Hfrhyu

I have a list of XY coordinates along multiple lines. The lines are separated by a single identifier at the start of each block of coordinates. How do I convert this to multiple lines linking the identifier to the lines in R so that I can output them as a shapefile? The problem is in parsing the data to link the ID with the subsequent coordinates.

The data is in the following format:

ID1
3285.48 -63.32
3285.14 -64.14
3284.67 -63.56
3285.00 -62.77
ID2
3299.84 -76.82
3299.25 -75.38
3299.96 -75.76
ID3
3299.76 -86.92
3299.77 -86.89
3299.76 -87.04
3299.76 -87.23
3299.74 -87.25
3299.68 -87.22
3299.68 -87.11

You can read in irregular data if you know the max number of columns by giving column names and a fill argument:

> data = read.table("./lines.txt",col.names=c("x","y"),fill=TRUE, stringsAsFactors=FALSE)
> data
 x y
1 ID1 NA
2 3285.48 -63.32
3 3285.14 -64.14
4 3284.67 -63.56
5 3285.00 -62.77
6 ID2 NA
7 3299.84 -76.82
8 3299.25 -75.38
9 3299.96 -75.76
10 ID3 NA

Then the lines are grouped by how many NA are in the second column so far:

> data$group = cumsum(is.na(data$y))
> data
 x y group
1 ID1 NA 1
2 3285.48 -63.32 1
3 3285.14 -64.14 1
4 3284.67 -63.56 1
5 3285.00 -62.77 1
6 ID2 NA 2
7 3299.84 -76.82 2
8 3299.25 -75.38 2
9 3299.96 -75.76 2
10 ID3 NA 3

Then you can pull out the IDS:

> IDS = data$x[is.na(data$y)]
> IDS
[1] "ID1" "ID2" "ID3"

And filter them out:

> data = data[!is.na(data$y),]
> data
 x y group
2 3285.48 -63.32 1
3 3285.14 -64.14 1
4 3284.67 -63.56 1
5 3285.00 -62.77 1
7 3299.84 -76.82 2
8 3299.25 -75.38 2
9 3299.96 -75.76 2
11 3299.76 -86.92 3

Then split by group gives you a list of data frames you can apply work to:

> par(ask=TRUE)
> lapply(split(data,data$group), function(d)plot(d$x,d$y,type="l"))
Hit <Return> to see next plot: 
Hit <Return> to see next plot: 
Hit <Return> to see next plot: 

Replace that function with whatever you want to do with each line data set, and attach the IDS extracted in the previous step as needed.

Note at this point the columns might still be character so coercion to numeric might be helpful via as.numeric.

If you want to make sf spatial lines data frame, then proceed:

data$x = as.numeric(data$x)
library(sf)
lfc = do.call(st_sfc,
 lapply(split(data, data$group),
 function(d)st_linestring(as.matrix(d[,1:2]))))
lfd = st_sf(data.frame(ID=IDS, geom=lfc))
plot(lfd,lwd=5)

to get...