I`m using the intersect function from turf (https://github.com/Turfjs/turf/tree/master/packages/turf-intersect).
It works nicely in the most cases, but I need to snap a polygon into another.
But, when I have two polygons and I grab a point of one polygon to snap in another, the funtion intersect returns a object, it means they intersects.
Ok, for me, this is a intersection and intersect function return "true":
But, in this case, intersect functions return "true" too.
image
Thanks!
Related
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.
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.
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.
I want to check whether a given point lies inside a polygon.
I have looked into the point in polygon method using ray casting, but I'm unsure of how it will work in cases where the given polygon in disjoint. For example, if the polygon has a "hole" right in the center. How do I use this function in such cases?
I've answered a similar question here, even though it wasn't clear if I understood the OP's question too well. Well , here it is again -
Let's take a very general case .
Polygon with a hole and Intersections
The ray casting (point in polygon) algorithm shoots off a ray counting the intersections with the sides of the POLYGON (Odd intersections = inside, Even = outside).
Hence it accurately gives the correct result regardless of whether you start from inside the disjoint trapezoidal hole or the triangular hole (inner edges?) or even if a part of the polygon is completely seperated and/or self intersecting.
However, in what order do you feed the vertices of the polygon such that all the points are evaluated correctly?
Though this is code specific, if you're using an implementation that is counting every intersection with the sides of the polygon then this approach will work -
- Break the master polygon into polygonal components. eg - trapezoidal hole is a polygonal component.
- Start with (0,0) vertex (doesn't matter where (0,0) actually lies wrt your polygon) followed by the first component's vertices, repeating its first vertex after the last vertex.
- Include another (0,0) vertex.
- Include the next component , repeating its first vertex after the last vertex.
- Repeat the above two steps for each component.
- End with a final (0,0) vertex.
Example -
Let's take the picture to the right, the shell with the hole and apply the above method. The vertices of a singular continuous polygon which is mathematically equivalent to the 2 components (polygon+hole) is -
(outside)-------(inside hole)
0,1,2,3,4,5,1,0,1,2,3,4,1,0
To understand how it works, try drawing this mathematically equivalent polygon on a scratch pad.-
1. Mark all the vertices but don't join them yet.
2. Mark the repeated vertices separately also. Do this by marking them close to the original points, but not on them. (at a distance e, where e->0 (tends to/approaches) ) (to help visualize)
3. Now join all the vertices in the right order (as in the example above)
You will notice that this forms a continuous polygon and only becomes disjoint at the e=0 limit.
You can now send this mathematically equivalent polygon to your ray casting function (and maybe even winding number function?) without any issues.
I need to evaluate the proximity of a Point to a LineString using MongoDB.
Because the $near operator can only compare a Point to another Point, I need to generate a polygon out of the LineString, so I can use the $within operator. The distance between the LineString and the edges of the polygon should represent the radius I want to search in, such as represented in red below:
What might be a useful algorithm in order to accomplish this?
I think much easier would be to write your own function
To find (perpendicular) distance between point and line and then creating thickness of poly-line by polygon means.
Where:
P0,P1 are line endpoints
P is point
d is distance between them
Line is defined as: p(t)=P0+(P1-P0)*t where t=<0.0,1.0>
So the function should do this:
create perpendicular line
q(t)=P+DQ*u where u=(-inf,+inf)
DQ is perpendicular vector to (P1-P0)
In 2D you can obtain it easily like this (x,y) -> (y,-x). In higher dimensions use cross product with some non coplanar vectors.
compute line vs. line intersection
there are tons of stuff about this so google or solve the equation yourself here you can extract mine implementation.
now after successful intersection
just compute d as distance between P and intersection point. Do not forget that parameter t must be in range. If not (or if no intersection) then return min(|P-P0|,|P-P1|)
[hints]
t and u parameters can be obtained directly from intersection test so if the perpendicular vector to (P1-P0) is normalized to size = 1 then the abs(u) parameter of intersection point is the distance
[notes]
I am not familiar with mongodb so if you have no means to use own tests inside then this answer is of coarse obsolete.
