Creating an interface for geofences

When we began prototyping geofences, our goal was to encourage people to geotag more photos without having to be too concerned about privacy.

Introducing people to geofences

The term “geofence” may sound complicated, but our implementation is quite simple. A geofence is a user-defined boundary around a specific area on a map. We decided to keep the creation and editing process similar to geotagging on the photo page. We understand from developing the photo page map that it is important to provide a way to search for a location as well as simply drop something in the right place on the map. The geofence is represented by a selector-circle on a modal map panel with simple edit controls on the side.


In previous map interfaces we used canvas elements to generate scalable components like our new selector-circle. One example of this would be the location circle in photos nearby. Our resident browser performance task-master, Scott Schiller, decided this time to compare the performance of continuing to use canvas with that of the now widely supported css border-radius feature. Using border-radius made for a smoother performing interface in our tests against the early prototypes of geofences. We loved the result.

The only noticeable downside to our current implementation is that it attaches a click handler to the entire element which is displayed as a selector-circle with css. This means that the actual click-able area is not a circle at all, it is a square.

box around selector-circle

We are currently working to find the most elegant way to get around this issue. The code below shows one possible solution:

var root_node   ='#circle_container_node'),
    circle_node ='.selector-circle');

function getCenterPoint() {
	var half_node_width = circle_node.get('offsetWidth')/2;
	return [(circle_node.getX()+half_node_width),(circle_node.getY()+half_node_width)];

function getDistanceFromCentroid(xy) {
	var centroid_point = getCenterPoint();

	return Math.sqrt(Math.round(Math.pow(centroid_point[0]-xy[0], 2.0)) + Math.round(Math.pow(centroid_point[1]-xy[1], 2.0)));

	if(getDistanceFromCentroid([e.pageX,e.pageY]) < half_node_width) {

Sustainably supporting a sill relevant, but older browser

Don’t you still have a fair amount of users on IE7, which does not support border radius? Yes, we do, and thanks for pointing that out so eloquently. We decided to use the CSS3PIE framework which uses clever tricks to make IE7 support modern css features like border-radius, box-shadow, border-image, multiple background images and linear-gradient as background image. What is awesome about using CSS3PIE, is that it allows us to give IE7 users a nearly identical experience as that enjoyed by modern browser users without a lot of branched code.

.selector-circle {
	border:solid 8px rgba(68, 68, 68, .4);
	background-color:rgba(0, 100, 220, .5);
	border-radius: 999px;
        /*Neeed for CSSPIE to support rgba*/
        -pie-background:rgba(0, 100, 220, .5);

Relative sizing

We determined a sensible size range of 50 to 10,000 meters for a geofence, then dreamt up a few ways to allow folks to select an appropriate size within this range on the map. A lot of people (myself included) are not very good at judging distance in kilometers or miles without a visual example. We establish a connection between changing to the radius dropdown menu, the size of the selector-circle, and the zoom level of the map by making sure that a change to one is reflected in all of them.

Zoom level of map in relation to the size of the selector-circle

Something had to be done to keep the geofence visible on the map even if the selector-circle has become too small to be seen in a selected zoom level. The selector-circle was given a center-point which would not change with changes of radius or zoom. It is always visible even if the surrounding fence is too small

Geofences are privacy defaults, not filters

A geofence is only applied as a default to at the time of uploading or geotagging a photo. It will not, on it’s own, affect photos already in that area. We created a process which one could follow to apply geofences to existing photos as a convenience. Nolan Caudill goes into detail about the code behind this process in his post from last week. Now, imagine if you did not realize that a large number of your photos, which you intended to stay public were in an area covered by a geofence. After applying the geofence settings, they would be suddenly hidden from the map. Changing all of those photos individually or even as a batch in the organizr would be, what we in the industry call, a massive honking bummer. A final step was added to the end of the creation and edit of geofences to show which photos could be affected. We display a list of potentially affected photos with a color-coded indicators of what their privacy will be after the geofence settings are applied to help people to make as informed a choice as possible.

Existing photo apply panel

In most cases you will get an idea of which photos will be affected right away, but areas with lots of photos can be carefully assessed as well.

Showing all of the fences at once

In the prototype phase, geofences were displayed in a list and the only way to see them on a map was individually. We decided to create a map which displays all of the circles, in one place, over the list, to make it easier to see how fences might interact. One great example is that if two fences of similar privacy value (like family and friends for instance) overlap, the intersecting area is considered private. With this in mind, it is nice to at least be able to see on the map where your fences cross. In some later release, we would love to show an appropriate red color-code in these intersecting areas.

