How to use python to lookup information at specific latitude/longitude in a ESRI shapefile?

Question

I have a ESRI shapefile (from here: http://pubs.usgs.gov/ds/425/). I looking to use python to lookup information from the shape file (surficial material in this case) at a given latitude/longitude.

What is the best way to go about solving this problem?

Thanks.

Final solution:

#!/usr/bin/python

from osgeo import ogr, osr

dataset = ogr.Open('./USGS_DS_425_SHAPES/Surficial_materials.shp')
layer = dataset.GetLayerByIndex(0)
layer.ResetReading()

# Location for New Orleans: 29.98 N, -90.25 E
point = ogr.CreateGeometryFromWkt("POINT(-90.25 29.98)")

# Transform the point into the specified coordinate system from WGS84
spatialRef = osr.SpatialReference()
spatialRef.ImportFromEPSG(4326)
coordTransform = osr.CoordinateTransformation(
        spatialRef, layer.GetSpatialRef())

point.Transform(coordTransform)

for feature in layer:
    if feature.GetGeometryRef().Contains(point):
        break

for i in range(feature.GetFieldCount()):
    print feature.GetField(i)

Answer 1

You can use the python bindings to the gdal/ogr toolkit. Here's an example:

from osgeo import ogr

ds = ogr.Open("somelayer.shp")
lyr = ds.GetLayerByName("somelayer")
lyr.ResetReading()

point = ogr.CreateGeometryFromWkt("POINT(4 5)")

for feat in lyr:
    geom = feat.GetGeometryRef()
    if geom.Contains(point):
        sm = feat.GetField(feat.GetFieldIndex("surface_material"))
        # do stuff...

Answer 2

Checkout the Python Shapefile Library

This should give you geometry and different info.

Answer 3

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).