Unfortunately, MongoDB provides very basic geospatial query, so you should create the buffer by your own. You can read how to do it here: Computing a polygon that surrounds a multi-point line
If you have longitude/latitude coordinates like WGS84 you must adjust this code; Read here how to calculate distance between point on a sphere https://en.wikipedia.org/wiki/Haversine_formula
It's usually popular to work with polygons with their vertices sorted CW or CCW in vectors(2*1 or 1*2 matrices). However, how to state polygons with holes in vectors?
I'm going to apply various process on these polygons, so I want a way of representing with which I could work easily or efficiently.(i.e how to state that kind of polygons in my program in order to ease my algorithms?)
polygons are 2D and I'm programming in MATLAB.
EDIT 1 : I'm going to calculate visibility graph of these polygons(with or without holes).
As others have mentioned, a polygon with holes can be represented as an exterior boundary, plus zero or more interior boundaries, all of which are mutually nonoverlapping*. If you use nonzero winding number to determine inside/outside, be sure to specify your interior boundaries in the opposite direction as the exterior boundaries (counterclockwise for exterior and clockwise for interior, or vice-versa) so that the contour integrals are zero inside the holes.
FYI, tis kind of definition/representation has been formalized in the OpenGIS Simple Features Specification (PDF).
As far as representation:
I'd probably have a cell array of K Nx2 matrices, where the first element in the cell array is the exterior boundary, and the remaining elements (if any) in the cell array are the interior boundaries. I would use a cell array because there may not be the same number of points on each boundary.
*nonoverlapping = except at individual points, e.g. a diamond inside a square:
You can break a polygon with a hole in it into two shapes without a hole. When you're doing contour integration in a complex plane, you can create a "cut" from one edge of the polygon that brings you to the edge of the hole; integrate around one side of the hole and back; then traverse around the other side for the second polygon. You end up with two path integrals along each cut that cancel each other out.
"visibility graph" - is this for a radiation view factor calculation with shading? Or a ray-tracing graphics algorithm?
A polygon, plus a list of polygonal holes. Just be sure the various polygons don't intersect.
What do you plan to do with this thing?
It sounds like each hole is just a polygon inside the polygon itself. Perhaps you could store a vector like you describe for the outer polygon, then a vector of more polygon vectors for the holes.
Presumably you'll want to have a tree structure if you want this to be as generic as possible (i.e. polygons with polygonal holes that have polygons inside them with holes inside that, ...). Matlab isn't really great at representing tree structures efficiently, but here's one idea...
Have a struct-array of polygons.
Each polygon is a struct with two fields, 'corners', and 'children'.
The 'corners' field contains a matrix of (x,y) coordinates of the corners, accessed as "data{polyIdx}.corners(:,cornerIdx)".
The 'children' field is a struct-array of polygons.
Here's an example of some code to make a triangle with bogus children that are holes (they aren't really valid though because they will likely overlap:
polygon = struct;
npoints = 3;
polygon.corners = rand(2,npoints);
polygon.children = struct;
nchildren = 5;
for c=1:nchildren
polygon.children(c).corners = rand(2,npoints);
polygon.children(c).children = struct;
end
You could continue to recursively define children that alternate between creating holes and filling them.
What exactly do you mean under "a visibility graph" ?
Two "full" poligons, two states possible, either +1 or -1.
If you're representing a hole, you've got one with state +1 and one with state -1, which represents a hole, resulting in state 0.
If you've got overlapping polygons, you'll end up with resultant state >1. Then you can calculate the borders of a new polygon.
If you've got two polygons with holes that intersect, then first calculate the state of a new polygon which consists of outer borders of the two old ones, then deal with holes.
Anyways, ... I think you get the general principle.
Have no idea how to do it in matlab, I used it only marginally so far, and even that for very simple things.
I have a table with thousands of addresses as points.
Is there a function in postgis that will allow me to get a bounding polygon around these points and return it as a polygon?
update I am looking for a more complex polygon than just a bounding rectangle
It may be not 100% clear from your question what you mean as more bounding polygon. It may be understood as
minimum bounding rectangle (also known as MBR or envelope) for which you can use ST_Envelope, ST_Box2D or ST_Extent,
as s a minimum geometry which encloses all points within given geometry, a convex envelope and in this case you can use ST_ConvexHull or even ST_ExteriorRing if your input would be a polygon,
As Pimin pointed in the comments below, ST_ConcaveHull is another option worth to consider, available since PostGIS 2.0.0.
ST_Extent provides an aggregate function that returns a bounding box that bounds a set of geometries.