Most people will create geofences with names like Home, Office, and School. For most people, these places are pretty close to each other. Some people might have places which they want to protect and are further apart. It is important for the map flexible enough to include the entire globe if you were to have one or two geofences abroad. We use a center-point to represent geofences which are too small to be seen, and we calculate the best zoom and center of the map when we load of the geo preferences page.

World view

The world view of the map

Neighborhood view

The neighborhood view of the map

The Code Behind Geofences

We launched geofences earlier this week. I want to give you a glimpse into some of the code that runs the feature.

Geofences are defined by three things: a coordinate pair, a radius, and a privacy level. We store the coordinates (ie, decimal latitude/longitude) in our database as integers, the radius as an integer representing meters, and the privacy level as an integer that maps to a constant in our code.

When an image is geotagged, we have to determine which geofences the point falls within. We do some complex privacy calculations that Trevor talks about in his introductory post.

First, we fetch all of your geofences from the database and then loop through each one, determining if the photo falls in any geofences. We limit the number of geofences you can create to ten, so running a point through 10 calculations is not that expensive.

We use the great-circle distance formula to figure out how far the image is from the center of a geofence. If this distance is less than the radius of the geofence, the geofence applies.

European detail map of Flickr and Twitter locations

MySQL is a Great Hammer

When you create a geofence and want to apply it to existing photos, the backfill is a more involved process. In order to grab just the photos we care about for a geofence, we have to limit the number of photos that we select due to performance.

As mentioned, we store the latitude and longitude as integers in MySQL. Since radial queries are next to impossible with this setup, we do a first pass with a bounding box formula that encompasses the geofence and then use our great-circle distance formula to cull the photos that don’t fall in the geofence.

The bounding box formula we use takes into consideration geographic gotchas, like the poles and the 180 degree discontinuity (ie, the International Date Line). Since a geofence could overlap the International Date Line, we have to modify the DB query in doing the longitude part of our query since doing a simple BETWEEN won’t work. When the bounding box doesn’t cross the IDL, we can do “SELECT id FROM Table WHERE longitude BETWEEN $lon1 and $lon2” and when it does we do a “SELECT id FROM Table WHERE longitude < $west_lon AND longitude > $east_lon”.

In short, we use a query that MySQL is a good at it to limit our initial dataset and then use a little post-processesing of the data to get the points we actually care about for the geofence.

After we have this bundle of photos that the geofence backfill applies to, we then run our normal geoprivacy calculations, lowering the privacy if the combination of fences calls for it.

Concurrency, or the world keeps turning

The introduction of the geofences backfill pane also introduced some concurrency issues, which were fun to deal with.

The contract that we wanted to create for the user was that the calculated privacy that they saw in the preview pane for the backfill would exactly match the result of running the backfill. This seemed intuitive but the implementation was tricky.

There are two things that can affect a photo’s geoprivacy: the user’s default privacy and the geofences that exist. These things can change through time, that is between the time that a user hits ‘apply’ on the preview pane and when the backfill finishes running.

Whenever you deal with mutable state through the lifetime of a process, you have to change how you treat this state. Who can modify it? What can they modify? And what processes see this updated object?

The easiest way to deal with mutable state is to make it immutable. We do this by storing the state of the user’s geoprivacy world (that is, the default geoprivacy and the geofences) alongside the backfill task, so that when the task runs it uses this state instead of querying the DB, which is mutable, possibly having changed since the user hit the ‘apply’ button.

Storing this state ensures that regardless of how the user modifies their geoprivacy settings, the result of the backfill will match exactly what they saw in the preview pane, providing a consistent view of the world.

We also added the restriction of only allowing one backfill task per user to be running at a time, which simplified our bookeeping, our mental model of the problem, and the user’s expecation of what happens.

Lessons Learned

Geo, by itself, is not easy. The math is complex (especially for someone like me a few years out of school). Privacy is even harder. Since the project evolved quite a bit while we were building it, having a large amount of automated testing around the privacy rules gave us the confidence that we could go forward with new privacy demands without introducing bad stuff into code that was “done”. We learned to give ourselves plenty of time to get all this stuff right, since geo has so many edge cases and violating privacy is an absolute no-no.

During the project, we had to keep balance between our users’ privacy and their expectations, all while keeping a complex and new feature understandable and even fun to play with. The only way we could do this was being open and honest between engineering, design and the product team about what we were building. This was a total team effort from Flickr, and we’re very proud of the end result and the control that it gives our users over their presence on the Internet.

Also, if you’re interested in working on fun projects like this one, we’re hiring!