I am running some geoprocessing tasks in R, in which I am trying to create some polygons for clipping rasters of environmental information. I am buffering somewhat complex polygons, and this leaves small subgeometries that I would like to get rid of. In ArcGIS, I think this would involve converting my polygon from multipart to singlepart (or something along those lines) and then dissolving, but I don't know how to do this in R.
Here's an example that illustrates the problem:
require(maptools)
require(rgeos)
data(wrld_simpl)
wrld_simpl[which(wrld_simpl#data$NAME=='Greece'),]->greece
proj4string(greece)<-CRS('+proj=lonlat +datum=WGS84')
gBuffer(greece,width=0.5)->buf
plot(buf)
What I really want is the outer boundary of the polygon, with nothing else inside. Any ideas?
If you just want to get the one ring that forms the boundary of your buffer, then this:
plot(SpatialPolygons(list(Polygons(list(buf#polygons[[1]]#Polygons[[1]]),ID=1))),lwd=2)
is a very ad-hoc way of doing it (and plotting it) for your case.
What you really really want is to get all the rings with ringDir=1, since the rest will be holes. You need all the rings because your buffer might still be two disconnected islands.
outerRings = Filter(function(f){f#ringDir==1},buf#polygons[[1]]#Polygons)
outerBounds = SpatialPolygons(list(Polygons(outerRings,ID=1)))
plot(outerBounds)
might do the trick... Try it with width=0.1 and you'll see it work with multiple islands, but still removing a hole.
If you want the convex hull that will fit Greece, you can use the gConvexHull function in the rgeos package. Note that this is not necessarily the approach to take if you are dealing with polygons with holes in them, as I thought was the case from the question's title. However, from your example, it looks like the below approach will get you where you want.
myCH <- gConvexHull(greece)
plot(myCH)
which will produce something like
And to check that everything fits,
plot(myCH)
plot(greece,add=TRUE)
Related
I'm currently working on a glsl shader (EDIT : I'm starting to think that a shader isn't necessarily the best solution and as I'm doing this in processing, I can consider a vectorial solution too) supposed to render something like this but filling the entire 2D space (or at least a larger surface):
To do so, I want to map the repeating patterns on the general leaves shapes that you can see on the top of the sketch below.
My problem is this mapping part : is it possible to find a function that project XY coordinates on the screen to another position in such a way that I can map my patterns the way I want? The leaves must have some kind of UV coordinates inside them (to be able to apply the repeating pattern) and the transformation must be a conformal map because otherwise, there would be some distortions in the pattern.
I've tried several lines of thought but I haven't managed to get the final result :
recursion :
the idea is to first cut the plane in stripes, then cut the stripes in leaves shapes that touch the top and the bottom of the stripes (because that's easier) and finally recursively cut the leaves in halves until the result looks more random. as long as the borders of the stripe aren't on the screen, it shouldn't be too noticeable. The biggest difficulty here is to avoid the distortion.
voronoi :
it may be possible to find a distance function guided by a vector field such that the Voronoi diagram looks more like what I'm looking for. However I don't think it will be possible to have the UV mapping I want. If it's the case, a good approximation woult do the trick, the result doesn't need to be exact as long as it isn't too noticable.
distortion :
it could also be possible to find a more direct way to do this projection. While desperately looking for a solution, I came across the fact that a continuous complex function is a conform map but I haven't managed to go any further.
Finaly, there may be another solution I haven't thought about and I would be glad if someone gave me a complete solution or just a new idea I haven't tried yet.
I am creating a line from point shapefile which is auto generated. First time when I create that line in ArcGIS, I got a line like this because the points are not in a order:
after that I ordered the points according to it's location and got a line like this:
But unable to create a line like this:
Please give me any solution to fix this in ArcGIS or R programming. If you need the shapefile I can provide you.
I think there is no bullet proof way to restore the line, as same dataset can obviously represent different lines, so you would need to use some heuristics to do this. What Rafael described is very good top-bottom heuristics if you can reliably detect start and end points.
Another heuristics could be a bottom-up process to stitch nearby segments into a line. Find nearby points for every point, sort and connect the two nearest points. Continue this process, making sure each point has at most two connections, and that there are no loops.
A simpler approach that might just work if the line follows in general some direction is your idea of sorting points. But instead of ordering by x or y coordinate, find a linear approximation of these points, project each point to this straight line, and sort using the coordinate of the projection.
