I've got a few shape-layers with some polygons which all join up. I want to create a new shape which is the the hole in the middle of the other layers.
I've tried 'snapping', but it seems to lock to vertex and requires a lot of manual accuracy. Ideally i'd like to select the lines where they join and then 'fill in' the area. Though I don't know how to do this in QGIS.
You could use an algorithmic approach like the following (it assumes a situation as the one in the question, so no other holes and polygons are from different layers):
Merge vector layers to combine your different layers into one layer
Dissolve to combine all your features into one feature with one hole
Delete holes to get a layer with hole in the middle filled
Symmetrical Difference of output of 3. and 2. to get a layer where overlapping areas are removed i.e. only the hole should remain as a new layer
Related
I'm trying to create an experience where I have a couple of detailed 3D models of buildings on the map with extruded building footprints of neighboring buildings via a vector tile source. The 3D models would be the main focus point and the extruded footprints would be for reference. One challenge I'm running into is that I have a global building footprint layer and it has a footprint for the 3D buildings which doesn't match up perfectly. Additionally, when extruded, it ends up merging/overlapping the nice 3D models.
I'd like to be able to hide the individual footprints that overlap the 3D models. My original thought was to grab the bounding box of the 3D model and then use the new within style expression, but it looks like this will only filter points and lines, not polygons. The building footprint polygons have no unique information in them that I could use to filter on.
I know I could monitor map movements and query the rendered features and manually detect intersecting polygons, but since there is no unique property on the footprint, I can't filter or use feature state.
Any ideas of how to efficiently avoid rendering individual polygons in a specific area that come in from a vector tile source?
It is a common issue that the buildings layer in Mapbox Streets don't contain any unique attributes to allow filtering or rendering differently.
The best solution is usually to source a different buildings layer, and in this case, remove those redundant buildings in pre-processing.
I can think of one rather crazy workaround that might work here, although the performance may be poor.
Add the building layer with very low opacity, of type fill, essentially invisible. (Maybe invisible would work.) Call your main source buildings`.
Create a secondary building source of type geojson, and a secondary fill-extrusion layer. Call this source buildings-copy.
On map move or moveend, use querySourceFeatures to obtain a copy of all buildings.
Process this copy using Turf to remove the buildings you don't want, and call setData to set the copy as the data for buildings-copy.
You may need to do some clever caching to get reasonable performance.
I have a number of spatial database, through which I have identified particular types of land-cover. The topographic layer [defining land-cover] is made up of multi-polygons and I am using a separate point layer in order to join a classification type to it. However, the areas portrayed in the topographic layer may be formed from several individual polygons, without a total perimeter outline to identify the area. Therefore, the classification point may sit in just one individual polygon of the overall area [see below].
Example. Image is of a park, which consists of over 20 individual polygons. The point that classifies the area as a park sits within one of the polygons and cannot be attached to the entire area.
I would like to be able to apply the point based classification to the whole park area. I have tried to use ST_UNION function in order to do such, but have been unable. Does anyone know of a way to dissolve the area into a single shape/remove the pathways? This is a small example of a much larger data set, where there is little scope of manually defining area sand buffers in order to classify the data, thus I wondered if there was a practicable solution.
If anyone can help, it would be hugely appreciated....
SELECT ST_Collect(t.the_geom) as singlegeom FROM TABLE t; http://postgis.net/docs/ST_Collect.html
Dynamically creating a mesh with a hole in it from two lists of vertices
I am currently attempting to dynamically create a mesh (2D) with a hole in it. I have a list of Vector3 vertices for both the outline and the hole's outline.
My question:
How would I go about merging these two lists of vertices into a single mesh?
More detail: I have two meshes that overlap, and I'm trying to do a boolean difference between the two, to create a new mesh that will eventually replace the bigger one, to get rid of clipping. Example
Using the Clipper-Library (see http://www.angusj.com/delphi/clipper.php) is of no use, because it returns the same two sets of vertices that I set as input.
I'm guessing I need to somehow fix the triangles for the mesh to create triangles between the outer and inner vertices? (The meshes can be any shape/size, so finding out which vertexes to combine into triangles is no easy task).
Can anybody tell me how I would create a single mesh out of the two vertex-loops?
If you need a generic boolean algorithm this is a very hard problem, for example 3D Studio Max has two seperate boolean mesh creators, each failing at different sets of objects.
If you only need to subtract rectangular, aligned shapes, which do not touch, its simpler. For your specific case you can just join the list of vertexes, and fill new list of triangles - you'll need two tris per quad, so thats eight triangles stretched across eight verices.
It gets a bit harder if they start to touch, as you need to find intersection points and basically re-triangulate the outline.
I have two data layers, one with points and one with polygons. Both layers have ID's and I would like to check whether the points with ID x lay inside or outside the polygon with ID x.
Does someone know how to do this?
Thanks,
Marie
One potential solution which gives you a comma separated list in the python console is to run a small script from the python console:
mapcanvas = iface.mapCanvas()
layers = mapcanvas.layers()
for a in layers[0].getFeatures():
for b in layers[1].getFeatures():
if a.geometry().intersects(b.geometry()):
print a.id(),",",b.id()
This should produce a result of cases where one feature intersects the other. The order of the layers did not matter in my testing, however, both layers had to use the same coordinate reference system, so you might need to re-project your data if both layers have different reference systems. This worked for points in polygons and polygons intersecting polygons (I'm sure it would work with lines as well).
Answers such as this: https://gis.stackexchange.com/questions/168266/pyqgis-a-geometry-intersectsb-geometry-wouldnt-find-any-intersections may help with further refinement of such a script, and was a primary source on this answer.
I have some data points which I have devided into them into some clusters with some clustering algorithms as the picture below:(it might takes some time for the image to appear)
alt text http://www.freeimagehosting.net/uploads/05a807bc42.png
Each color represents different cluster. I have to draw polygons around each cluster. I use convhull for this reason. But as you can see the polygon for the red cluster is very big and covers a lot of areas, which is not the one I am looking for. I need to draw lines(ploygons) exactly around my data sets. For example in the picture above I want a polygon that is drawn exactly the same(and around) as the red cluster with the 3 branches. In other words, in this case I need a polygon with 3 branches to cover my red clusters not that big polygon that covers the whole area. Can anyone help me with this?
Please Note that the solution should be general, because the clusters will change in each run of the algorithm, so it needs to be in a way that is general.
I am not sure this is a fully specified question. I see this variants on this question come up quite often.
Why this can not really be answered here: Imagine six points, three in an equilateral triangle with another three in an equilateral triangle inside it in the same orientation.
What is the correct hull around this? Is it just the convex hull? Is it the inner triangle with three line spurs coming out from it? Does it matter what the relative sizes of the triangles are? Should you have to specify that parameter then?
If your clusters are very compact, you could try the following:
Create a grid, say with a spacing of 0.1.
Set every pixel in the grid to 1 if there's at least one data point covering it, set the pixel to 0 if there is no data point covering the pixel.
You may need to run imclose on your mask in order to fill little holes inside that have not been colored due to sheer bad luck.
Extract the border pixels using, e.g. bwperim. This is the outline of the polygon you're looking for.