Adds latitude and longitude post meta field values to REST API response.

REST-API-my-meta-fields.php

<?php
/**
 * @package REST_API_my_meta_fields
 * @version 1.0
 */
/*
Plugin Name: REST API my meta fields
Plugin URI: http://wordpress.org/extend/plugins/#
Description: This is an example plugin 
Author: Carlo Daniele
Version: 1.0
Author URI: http://carlodaniele.it/en/
*/

/**
 * Register custom REST API fields 
 *
 * @link http://v2.wp-api.org/extending/modifying/
 */
function REST_API_register_my_fields(){
	register_rest_field( 'post',
	    'latitude',
	    array(
	       'get_callback'    => 'get_my_field_cb',
	       'update_callback' => null,
	       'schema'          => null
	    )
	);
	register_rest_field( 'post',
	    'longitude',
	    array(
	       'get_callback'    => 'get_my_field_cb',
	       'update_callback' => null,
	       'schema'          => null
	    )
	);
}
add_action( 'rest_api_init', 'REST_API_register_my_fields');

/**
 * Get the value of a REST API custom field
 *
 * @param array $object Details of current post.
 * @param string $field_name Name of field.
 * @param WP_REST_Request $request Current request
 *
 * @return mixed
 */
function get_my_field_cb( $post, $field_name, $request ){
	return get_post_meta( $post['id'], $field_name );
}