Using the WordPress Featured Image / Post Thumbnail as a Google Maps Marker

This is a follow-up to my previous post on combining WordPress and Google Maps. Jayc asks:

“Is there any way to change the MarkerImage to the featured image for the post?”

Yes, there is!

But one thing to keep in mind is that markers are not one image but two — the marker itself and the shadow. What we’re going to cover here will show how to use the Featured Image as the marker, while continuing to use a single shadow across all posts. This means it is appropriate for markers that share the same size and silhouette.

We’ll be using the code from the previous post as the starting point. The complete example with these changes applied is also available for download from GitHub.

Initiate Featured Images

Create a new file in your theme directory, called functions.php. In it, write:

// Featured Images / Post Thumbnails
if ( function_exists( 'add_theme_support' ) ) {
    add_theme_support( 'post-thumbnails' );

This is the basic code. For more features see Post Thumbnails in the Codex.

When you go to your Post Edit screen you will now see a metabox called Featured Image. Clicking the link, you will be prompted to upload a file. Once you have done that, click the “Use as featured image” link (it’s next to “Insert into post”).

Make a marker declaration in the loop

We’re going to move our marker declaration into the WordPress loop. We are going to add another parameter to our locations variable, so in index.php look for the line:

info : document.getElementById('item<?php echo $i; ?>')

And place a comma at the end of it.

Because Featured Images are not required by WordPress when publishing a post, we will test if one has been set and to provide a fallback. On the next line include the following test:

<?php if ( has_post_thumbnail() ) { ?>
	// There is a Featured Image
<?php } else { ?>
	// No Featured Image, use fallback
<?php } ?>

And under “use fallback”, define a fallback image

// No Featured Image, use fallback
marker : new google.maps.MarkerImage('<?php echo get_stylesheet_directory_uri() ?>/pink_Marker.png', new google.maps.Size(20, 34) )

Get the path to the Featured Image

The normal way to access the post thumbnail is to use the the_post_thumbnail() function, but this returns a full img tag. We want only the image path, so let’s make our own function that does this. Open functions.php and add:

function get_thumbnail_path($post_ID) {
	$post_image_id = get_post_thumbnail_id($post_ID->ID);
	if ($post_image_id) {
		$thumbnail = wp_get_attachment_image_src( $post_image_id, 'post-thumbnail', false);
		if ($thumbnail) (string)$thumbnail = $thumbnail[0];
		return $thumbnail;

And back in index.php, under the “There is a Featured Image” comment, create a marker declaration that includes a call to the function:

// There is a Featured Image
marker : new google.maps.MarkerImage('<?php echo get_thumbnail_path($post->ID); ?>', new google.maps.Size(20, 34) )

Update the map JavaScript

Last but not least, we’ll modify the maps.js to reflect our changes. Find the line that declares the “pinkmarker”:

var pinkmarker = new google.maps.MarkerImage('/wp-content/themes/mapdemo/pink_Marker.png', new google.maps.Size(20, 34) );

and remove it; it’s no longer needed. Next, look for the line:

icon: pinkmarker,

And change it to:

icon: locations[i].marker,

And that’s it! Save everything and you should now see the featured image loaded as the marker.

Posted March 2012

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