按面积过滤shapefile多边形 [英] Filter shapefile polygons by area

问题描述

我具有以下英国的边界数据集，其中显示了所有县:

I have the following boundary dataset for the United Kingdom, which shows all the counties:

library(raster)
library(sp)
library(ggplot)

# Download the data
GB <- getData('GADM', country="gbr", level=2)

使用subset函数，很容易通过数据中的属性过滤shapefile多边形.例如，如果我要排除北爱尔兰:

Using the subset function it is really easy to filter the shapefile polygons by an attribute in the data. For example, if I want to exclude Northern Ireland:

GB_sub <- subset(UK, NAME_1 != "Northern Ireland")

但是，有许多小岛扭曲了比例数据范围，如下图所示:

However, there are lots of small islands which distort the scale data range, as shown in the maps below:

关于如何以最小大小优雅地对数据集进行子集化的任何想法?具有与subset参数一致的格式的某些内容将是理想的.例如:

Any thoughts on how to elegantly subset the dataset on a minimum size? It would be ideal to have something in the format consistent with the subset argument. For example:

GB_sub <- subset(UK, Area > 20) # specify minimum area in km^2

推荐答案

这是另一个潜在的解决方案.由于您的数据是经纬度投影，因此直接根据纬度和经度计算面积会产生偏差，因此最好根据geosphere包中的函数来计算面积.

Here is another potential solution. Because your data is in lat-long projection, directly calculating the area based on latitude and longitude would cause bias, it is better to calculate the area based on functions from the geosphere package.

install.packages("geosphere")
library(geosphere)

# Calculate the area
GB$poly_area <- areaPolygon(GB) / 10^6

# Filter GB based on area > 20 km2
GB_filter <- subset(GB, poly_area > 20)

poly_area包含所有多边形的以km2为单位的面积.我们可以按阈值过滤多边形，例如您的示例中的20. GB_filter是最终输出.

poly_area contains the area in km2 for all polygons. We can filter the polygon by a threshold, such as 20 in your example. GB_filter is the final output.

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