One way to go about this would be to treat it as a graph problem.
Create a weighted fully connected graph where the nodes are the points and the edge weight distance between its endpoints. Heuristically identify the “starting” and “ending” points of the line (for example, pick the bottom-leftmost point as start and top-rightmost and end).
Then you can use a shortest path algorithm to generate the order in which you connect the points.
I have got a table with coordinates of points and want to get the smalest polygon around them. I tried different functions and so far alphahull works best for my purposes. My major interest is in the area of the hull. I have got approximately 3500 datasets, so I have to find a reliable method for my analysis.
I analysed some datasets and realised that in some cases I get a hull in a hull and areahull() is not able to return an area. A higher alpha-value would avoid this but would overestimate my area by far.
Is there a possibility to post-process my alpha-hull to remove the second hull? Or a better method to get the size of the area?
library(alphahull)
tmp <- ahull(path.points.1$x, path.points.1$y, alpha = 50)
plot(tmp, wpoints = F)
lin to example dataset
I found a solution which seems to work for my purposes: the function ahull_track() returns only the boundary as a geom_path()-object. the coordinates of the single boundary segments are stored in a list. unfortunately they are not in the correct order, so it is no straight-forward solution. I had to write a function which rearranges the segments into the correct order and generates a polygon.
I am working in aspnetcore using the most up to date GeoAPI and NetTopologySuite version for core. What I'm trying to do should be fairly simple but I can't seem to find the proper way to do it either through experimentation of googling. Or even what to call it, to be honest, which makes googling harder.
Hopefully someone can kick me in the right direction.
I have a multipolygon which may be made up of one or more polygons. I want to create a buffer around that multipolygon's points out to X distance. This is basically a map overlay with concentric areas of interest. A given point of interest may fall in the original multi polygon's shapes... or it might fall in the first or second buffer area. Kinda like an onion if the core of an onion had random shapes in it.
That first part is simple. Just iterate the multipolygon's points and apply a buffer to each point using the buffer method:
var bufferZonePoints = new List<IGeometry>();
foreach(var point in multiPolygon.Coordinates)
{
bufferZonePoints.Add(point.Buffer(x));
}
var bufferZone = this.geometryFactory.CreateMultiPolygon(bufferZonePoints);
That's fine. But it's giving me another multipolygon made up of thousands of points. When I use this as a map overlay, I get a hurricane of circles following the vague outlines of the original shape sort of looking like a spirograph drawing. All I want is basically the outer boundary of all the buffer circles without all the points in the center.
I tried doing a ConvexHull on the multipolygon and it looked correct at first until I realize that it was shaving off the angles on the outside in order to get the smallest polygon all those points fit into (which is what convex hulls do after all). But that causes problems in the stuff I'm overlaying. Some points of interest may be outside the actual buffer, but be inside if the convex hull decides to round off a bumpy area of the zone. (I hope that makes sense).
Basically what I'm trying to do is take that multipolygon made up of all those buffered points and squash it down into a single polygon made up all the outermost boundaries of the buffers. But without all the spirograph garbage in the middle. I don't really want a ConvexHull. I've also tried Union and the GeometryCombiner class, but none of these are doing what I want.
I don't know if this helps makes this mud any clearer but there is a setting in QGIS that when you plunk down two circles and the circles would overlap they combine into one big blob like soap bubbles and the boundaries in between vanish. That's kinda what I'm trying to do via code.
Does that make sense? Can anyone help?
After continuing to experiment with my mapping tool. It would appear that Union DOES actually give me the result I wanted.
I started with two polygons that were far enough apart to make it obvious what was going on, did a union on them and got back just the shell of the combination of them. As I added more of the buffered points to it, the shame became a bit more obvious.
That's pretty well what I wanted.
Thanks anyway though! Hopefully this will help someone else.
In my project I am using different projections for my data. I want to learn what happens when you create shapes in a equivalent (equal area) projection and transform them to a conformal projection.
I have a SPDF in an equal area projection. I am trying to create some shapes in this SPDF to later transform them. I did not expect that this is a difficult task. I would like to work with vectors.
The main problem I think I have is that I am unaware of the package and function I should use. I have searched for a couple of hours this morning but can not find the right package or function. Also I can not find people asking questions about this exact topic, which surprises me.
So the question is: How do I make a shape in a SPDF using R?