Average geodetic distance between subsets of points (same ID) using distm(distVincentyEllipsoid) and storing the results in a new dataframe in R

Question

My database has the following structure:

    > long <- c(13.2345, 14.2478, 16.2001, 11.2489, 17.4784, 27.6478, 14.2500, 12.2100, 11.2014, 12.2147)
    > lat <- c(47.1247, 48.2013, 41.2547, 41.2147, 40.3247, 46.4147, 42.4786, 41.2478, 48.2147, 47.2157)
    > hh_id <- 1:10
    > vill_id <- c(rep(100, 4), rep(101, 3), rep(102, 2), 103)

    > df <- matrix(c(long, lat, hh_id, vill_id), nrow = 10, ncol = 4)
    > colnames(df) <- c("longitude", "latitude", "hh_id", "vill_id") 
    > df <- as.data.frame(df)
    > df
       longitude latitude hh_id vill_id
       13.2345  47.1247     1     100
       14.2478  48.2013     2     100
       16.2001  41.2547     3     100
       11.2489  41.2147     4     100
       17.4784  40.3247     5     101
       27.6478  46.4147     6     101
       14.2500  42.4786     7     101
       12.2100  41.2478     8     102
       11.2014  48.2147     9     102
       12.2147  47.2157    10     103

hh_id - households IDs

vill_id - village IDs

Households with identical ID belong to the same village.

My aim: calculate the mean distance between all points with the same vill_id and store the result in a new data frame:

vill_id    mean_dist
100        587553.5
101        …………………
102        …………………
103        ………………

My approach: To calculate the geodetic distance between points I have used the distm command from the geosphere package (distVincentyEllipsoid should be most accurate)

> library(geosphere)
> df_100 <- df[df$vill_id == 100, ]
> dist_100 <- distm(df_100, fun = distVincentyEllipsoid)
Error in .pointsToMatrix(p1) : Wrong length for a vector, should be 2 --> 
> df_100_2 <- df_100[, c(1, 2)]
> dist_100_2 <- distm(df_100_2, fun = distVincentyEllipsoid)
> dist_100_2
         [,1]     [,2]     [,3]     [,4]
[1,]      0.0 141844.7 693867.8 675556.9
[2,] 141844.7      0.0 787217.4 811777.4
[3,] 693867.8 787217.4      0.0 415056.6
[4,] 675556.9 811777.4 415056.6      0.0

So a symmetric distance matrix for all points with vill_id = 100 was generated. To calculate the mean distance I need to to decompose this matrix (or drop all of the diagonal values (0)).

> diag(dist_100_2) = NA
> dist_100_2_final <- dist_100_2[!is.na(dist_100_2)]
> dist_100_2_final
 [1] 141844.7 693867.8 675556.9 141844.7 787217.4 811777.4 693867.8 787217.4 415056.6 675556.9
[11] 811777.4 415056.6
> mean(dist_100_2_final)
[1] 587553.5 (in m)

So far so good. Now I need to create a new dataframe which stores the mean distances for all subsets with the same ID (my original database has over 200 villages (vill_id) and almost 2000 households (hh_id)). Can you please help me how to finish the code? I think I have to use loops (or maybe there is another package to solve this problem)? Many thanks for your help.

Yesterday I have posted similar question with the difference that the mean_dist were already part of my original dataframe (computed in ArcGIS) but now I want to calculate these in R to compare the results. I have tried to implement the recommended codes from my previous question but without success.

Answer 1

Consider base R's by since you need to run an operation across different levels of factors (ie, vill_id ). Inside by , you can call a defined or anonymous function which will return a list of dataframes that you can row bind back to one dataframe:

dfList <- by(df, df[c("vill_id")], FUN = function(i){
     sub <- i[, c(1, 2)]
     tmp <- distm(sub, fun = distVincentyEllipsoid)
     diag(tmp) = NA
     i$mean_dist <- mean(tmp[!is.na(tmp)])                  # NEW COLUMN ADDED
     return(i)
})

finaldf <- do.call(rbind, dfList)

Should you need vill_id and hh_id subset, add to the factor list:

dfList <- by(df, df[c("vill_id", "hh_id")], FUN = function(i){ ... })

And if you only need vill_id and mean_dist returned from function, change return value:

newdf <- unique(i[c("vill_id", "mean_dist")]
return(newdf)

---

Specifically, the following block of code:

df_100 <- df[df$vill_id == 100, ]                            # BY REPLACES THIS LINE
df_100_2 <- df_100[, c(1, 2)]
dist_100_2 <- distm(df_100_2, fun = distVincentyEllipsoid)                 
diag(dist_100_2) = NA
dist_100_2_final <- dist_100_2[!is.na(dist_100_2)]
mean(dist_100_2_final)

Is translated as the following where i is the by function variable:

sub <- i[,c(1, 2)]
tmp <- distm(sub, fun = distVincentyEllipsoid)
diag(tmp) = NA
i$mean_dist <- mean(tmp[!is.na(tmp)])

Answer 2

Another way would be to use lapply() . I basically revised your code. One thing I added was to split your data by vill_id and create a list. Then, I applied your chunk of code for calculating distance to each split data frame in lapply() . Finally, I created a data frame with mean values.

library(geosphere)

mylist <- split(df, f = df$vill_id)

unlist(lapply(mylist, function(x){

        foo <- x[, 1:2]
        foo <- distm(foo, fun = distVincentyEllipsoid)
        diag(foo) = NA
        out <- foo[!is.na(foo)]
        average <- mean(out)
        average
      })
) -> mean_dist

data.frame(vill_id = unique(df$vill_id),
           mean_dist = mean_dist)

#    vill_id mean_dist
#100     100  587553.5
#101     101  858785.6
#102     102  778299.1
#103     103       NaN