在边界框的边缘剪裁的 Voronoi 多边形

Question

抱歉，如果这个 post 的标题让您感到困惑，我想不出一个更准确的标题。我正在尝试以一定的缩放级别在休斯顿城市地图上叠加使用休斯顿犯罪数据生成的 Voronoi 多边形。当我自己绘制多边形并使用 coord_cartesian(xlim =, ylim =), 将可见区域限制为地图的边界框时，绘图看起来很好（请参见下面的第一个图）。但是当它们被绘制在城市地图上时，框边缘附近的多边形被剪切和扭曲（第二个图），据我所知 from this cheatsheet 不应该发生。我做错了什么？

# load Houston crime data
suppressMessages(library(ggmap))
data(crime)
set.seed(42)
crime <- crime[sample(1:nrow(crime), 2000), ] # truncated for fast run

# convert to SpatialPointsDataFrame
suppressMessages(library(sp))
coords <- SpatialPoints(crime[, c("lon", "lat")])
crime_spdf <- SpatialPointsDataFrame(coords, crime)
proj4string(crime_spdf) <- CRS("+proj=longlat +ellps=WGS84")

# create Voronoi polygons
suppressMessages(library(spatstat))
suppressMessages(library(maptools))
vor_pp <- as(dirichlet(as.ppp(crime_spdf)), "SpatialPolygons")
proj4string(vor_pp) <- CRS("+proj=longlat +ellps=WGS84")

# get Houston map
houston_map <- get_map(location = geocode("Houston"),
                       zoom = 16,
                       maptype = "satellite")

# get map bounding box
xlim <- bb2bbox(attr(houston_map, "bb"))[c(1, 3)]
ylim <- bb2bbox(attr(houston_map, "bb"))[c(2, 4)]

# create Voronoi polygon plot
vor_df <- fortify(vor_pp)
plt1 <- ggplot(data = vor_df) +
  geom_polygon(aes(x = long, y = lat, group = group),
               color = "black",
               fill = NA) +
  coord_cartesian(xlim = xlim, ylim = ylim) +
  ggtitle("Voronoi Polygon Plot") +
  theme(aspect.ratio = 1,
        axis.title = element_blank(),
        axis.text = element_blank(),
        axis.ticks = element_blank())

# create map + polygon plot
plt2 <- ggmap(houston_map) +
  geom_polygon(data = vor_df,
               aes(x = long, y = lat, group = group),
               color = "gray80",
               fill = "red",
               alpha = 0.2) +
  coord_cartesian(xlim = xlim, ylim = ylim) +
  ggtitle("Map + Polygon Plot") +
  theme(aspect.ratio = 1,
        axis.title = element_blank(),
        axis.text = element_blank(),
        axis.ticks = element_blank())

# plot side-by-side
suppressMessages(library(grid))
pushViewport(viewport(layout = grid.layout(1,2)))
print(plt1, vp=viewport(layout.pos.col = 1, layout.pos.row = 1))
print(plt2, vp=viewport(layout.pos.col = 2, layout.pos.row = 1))

谢谢

Answer 1

可以通过将参数 extent = "normal" 和 maprange = FALSE 添加到 ggmap:

来解决问题

ggmap(houston_map, extent = "normal", maprange = FALSE) +
  geom_polygon(data = vor_df,
               aes(x = long, y = lat, group = group),
               color = "gray80",
               fill = "red",
               alpha = 0.2) +
  coord_cartesian(xlim = xlim, ylim = ylim) +
  ggtitle("Map + Polygon Plot") +
  theme(aspect.ratio = 1,
        axis.title = element_blank(),
        axis.text = element_blank(),
        axis.ticks = element_blank())

maprange: logical for use with base_layer; should the map define the x and y limits?

extent: how much of the plot should the map take up? "normal", "device", or "panel" (default)

将 expand = 0 添加到 coord_cartesian 结果：

在边界框的边缘剪裁的 Voronoi 多边形

Voronoi polygons clipped at the edges of bounding box

voronoi

r

polygon

ggplot2

ggmap