <- smooth(sp, method = "ksmooth", smoothness=0.7)
plot(sps, add=T, border="blue")

【讨论】：