Create clipping_featurecollection.md

docs/src/tutorials/clipping_featurecollection.md

@@ -0,0 +1,41 @@
+# Clipping a FeatureCollection by a polygon
+
+Often, one might want to clip a feature collection (maybe from Shapefile or GeoJSON)
+
+```@example clipping_featurecollection
+function clip_or_empty(polygon, clipper)
+    #=
+    result = GO.intersection(polygon, clipper; target = GI.PolygonTrait())
+    if isempty(result)
+        null_point = GI.is3d(polygon) ? (GI.ismeasured(polygon) ? (NaN, NaN, NaN, NaN) : (NaN, NaN, NaN)) : (NaN, NaN)
+        contents = GI.LinearRing.([[null_point, null_point, null_point]])
+        return GI.Polygon{GI.is3d(polygon),GI.ismeasured(polygon),typeof(contents),Nothing, typeof(GI.crs(polygon))}(contents, nothing, GI.crs(polygon))
+    else
+        return GI.MultiPolygon(result; crs = GI.crs(polygon))
+    end
+    =#
+    return GO.intersection(GO.GEOS(), polygon, clipper)
+end
+```
+First, let's load our data:
+```@example clipping_featurecollection
+df = nothing # DataFrame(Shapefile.Table(...))
+```
+and plot it:
+```@example clipping_featurecollection
+f, a, p = poly(df.geometry)
+```
+Now, we can define some polygon in that space, that we want to use to clip all geometries by!
+```@example clipping_featurecollection
+clipping_poly = GI.Polygon([[(880_000, 990_000), (910_000, 990_000), (910_000, 1030_000), (880_000, 1030_000), (880_000, 990_000)]])
+poly!(a, clipping_poly; color = Makie.Cycled(2))
+f
+```
+Finally, we clip, and show the output:
+```@example clipping_featurecollection
+clipped_geoms = clip_or_empty.(df.geometry, (clipping_poly,))
+```
+```@example clipping_featurecollection
+poly!(a, clipped_geoms; color = Makie.Cycled(3), strokewidth = 0.75, strokecolor = :forestgreen)
+f
+```