问题
Is there a way in Turfjs to determine the co-ordinates at which a LineString intersects with the border of a polygon?
There's a number of ways to find out if a point is within a polygon and a number of ways to find out if a point is on a line and so on, but I can't seem to figure out a way to ask "at what point does this line intersect this polygon's border".
I could enumerate the points in the polygon using a line intersection algorithm to find that point but I was wondering if there's a more "turf" way of doing this.
For context, I've loaded a GPX track and want to estimate the location/time at which the track enters/exits a defined area.
Because a GPX track only records locations at specific intervals it usually the case that pN recorded at time tN is outside the area and pN+1 recorded at time tN+1 is inside the area. If I can get the point at which line (pN, pN+1) intersects the polygon's border I can estimate the exact time the track crosses into the polygon.
回答1:
Ultimately, turfjs does not seem to have an API for doing this.
I was able to get the answer I wanted by enumerating the points in the polygon from the GeoJSON object to construct a sequence of line segments and then I used maxogden/geojson-js-utils linesIntersect function to test for intersection points.
回答2:
I don't see a Turf function that does exactly that, but there is intersect, which finds the area of intersection between two polygons.
You could:
- Construct a polygon by joining the line to itself reversed (so ABC becomes ABCBA)
- Find the intersection of ABCBA and P, the original polygon using Intersect.
- The intersection should be a zero-area polygon that is the part of ABCBA inside P. Somehow compute the length of it (strangely there's no perimeter function).
- Subtract that length from the original length of ABC.
Not exactly elegant, true. :)
EDIT
Tried this. Turns out Turf-intersect doesn't return intersections if one of the polygons is zero-area.
来源：https://stackoverflow.com/questions/33629369/find-co-ordinates-where-linestring-intersects-a-polygon-border-in-turfjs