Question

I have a textbox, in which user will enter a Landmark and I have to retrieve a postal code of that landmark. I have no idea how to complete this task. I need some assisstance

Answer 1

Google is your best bet. Use Reverse Geocoding (Address Lookup) on Google Map API to retrieve the postal code

The term geocoding generally refers to translating a human-readable address into a location on a map. The process of doing the converse,
translating a location on the map into a human-readable address, is
known as reverse geocoding.

The Geocoding API supports reverse geocoding directly using the latlng
parameter. For example, the following query contains the
latitude/longitude value for a location in Brooklyn:

http://maps.googleapis.com/maps/api/geocode/json?latlng=40.714224,-73.961452&sensor=true_or_false

This query returns the following result:

{
  "status": "OK",
  "results": [ {
    "types": street_address,
    "formatted_address": "275-291 Bedford Ave, Brooklyn, NY 11211, USA",
    "address_components": [ {
      "long_name": "275-291",
      "short_name": "275-291",
      "types": street_number
    }, {
      "long_name": "Bedford Ave",
      "short_name": "Bedford Ave",
      "types": route
    }, {
      "long_name": "New York",
      "short_name": "New York",
      "types": [ "locality", "political" ]
    }, {
      "long_name": "Brooklyn",
      "short_name": "Brooklyn",
      "types": [ "administrative_area_level_3", "political" ]
    }, {
      "long_name": "Kings",
      "short_name": "Kings",
      "types": [ "administrative_area_level_2", "political" ]
    }, {
      "long_name": "New York",
      "short_name": "NY",
      "types": [ "administrative_area_level_1", "political" ]
    }, {
      "long_name": "United States",
      "short_name": "US",
      "types": [ "country", "political" ]
    }, {
      "long_name": "11211",
      "short_name": "11211",
      "types": postal_code
    } ],

Read more : https://developers.google.com/maps/documentation/geocoding/#Types