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pythonpolygonpointshapefile

Find enclosing ploygon for point in Python, using shapefile

I have a shapefile with a series of polygons which represent counties. I would like to know of a function/module in Python which can derive the 'bounding' polygon given the coordinates of a single point.

In other words, a function which uses a lat/lon coordinate of a point and the shapefile, and returns the polygon within this shapefile which encompasses the point. See diagram:

enter image description here

As you can see, the point is within the 'blue' polygon and this is what I need for any given point.

I understand that there may not be a built-in function to do this but any advice of how to go about this would be excellent, thanks!

Solution

  • The answer from Richard on a similar question is the best.

    Found here: https://stackoverflow.com/a/13433127/7019148

    Copied below for future reference:

    #!/usr/bin/python
import ogr
from IPython import embed
import sys

drv = ogr.GetDriverByName('ESRI Shapefile') #We will load a shape file
ds_in = drv.Open("MN.shp")    #Get the contents of the shape file
lyr_in = ds_in.GetLayer(0)    #Get the shape file's first layer

#Put the title of the field you are interested in here
idx_reg = lyr_in.GetLayerDefn().GetFieldIndex("P_Loc_Nm")

#If the latitude/longitude we're going to use is not in the projection
#of the shapefile, then we will get erroneous results.
#The following assumes that the latitude longitude is in WGS84
#This is identified by the number "4326", as in "EPSG:4326"
#We will create a transformation between this and the shapefile's
#project, whatever it may be
geo_ref = lyr_in.GetSpatialRef()
point_ref=ogr.osr.SpatialReference()
point_ref.ImportFromEPSG(4326)
ctran=ogr.osr.CoordinateTransformation(point_ref,geo_ref)

def check(lon, lat):
    #Transform incoming longitude/latitude to the shapefile's projection
    [lon,lat,z]=ctran.TransformPoint(lon,lat)

    #Create a point
    pt = ogr.Geometry(ogr.wkbPoint)
    pt.SetPoint_2D(0, lon, lat)

    #Set up a spatial filter such that the only features we see when we
    #loop through "lyr_in" are those which overlap the point defined above
    lyr_in.SetSpatialFilter(pt)

    #Loop through the overlapped features and display the field of interest
    for feat_in in lyr_in:
        print lon, lat, feat_in.GetFieldAsString(idx_reg)

#Take command-line input and do all this
check(float(sys.argv[1]),float(sys.argv[2]))
#check(-95,47)