ggsn全球地图比例尺看起来不对

Question

我需要在R中的ggplot2中制作的全局地图中添加比例尺，而easiest way似乎是通过ggsn包。

但是当我对地图进行观察时，比例尺看起来并不正确 - 例如，它在整个澳大利亚E-W的距离大约为4000公里，比非洲赤道的距离要小一些。 2000公里规模的酒吧在赤道看起来比非洲宽，几乎和澳大利亚一样宽。

感谢您的想法！

可重复的代码：

library(ggplot2)
library(ggsn)
library(dplyr)

GG.GlobalMap = map_data("world") %>%
  filter(region != "Antarctica")

ggplot(GG.GlobalMap, aes(long, lat, group = group)) +
  geom_path(color = "black") +
  scalebar(data = GG.GlobalMap, dist = 1000, dd2km = TRUE, model = "WGS84",
           st.size = 3, location = "bottomright") +
  theme_void()

Answer 1

在您使用的简单的长度投影中，距离随纬度而变形，随着它们越来越接近而逐渐拉伸。在极点处，单个点伸展到矩形地图的整个宽度。因此，不建议在此类地图上使用比例尺，因为它们的大小仅在特定纬度上是正确的，并且在地图上的其他任何位置都会产生误导。我们可以通过在不同纬度绘制一系列比例尺来看到这一点，以显示它们的大小变化：

ggplot() +
  geom_path(data = map_data("world"), aes(long, lat, group = group), color = "grey80") +
  scalebar(dist = 1000, dd2km = TRUE, model = "WGS84", y.min = -80, y.max = 90, x.min = -180, x.max = 180, st.size = 2, location = "bottomright") +
  scalebar(dist = 1000, dd2km = TRUE, model = "WGS84", y.min = -60, y.max = 90, x.min = -180, x.max = 180, st.size = 2, location = "bottomright") +
  scalebar(dist = 1000, dd2km = TRUE, model = "WGS84", y.min = -40, y.max = 90, x.min = -180, x.max = 180, st.size = 2, location = "bottomright") +
  scalebar(dist = 1000, dd2km = TRUE, model = "WGS84", y.min = -20, y.max = 90, x.min = -180, x.max = 180, st.size = 2, location = "bottomright") +
  scalebar(dist = 1000, dd2km = TRUE, model = "WGS84", y.min = 0, y.max = 90, x.min = -180, x.max = 180, st.size = 2, location = "bottomright") +
  scalebar(dist = 1000, dd2km = TRUE, model = "WGS84", y.min = 20, y.max = 90, x.min = -180, x.max = 180, st.size = 2, location = "bottomright") +
  scalebar(dist = 1000, dd2km = TRUE, model = "WGS84", y.min = 40, y.max = 90, x.min = -180, x.max = 180, st.size = 2, location = "bottomright") +
  scalebar(dist = 1000, dd2km = TRUE, model = "WGS84", y.min = 60, y.max = 90, x.min = -180, x.max = 180,  st.size = 2, location = "bottomright") +
  scalebar(dist = 1000, dd2km = TRUE, model = "WGS84", y.min = 80, y.max = 90, x.min = -180, x.max = 180, st.size = 2, location = "bottomright") +
  theme_void()

出于这个原因，通常建议在小规模的全球地图上使用刻度而不是比例尺。

ggplot() +
  geom_path(data = map_data("world"), aes(long, lat, group = group), color = "black") +
  scale_x_continuous(breaks = (-9:9)*20) +
  scale_y_continuous(breaks = (-9:9)*10) +
  theme_bw() +
  theme(panel.grid.major = element_line(colour = 'grey50', size = 0.3, linetype = 3))

如果您真的想使用比例尺，则应首先将数据（使用spTransform）重新投影到等面积投影，其距离最小或完全没有扭曲。