polygon area in R with sf

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Readme.md

Polygon are in R with sf

Enrico Spinielli
October 15, 2017

Source polygon_area.R and you will get the answer in ${km}^2$.

There are two CSV files for the polygons. Polygons are "closed", i.e. first and last point are the same.

The area of the full polygon is $22597969\ {km}^2$ while the area of the simplified one is $27412031\ {km}^2$.

Here is the map of the two (red = full polygon, blue = simple polygon).

Readme.Rmd

---
title: "Polygon are in R with sf"
author: "Enrico Spinielli"
date: "October 15, 2017"
output:
  html_document:
    keep_md: TRUE
---

```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = TRUE)
```


Source `polygon_area.R` and you will get the answer in ${km}^2$.

There are two CSV files for the polygons.
Polygons are "closed", i.e. first and last point are the same.

The area of the full polygon is $22597969\ {km}^2$
while the area of the simplified one is $27412031\ {km}^2$.

Here is the map of the two (red = full polygon, blue = simple polygon).

![](polys.png)

full_polygon.csv

73.00000;0.00000
73.00000;0.00000
82.00000;0.00000
82.00000;32.00000
81.00000;32.00000
81.00000;35.00000
74.04500;35.00000
74.00000;32.08333
70.16667;32.08333
70.13333;31.98333
70.00000;31.13333
69.79472;30.81861
69.53333;30.83333
69.54167;30.56667
69.67222;30.13333
69.65833;30.08333
69.42194;30.02778
69.33333;29.53333
69.30833;29.49444
69.01667;29.05000
68.94083;28.57083
68.91667;28.41667
68.87500;28.80333
68.50333;28.45833
68.10000;29.33333
67.70000;30.00000
67.20389;29.40111
66.96667;29.08333
66.70000;29.61667
66.39722;29.75861
66.16667;29.86694
66.02194;29.95444
65.66667;30.16667
65.58361;29.81556
65.05639;29.62583
64.93333;29.61667
64.70000;30.08333
64.23333;30.50000
63.73333;30.01667
63.24111;31.17444
63.14667;31.26500
63.00000;31.50000
62.95000;31.55000
62.42167;31.10833
61.91944;30.27861
61.80000;30.08333
61.72167;29.94750
61.55111;29.65250
61.43333;29.45000
61.25333;29.11361
61.07583;28.75694
60.85833;28.33833
60.75000;28.16667
60.36528;27.67528
60.34500;27.64944
60.33167;27.62806
60.23333;27.47500
60.22333;27.45667
60.20028;27.29306
60.19389;27.22111
60.17167;26.94639
60.16139;26.83861
60.15250;26.75889
60.14972;26.71694
60.14417;26.66139
60.13333;26.55000
60.07611;26.39028
60.07028;26.37444
59.94556;26.03500
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59.85000;26.08917
59.83750;26.17389
59.79639;26.44806
59.78944;26.49333
59.75528;26.72139
59.74694;26.77667
59.61167;27.63667
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59.18639;27.88917
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58.84333;27.46778
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58.48861;27.49611
58.32944;27.51472
58.03833;27.62000
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57.83000;27.55972
57.60000;27.38333
57.55000;27.70000
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56.50500;28.12833
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55.87417;30.01889
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54.66833;31.04139
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54.21667;31.33139
54.09472;31.49333
53.50278;32.27333
53.21667;32.46667
52.96833;31.35972
52.08139;31.95139
52.27500;32.67333
52.35000;33.31000
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52.00000;34.11667
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51.70000;34.50000
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51.23333;34.45000
51.07167;35.33000
50.85000;35.43333
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50.35500;35.69833
50.32139;36.29333
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50.31667;36.82833
50.43333;37.46667
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50.30083;37.64083
50.09194;37.76000
49.90861;38.04278
49.93028;38.14667
49.95000;38.17861
50.06611;38.19250
50.07500;38.22667
49.74167;39.50000
49.61250;40.15167
49.56306;40.20750
49.53194;40.07000
49.36667;40.16667
48.51833;39.86333
48.19500;39.94333
48.12833;39.89167
47.95000;39.80833
47.83333;39.67500
47.85667;39.21833
47.81667;38.91667
47.55500;38.31333
47.11167;38.22333
46.98333;38.33333
46.78333;37.50000
46.65833;37.42167
46.57333;37.36833
45.63833;36.79667
45.50444;36.71611
45.45000;36.68333
45.37833;36.68333
45.35000;36.64667
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45.24500;36.59500
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45.15333;36.54500
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45.00000;36.59167
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44.51667;36.66667
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43.25000;36.38333
42.73222;36.26639
42.72500;36.50000
42.72000;36.61667
42.70889;36.91472
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42.68333;37.66667
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42.65333;37.80833
42.18389;39.40306
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41.79917;40.65639
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41.60000;41.28194
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41.42806;42.97889
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41.24694;43.19778
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38.32222;44.50417
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37.89722;44.23611
37.70833;44.62667
37.10833;44.66667
36.94167;44.26667
37.30556;44.16667
37.22472;43.64056
37.35000;43.21667
37.37639;43.11833
37.30556;42.82361
37.24889;42.64972
37.08333;42.33333
37.26667;42.25000
37.10333;41.79111
37.00000;41.50000
37.07222;41.23028
37.03333;41.19861
37.03333;40.83333
36.66667;39.33333
36.70861;39.02833
36.66667;38.75000
36.90000;38.36667
36.64889;37.38306
36.65000;37.06389
36.83333;36.66667
36.78444;36.64139
36.50000;36.58750
36.23333;36.70000
36.16667;36.36667
35.86667;36.18333
35.88333;36.03333
35.91667;35.66667
35.55000;35.53333
35.19333;35.71667
34.58333;35.75000
34.53833;35.28667
34.48333;35.05000
34.36667;34.87444
34.24500;34.74667
34.03167;34.62139
33.94556;34.60389
33.89167;34.59167
33.83194;34.58333
33.64889;34.63306
33.16667;34.93333
33.10000;34.86667
33.10000;34.71667
33.09083;34.60306
32.85750;34.55361
32.78167;34.54389
32.71667;34.53333
32.68056;34.42750
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32.50583;34.18806
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32.33500;34.05528
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31.91667;33.98333
31.83333;34.00000
31.82500;34.00194
32.21667;33.10000
32.63333;32.08333
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33.10000;30.95000
33.50000;30.00000
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33.95167;27.33333
34.00000;27.16667
34.00000;24.16667
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37.50000;11.50000
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37.68333;11.10000
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36.50000;-15.00000
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38.00000;-15.00000
38.50000;-15.00000
39.00000;-15.00000
39.50000;-15.00000
40.00000;-15.00000
41.00000;-15.00000
42.00000;-15.00000
42.50000;-14.00000
43.00000;-13.00000
45.00000;-13.00000
45.00000;-8.75000
48.57333;-8.75000
49.00000;-15.00000
57.00000;-15.00000
57.00000;-10.00000
61.00000;-10.00000
61.00000;0.00000
73.00000;0.00000

