Check if a geopoint with latitude and longitude is within a shapefile

Question

How can I check if a geopoint is within the area of a given shapefile?

I managed to load a shapefile in python, but can\'t get any further.

Answer 1

If you want to find out which polygon (from a shapefile full of them) contains a given point (and you have a bunch of points as well), the fastest way is using postgis. I actually implemented a fiona based version, using the answers here, but it was painfully slow (I was using multiprocessing and checking bounding box first). 400 minutes of processing = 50k points. Using postgis, that took less than 10seconds. B tree indexes are efficient!

shp2pgsql -s 4326 shapes.shp > shapes.sql

That will generate a sql file with the information from the shapefiles, create a database with postgis support and run that sql. Create a gist index on the geom column. Then, to find the name of the polygon:

sql="SELECT name FROM shapes WHERE ST_Contains(geom,ST_SetSRID(ST_MakePoint(%s,%s),4326));"
cur.execute(sql,(x,y))

Answer 2

This is an adaptation of yosukesabai's answer.

I wanted to ensure that the point I was searching for was in the same projection system as the shapefile, so I've added code for that.

I couldn't understand why he was doing a contains test on ply = feat_in.GetGeometryRef() (in my testing things seemed to work just as well without it), so I removed that.

I've also improved the commenting to better explain what's going on (as I understand it).

#!/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)

This site, this site, and this site were helpful regarding the projection check. EPSG:4326

Answer 3

One way to do this is to read the ESRI Shape file using the OGR library http://www.gdal.org/ogr and then use the GEOS geometry library http://trac.osgeo.org/geos/ to do the point-in-polygon test. This requires some C/C++ programming.

There is also a python interface to GEOS at http://sgillies.net/blog/14/python-geos-module/ (which I have never used). Maybe that is what you want?

Another solution is to use the http://geotools.org/ library. That is in Java.

I also have my own Java software to do this (which you can download from http://www.mapyrus.org plus jts.jar from http://www.vividsolutions.com/products.asp ). You need only a text command file inside.mapyrus containing the following lines to check if a point lays inside the first polygon in the ESRI Shape file:

dataset "shapefile", "us_states.shp"
fetch
print contains(GEOMETRY, -120, 46)

And run with:

java -cp mapyrus.jar:jts-1.8.jar org.mapyrus.Mapyrus inside.mapyrus

It will print a 1 if the point is inside, 0 otherwise.

You might also get some good answers if you post this question on https://gis.stackexchange.com/

Answer 4

Checkout http://geospatialpython.com/2011/01/point-in-polygon.html and http://geospatialpython.com/2011/08/point-in-polygon-2-on-line.html

Answer 5

i did almost exactly what you are doing yesterday using gdal's ogr with python binding. It looked like this.

import ogr

# load the shape file as a layer
drv    = ogr.GetDriverByName('ESRI Shapefile')
ds_in  = drv.Open("./shp_reg/satreg_etx12_wgs84.shp")
lyr_in = ds_in.GetLayer(0)

# field index for which i want the data extracted 
# ("satreg2" was what i was looking for)
idx_reg = lyr_in.GetLayerDefn().GetFieldIndex("satreg2")


def check(lon, lat):
  # create point geometry
  pt = ogr.Geometry(ogr.wkbPoint)
  pt.SetPoint_2D(0, lon, lat)
  lyr_in.SetSpatialFilter(pt)

  # go over all the polygons in the layer see if one include the point
  for feat_in in lyr_in:
    # roughly subsets features, instead of go over everything
    ply = feat_in.GetGeometryRef()

    # test
    if ply.Contains(pt):
      # TODO do what you need to do here
      print(lon, lat, feat_in.GetFieldAsString(idx_reg))

Answer 6

Another option is to use Shapely (a Python library based on GEOS, the engine for PostGIS) and Fiona (which is basically for reading/writing files):

import fiona
import shapely

with fiona.open("path/to/shapefile.shp") as fiona_collection:

    # In this case, we'll assume the shapefile only has one record/layer (e.g., the shapefile
    # is just for the borders of a single country, etc.).
    shapefile_record = fiona_collection.next()

    # Use Shapely to create the polygon
    shape = shapely.geometry.asShape( shapefile_record['geometry'] )

    point = shapely.geometry.Point(32.398516, -39.754028) # longitude, latitude

    # Alternative: if point.within(shape)
    if shape.contains(point):
        print "Found shape for point."

Note that doing point-in-polygon tests can be expensive if the polygon is large/complicated (e.g., shapefiles for some countries with extremely irregular coastlines). In some cases it can help to use bounding boxes to quickly rule things out before doing the more intensive test:

minx, miny, maxx, maxy = shape.bounds
bounding_box = shapely.geometry.box(minx, miny, maxx, maxy)

if bounding_box.contains(point):
    ...

Lastly, keep in mind that it takes some time to load and parse large/irregular shapefiles (unfortunately, those types of polygons are often expensive to keep in memory, too).