Determine if points fall upstream or downstream from a point on a line (river)

Question

I have a set of fish species occurrences (lat/long) as points and set of rivers as lines. The fish species can of course only occur in the rivers (and not on land).

I would like to determine whether these points fall up or downstream of a point on the river (like a dam for example).

The way I think may work is to convert the river to a polygon with a small buffer around the polygon to include points that weren't exactly in the river (but should be). Then break up the new polygon to that below the dam, and that above.

Then do a spatial overlay, e.g., over(points, line_downstream) and over(points, line_upstream) to get that set of points down and upstream of the dam.

Is this the correct approach. If so, any tips on the code for this? I have tried converting lines to polygons and creating a buffer, but no success yet.

p.s. I did notice a similar question done in ArcGIS - I need to do this in R.

Answer 1

You can use function snapPointsToLines from the package maptools. It will not only snap the fish location to the nearest river, but also give you the id of the nearest river.

snappedFish <- snapPointsToLines(fish, rivers)
snappedFish$nearest_line_id

Than, you can calculate stationing of point on that line

# function to decide, if point is on the line segment
pointOnLine <- function(point,line_start,line_end){ # each of parameters is pair x;z
  if(identical(point,line_start) | identical(point,line_end)){return(TRUE)}
  if(point[1]>max(c(line_start[1],line_end[1])) | point[1]<min(c(line_start[1],line_end[1])) | point[2]>max(c(line_start[2],line_end[2])) | point[2]<min(c(line_start[2],line_end[2]))){
    return(FALSE)
  }
  if(line_start[2]==line_end[2]){
    slope<-0
  } else if(line_start[1]==line_end[1]){
    return(T)
  } else {
    slope <- (line_start[2]-line_end[2])/(line_start[1]-line_end[1])
  }
  intercept <- -slope*line_start[1] + line_start[2]
  onLine <- round(point[2],digits=0)==round((slope*point[1]+intercept),digits=0)
  return(onLine)
}

#### here is just a pseudocode, you will have to iterate through all fish locations
pointCrds <- coordinates(fishLocation)
# get the river a fish location is on
coords <- coordinates(nearestRiver)[[1]][[1]]

position <- 0
# calculate stationing from the start
d <- 0
for(j in 2:nrow(coords)){
  onLine <- pointOnLine(point=pointCrds,line_start=coords[j-1,],line_end=coords[j,])
  if(onLine){
    d0 <- sqrt((pointCrds[1]-coords[j-1,1])^2 + (pointCrds[2]-coords[j-1,2])^2)
    position <- round(d+d0)
    break
  }
  d <- d + sqrt((coords[j,1]-coords[j-1,1])^2 + (coords[j,2]-coords[j-1,2])^2)
}

#d is the distance of fish location from the start on given river

The second task is to decide, whether that nearest river is upstream or downstream, which is more difficult and the approach would depend on the structure of your data.

Answer 2

I've never used R so not sure what it is capable of with regards to hydrological tools, but it can be done in one of two ways:

1. Snap your sites to the network then do a network search from your test point (e.g. dam). Any points intersecting the network as you search upstream must be upstream of the dam. The rest of your sites are either in entirely different catchments or are downstream of your dam but could potentially be up another tributary.
2. The second approach is to generate a catchment boundary from a DEM for your dam. Then it is a simple point in polygon test to determine if the point is upstream of your dam. Again the rest of your sites are either in entirely different catchments or are downstream of your dam but could potentially be up another tributary.

RivEX is an ArcGIS Pro toolbox that can generate these sorts of metrics, worth a look to give you some ideas.