Chris Essig

Walkthroughs, tips and tricks from a data journalist in eastern Iowa

Multiple layers and rollover effects for Fusion Table maps

with 3 comments

Note: Because of a change in the Fusion Tables API, the method for using rollover effects no longer works.

Note: This is cross-posted from Lee’s data journalism blog. Reporters at Lee newspapers can read my blog over there by clicking here.

If you haven’t noticed by now, a lot of journalists are in love with Google Fusion Tables. I’m one of them. It’s helped me put together a ton of handy maps on deadline with little programming needed.

For those getting started, I suggest these two walkthroughs on Poynter. For those who have some experience with FT, here are a couple of options that may help you spruce up your next map.

Multiple Fusion tables on one map

Hypothetically speaking, let’s say you have two tables with information: one has county data and the other city data. And you want to display that data on just one map.

Fusion Tables makes it very easy to display both at the same time. We’ll start from the top and create a simple Javascript function to display our map (via the Google Maps API):

function initialize() {
	map = new google.maps.Map(document.getElementById('map_canvas'), {
	    center: new google.maps.LatLng(42.5, -92.2),
		zoom: 10,
		minZoom: 8,
		maxZoom: 15,
	    mapTypeId: google.maps.MapTypeId.TERRAIN

At the end of the initialize function we call a function “loadmap(); With this function, we will actually pull in our Fusion Tables layers. For this example we’ll bring in two layers instead of one. Notice how strikingly similar the two are:

function loadmap() {
	layer2 = new google.maps.FusionTablesLayer({
		query: {
			select: 'geometry',
			from: 2814002

	layer = new google.maps.FusionTablesLayer({
		query: {
			select: 'Mappable_location',
			from: 2813443

That’s it! You now have one map pulling in two sets of data. To see the full code for this map, click here.

Rollover effects for Fusion Tables

One feature often requested in the Fusion Tables forums is to enable mouse rollover events for Fusion Table layers. Typically, readers who look at a map have to click on a point, polygon, etc. to open up new data about that point, polygon, etc. A mouseover event would allow new data to pop up if a reader hovers over a point, polygon, etc. with their mouse.

A few months ago, some very smart person rolled out a “workable solution” for the rollover request. Here’s the documentation and here’s an example of it in the wild.

Another example is this map on poverty rates in Iowa. The code below is from this map and is very similar to the code on the documentation page:

		select: "'Number', 'Tract', 'County', 'Population for whom poverty status is determined - Total', 'Population for whom poverty status is determined - Below poverty level', 'Population for whom poverty status is determined - Percent below poverty level', 'One race - White', 'One race - Black', 'Other', 'Two or more races'", // list of columns to query, typially need only one column.
		from: 2415095, // fusion table name
		geometryColumn: 'geometry', // geometry column name
		suppressMapTips: true, // optional, whether to show map tips. default false
		delay: 1, // milliseconds mouse pause before send a server query. default 300.
		tolerance: 6 // tolerance in pixel around mouse. default is 6.

	//here's the pseudo-hover
	google.maps.event.addListener(layer, 'mouseover', function(fEvent) {
var NumVal = fEvent.row['Number'].value;
		styles: [{
			where: "'Number' = " + NumVal,
			polygonOptions: {
				fillColor: "#4D4D4D",
				fillOpacity: 0.6

Note: It’s easiest to think of your Fusion Table as a list of polygons with certain values attached to it. For this poverty map, each row represents a Census Tract with values like Tract name, number of people within that tract that live in poverty, etc. And for this map, I made it so each polygon in the Fusion Table has its own, unique number in the “Number” column.

Here’s a run through of what the above code does:

1.  We’ve already declared “layer” as a particular Google Fusion Table layer (see the second box of code above). Now the “layer.enableMapTips” will allow us to add rollover effects to that Fusion Table layer. The “select” option represents all the columns in that Fusion Table layer that you want to use with the rollover effect.

For instance, here’s the Fusion Table I’m calling in the above “enableMapTips” function. Notice how I’ve called all the columns with data (‘Tract’, ‘County’, etc.). I then told it which Fusion Table to look for with “from: 2415095.” Each Fusion Table has its own unique number. The number for my poverty Fusion Table is 2415095, which is called. To find out what number your Fusion Table is, click File > About.

