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Calculate the geographic midpoint given a list of geographic coordinates in R.
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| library(tidyverse) | |
| geomid <- function(lat, lon) | |
| { | |
| # http://www.geomidpoint.com/calculation.html | |
| lat <- lat * pi / 180 | |
| lon <- lon * pi / 180 | |
| x <- mean(sum(cos(lat) * cos(lon))) | |
| y <- mean(sum(cos(lat) * sin(lon))) | |
| z <- mean(sum(sin(lat))) | |
| lat <- atan2(y, x) | |
| hyp <- sqrt(x * x + y * y) | |
| lon <- atan2(z, hyp) | |
| c(lat = lat * 180 / pi, lon = lon * 180 / pi) | |
| } | |
| # calculate the geographic midpoint for each county in the US | |
| map_data("county") %>% | |
| mutate(polyname = str_c(region, subregion, sep = ",")) %>% | |
| group_by(polyname) %>% | |
| group_modify(~ { | |
| as.data.frame(t(geomid(.x$lat, .x$long))) | |
| }) |
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