polygon_area.R

#  install.packages(c("tidyverse", "sf", "units", "mapview")

library(tidyverse)
library(sf)
library(units)
library(mapview)

# list of point defining the polygon (last point equal to first one)
poly_full_csv <- read_delim(
  "full_polygon.csv",
  delim = ";",
  col_names = c("lat", "lon"),
  col_types = "dd")

# from list of points to SF polygon
poly_full <- poly_full_csv %>%
  select(lon, lat) %>% 
  as.matrix() %>%
  list() %>%
  st_polygon()


poly_simple_csv <- read_delim(
  "simple_polygon.csv",
  delim = ";",
  col_types = "dd", col_names = FALSE
) %>% rename(lon = X2, lat = X1)


poly_simple <- poly_simple_csv %>%
  select(lon, lat) %>% 
  as.matrix() %>%
  list() %>%
  st_polygon()

ps <- st_sf(a = 1, geometry = st_sfc(poly_simple)) %>%
  st_set_crs(4326)

pf <- st_sf(a = 1, st_sfc(poly_full)) %>%
  st_set_crs(4326)

pf %>% st_area() %>% set_units(km^2)
ps %>% st_area() %>% set_units(km^2)

mapview(pf, color = "white", col.regions = "red") + mapview(ps)

polygonarea.Rproj

Version: 1.0

RestoreWorkspace: Default
SaveWorkspace: Default
AlwaysSaveHistory: Default

EnableCodeIndexing: Yes
UseSpacesForTab: Yes
NumSpacesForTab: 2
Encoding: UTF-8

RnwWeave: knitr
LaTeX: pdfLaTeX

polys.png

simple_polygon.csv

81.07095264;-1.249340377
82.15928065;20.49622637
79.98098991;37.36859498
36.99425199;44.74575178
33.9395891;25.52224348
35.61426157;-6.585536566
36.32646554;-12.62765374
39.68569469;-15.69204363
81.07095264;-1.249340377