Compare the distance of same individual between groups [R]

Question

I would like to know difference of position of same individuals between different groups. In other words, the difference of position between the "B150" (df$ID) of the groups "1" & and the "B150" of the group "2" (df$date), and repeat that for all individuals ("B145", "B140",...). The distance between different individuals (e.g. "B150" & "B145") does not interest me.

Here is the sample of the dataset:

df <- structure(list(ID = c("B150", "B145", "B140", "B136", "B150", 
"B145", "B140", "B136"), Ellipsoid_height_m = c(155.5, 155.5, 
155.4, 155.3, 155.5, 155.5, 155.4, 155.3), X_Lambert_72_m = c(232762.455, 
232763.271, 232764.444, 232765.093, 232764.955, 232765.771, 232766.944, 
232767.593), Y_Lambert_72_m = c(125994.937, 125996.489, 125997.991, 
125998.854, 125994.937, 125996.489, 125997.991, 125998.854), 
    Z_DNG_plus130cm = c(111.102, 111.102, 111.002, 110.902, 111.102, 
    111.102, 111.002, 110.902), Z_DNG = c(109.802, 109.802, 109.702, 
    109.602, 109.802, 109.802, 109.702, 109.602), Validite_Z = c("Non", 
    "Non", "Non", "Non", "Non", "Non", "Non", "Non"), Type = c("Pittag", 
    "Pittag", "Pittag", "Pittag", "Pittag", "Pittag", "Pittag", 
    "Pittag"), date = structure(c(1L, 1L, 1L, 1L, 2L, 2L, 2L, 
    2L), .Label = c("1", "2"), class = "factor")), class = c("tbl_df", 
"tbl", "data.frame"), row.names = c(NA, -8L))

Answer 1

Here is a way.
In order to make things simple, keep only the variables relevant to the distance, reshape to wide format putting the same individuals in the same row and compute the distances.

suppressPackageStartupMessages({
  library(dplyr)
  library(tidyr)
})

euclid <- function(x1, y1, x2, y2) {
  sqrt( (x1-x2)^2 + (y1-y2)^2 )
}
df %>%
  select(ID, contains("Lambert"), date) %>%
  pivot_wider(
    id_cols = ID,
    names_from = date,
    values_from = contains("Lambert")
  ) %>%
  mutate(Dist = euclid(X_Lambert_72_m_1, Y_Lambert_72_m_1, 
                       X_Lambert_72_m_2, Y_Lambert_72_m_2)) %>%
  select(ID, Dist)
#> # A tibble: 4 × 2
#>   ID     Dist
#>   <chr> <dbl>
#> 1 B150    2.5
#> 2 B145    2.5
#> 3 B140    2.5
#> 4 B136    2.5

^{Created on 2022-10-01 with reprex v2.0.2}