Verify if a point is Land or Water in Google Maps

Question

..and then Google-maps \"divide the waters from the waters\"

Well, not in the biblical sense but..

I would like to know what options I have in order to verif

Answer 1

I thought it was more interesting to do this query locally, so I can be more self-reliant: let's say I want to generate 25000 random land coordinates at once, I would rather want to avoid calls to possibly costly external APIs. Here is my shot at this in python, using the python example mentionned by TomSchober. Basically it looks up the coordinates on a pre-made 350MB file containing all land coordinates, and if the coordinates exist in there, it prints them.

import ogr
from IPython import embed
import sys

drv = ogr.GetDriverByName('ESRI Shapefile') #We will load a shape file
ds_in = drv.Open("land_polygons.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 "4236", 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:
        # success!
        print lon, lat

check(-95,47)

I tried a dozen coordinates, it works wonderfully. The "land_polygons.shp" file can be downloaded here, compliments of OpenStreetMaps. (I used the first WGS84 download link myself, maybe the second works as well)