Derived spatial lines pointing to incorrect polygons

Question

I'm currently trying to make a flow map in leaflet for R to map the movement of something from one area (polygon) to another.

library(sp)
library(rgdal)
library(tidyverse)
library(leaflet)

I'm using this shapefile of local authorities (LAs) in England for the polygons, and I'm deriving the lines coordinates from those.

Map <- readOGR(dsn = "Map", layer = "LEA_Boundaries")
Map <- Map[!is.na(Map$lad16cd),]

I'm wanting to create lines from the centre of one polygon to another so I used the origin and destination polygons latitude and longitude(s) to derive the beginning and end point of my line. I did this through sampling the LA names from the Map's data slot, then sampling them again to create my starting and ending LAs, then merging the coordinates for the starting LA on, followed by the coordinates for the ending LA, as follows:

StartingLA <- sample(Map@data$lad16nm, 100000, replace = TRUE)
NextLA <- sample(StartingLA, 100000, replace = TRUE)
Movement <- data.frame(StartingLA, NextLA)
LD <- merge(Movement, Map@data[,c("lad16nm", "long", "lat")], by.x = "StartingLA", by.y = "lad16nm") %>%
      merge(Map@data[,c("lad16nm", "long", "lat")], by.x = "NextLA", by.y = "lad16nm", suffixes = c("",".y"))

After this I have tried two separate ways to create spatial lines, one was a custom function to add a bezier line, and the other was gcIntermediate(), which I'll use here a it's more accessible and rules out programming errors in the function.

flow <- gcIntermediate(LD[,c("long", "lat")], LD[,c("long.y", "lat.y")], sp = TRUE, addStartEnd = TRUE)

And now for the map itself; we'll add popup labels to the polygons so we can see which LAs are which.

leaflet() %>%
  addProviderTiles(providers$CartoDB.PositronNoLabels) %>%
  addPolygons(data = Map, weight = 1, col = "#000000", fillOpacity = 0, popup = Map@data$lad16nm) %>%
  addPolylines(data = flow)

And we'll take a look at the data that was used to create the line.

LD

On my map I now have a blue line joining one polygon to the next. However for the StartingLA in LD, I have 'East Sussex', and the NextLA is 'Hillingdon', but the line points to/from Birmingham and Wandsworth.

This was also true using my custom function to fit bezier lines, which makes me think that it could be something that happened before, but all the data is correct right up until the lines are made into SpatialLines, and after that it all goes haywire.

Does anyone have any insight as to where I'm going wrong here?

Thank you.

Answer 1

The co-ordinates in the Map@Data object are not appropriate for your use case.

Greenwich, for instance, has the co-ordinates -1.108940, 53.52697, which is firmly located in Doncaster. Perhaps at an earlier processing step the co-ordinates and IDs got unmatched somehow?

In any case, it's probably quickest/easiest now to re-calculate the co-ordinates using using rgeos::gCentroid on the Map object and then replace the offending columns in the data file:

rgeos::gCentroid(Map,byid = T)