Finally, I’ve told it what column contains the geometry information for this Fusion Table (Again, go through this Poynter walkthrough to find out all you need to know about the geometry field). Mine is simply called “geometry.” Each row in the “geometry” column represents one polygon.

2. The second step is the “google.maps.event.addListener(layer, ‘mouseover’, function(fEvent).” Basically this says “anytime the reader rollovers a polygon, the following will happen.”

In this function, “fEvent”represents the polygon that the reader is currently hovering over. Let’s say I’m rolling over the polygon that is represented by the first row in the Fusion Table. It’s Census Tract 9601 and has the value of “1” in the “Number” column.

Every time a reader rolls over Census Tract 9601, the code “fEvent.row[‘Number’].value” goes into the Google Fusion Table, finds the Census Tract 9601 row and returns the value of the Number column, which is “1.” So var “NumVal” would become “1” when a reader rolls over Census Tract 9601.

The next part changes that polygon’s color. This happens with the “where” statement. This is saying, “when I rollover a polygon, find the polygon in the Fusion Table that represents ‘NumVal’ and change its color.” Since the variable “NumVal” represents the polygon currently being hovered over, this is the polygon that changes colors. For a reader, the output is simple: I rollover a polygon. It changes colors.

In short: I roll over Census Tract 9601, firing of the “google.maps.event.addListener” function. This function finds the value of the “Number” column. It returns “1.” The code then says change the color of the polygon that has a value of “1” in the “Number” column. This is Census Tract 9601, which I rolled over. It changes colors and life is good.


If you go back up to “layer.enableMapTips” in the third box of code, you’ll notice there is an option for “suppressMapTips.” For the poverty map, I have it set to true. But what if you set it to false? Basically, any time a reader hovers over a point or polygon, a small box shows up next to it containing information on that point or polygon. Notice the small yellow box that pops on this example page.

This is a nifty feature and a great replacement for the traditional InfoBox (the box that opens when you click on a point in a Google map). The only problem is the default text size is almost too small to read. How do we change that? Fairly easily:

1. Download a copy of the FusionTips javascript file.

2. Copy the file to the same folder your map is in and add this at the top of your document header:

<script type="text/javascript" src="fusiontips.js"></script>

3. Open the FusionTips file and look for “var div = document.createElement(‘DIV’).” It’s near the top of the Javascript file.

4. This ‘DIV’ represents the MapTips box. By editing this, you can change how the box and its text will display when a reader hovers over a point on the map. For instance, this map of historical places in Iowa used MapTips but the text is larger, the background is white, etc. Here’s what the DIV looks like in my FusionTips javascript file:

FusionTipOverlay.prototype.onAdd = function() {
    var div = document.createElement('DIV'); = "1px solid #999999"; = ".85"; = "absolute"; = "nowrap"; = "#ffffff"; = '13px'; = '10px'; = 'bold'; = '10px'; = '1.3em';
    if (this.style_) {
      for (var x in this.style_) {
        if (this.style_.hasOwnProperty(x)) {
[x] = this.style_[x]

Much better! Here’s the code for this map. And here are my three Fusion Tables.

I hope some of these tips help and if you have any questions, send me an e-mail. I’d be more than happy to help.


Written by csessig

February 7, 2012 at 4:34 pm

3 Responses

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  1. […] Multiple layers and rollover effects for Fusion Table maps « Chris Essig's blog. This entry was posted in Uncategorized and tagged maps. Bookmark the permalink. ← MapBox | MapBox […]

  2. here is the simple way to add/remove a fusion table layer.
    To Add a Layer on Google Maps:

    function ShowLayer() {

    layer = new google.maps.FusionTablesLayer({
    query: {
    select: ‘\’geometry\”,
    from: ‘1WV5tK0OkWeAxNqXkoqKwAxqucdcZnfhIseuZCZk’

    To remove a Layer on Google Maps:

    function RemoveLayer() {

    for more visit


    April 3, 2013 at 6:05 am

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