arcgis-10.0arcgis-9.3arcgis-desktopdonut-polygonsselect
i have a dataset containing many polygons. In this dataset some polygons have other polygons (as a donut) inside of them. So those island polgons are contained by only one polygon but it originated from the same layer and has the same attributes. I want to select and merge them but i dont know how to select/find them. It is possible to select polygons based on the number of donuts inside of them?
Yet another option, this is more of a theory and programmatic one, using arcpy.
A polygon can consist not only of a single outer ring with a single inner donut hole -- they can be nested to an arbitrary number of levels.
Consider the following:
A topologically correct polygon's rings are ordered according to their containment relationship (source). Based on my results below this appears to be in order of innermost to outermost with exterior rings being listed before the interior rings within them.
Additionally interior rings (green lines) are always within exterior rings (red lines). It is possible to have rings that overlap each other, self-intersections, etc., but typically these are considered topologically incorrect and are simplified before they are stored.
Another important point is the distinction between parts and rings. A feature can have multiple parts, and a part can have multiple rings. In the picture below, think of each solid red shape as an individual part, each having a single exterior ring and 0, 1, or more inner rings.
(source: arcgis.com)
For each part, the first ring is the outer ring, while all subsequent rings are inner rings. The vertices of outer rings are oriented in a clockwise fashion while inner rings are oriented counter-clockwise.
Now to get practical:
You can use the geometry objects in arcpy to access the parts, rings, and vertices of a feature. There is a null point between the rings of a part. You could iterate over the parts and points, checking for the null point to see if there are interior rings.
See the Python script below. This defines a generator function to list the X, Y, FID, part, ring, and vertex indexes which is called repeatedly within a SearchCursor to write to a CSV file using the csv module.
The FID, part, and ring indices uniquely identify each ring, and you know that if the ring index is 0 it's an exterior ring. If the ring index is greater than 0, it's an interior ring. One tweak you might want to make is to remove the last point of each ring as it will always be the same as the first point, to make a closed ring. To do that just set skiplastvertex = True near the top of the script. I used True in the CSV output listed below.
import arcpy, csv
fc = r"C:\GISData\test.gdb\ringtest2"
csvfile = r"C:\GISData\ringtest2.csv"
header = ['X', 'Y', 'FID', 'PART', 'RING', 'VERTEX']
skiplastvertex = False
def iterateRingsAndVertices(shape, fid, skiplastvertex=False):
for partindex, part in enumerate(shape):
ringindex = 0
vertexindex = 0
pnt = part.next()
while pnt:
output = [pnt.X, pnt.Y, fid, partindex, ringindex, vertexindex]
pnt = part.next()
if pnt is None: # Check if this is last point in ring
if not skiplastvertex:
yield output # Return the last point in ring
pnt = part.next() # Check for inner ring
if pnt:
vertexindex = 0
ringindex += 1
else:
yield output
vertexindex += 1
if __name__ == "__main__":
# Open text file for writing
with open(csvfile, 'wb') as f:
w = csv.writer(f)
w.writerow(header) # Write header row
desc = arcpy.Describe(fc)
shapeField = desc.shapeFieldName
oidField = desc.OIDFieldName
rows = arcpy.SearchCursor(fc)
for row in rows:
oid = row.getValue(oidField)
shape = row.getValue(shapeField)
w.writerows(iterateRingsAndVertices(shape, oid, skiplastvertex))
Example output with screenshot of test dataset:
Screenshot of test dataset http://img406.imageshack.us/img406/6293/3df0e6d59ae3480d82effac.png
X Y FID PART RING VERTEX ------------------------------------------------- 6.25 3.75 1 0 0 0 3.75 3.75 1 0 0 1 3.75 6.25 1 0 0 2 6.25 6.25 1 0 0 3 10.00 10.00 1 1 0 0 10.00 0.00 1 1 0 1 0.00 0.00 1 1 0 2 0.00 10.00 1 1 0 3 2.50 7.50 1 1 1 0 2.50 2.50 1 1 1 1 7.50 2.50 1 1 1 2 7.50 7.50 1 1 1 3
I was able to import the CSV file into ArcMap, display it as XY data, and label it without much fuss. You could of course also join it back to your original feature class and work with it that way, export it to another feature class or table, etc. Hopefully this helps!
Two ways spring to mind. The most efficient depends on how many inner polygons you have.
Clip: Select one of your existing 'lake' polygons then choose clip from the editor menu. Use a buffer distance of 0 and discard the area that intersects. This will clip a hole in any overlapping polygons. You will have to do this for each inner polygon.
Cut Polygon Tool: Select the overlapping polygon then choose 'cut polygon tool' from the editor toolbar. Use the trace tool to draw the outer limits of your inner polygons. This will cut the outer polygon into two pieces. Delete the inner piece to have a hole.
Best Answer
If your ultimate aim is to remove all of the islands and if the island polygons have the same attributes and assuming you are using ArcGIS then you can use the Dissolve tool and bypass the need to select them:
QGIS has a tool also called dissolve which will also allow you to select and attribute field to base the dissolve on: