Zooming In On History

Last weekend saw the close of a six-month exhibition by the Norman B. Leventhal Map Center at the Boston Public Library titled “We Are One: Mapping America’s Road from Revolution to Independence,” a piece of which we at Axis Maps had the pleasure of creating. A commemoration of the 250th anniversary of the Stamp Act, the exhibition featured an impressive collection of contemporary maps from the years prior to the American Revolution through the early years of the new nation, documenting and providing context for the events leading to American independence.

A few maps from 'We Are One' 'We Are One' exhibition

As part of the exhibit, we designed interactive features for two large touchscreen displays, which allowed visitors to explore many of the maps on display more closely. (This was handy, because one of the display cases literally yelled at you to stand back if you got too close.) The two interactive displays featured maps of different themes, georectified and overlaid on a modern map. Visitors could search for locations and see them in the context of the historic maps, and also browse a curated set of map details.

Touchscreen at 'We Are One' exhibition

If you didn’t have the great fortune of being in Boston during the past six months, don’t worry! Though the touchscreens are gone, the interactive map browser is on the web—and will work nicely on your phone if you still want to touch it: zoominginonhistory.com. (The exhibition itself will be at Colonial Williamsburg in 2016 and at the New York Historical Society after that.)

Zooming In On History

This was a fun project to work on, not least because as the lucky local Axis Mapper, I got to pay a few visits with behind-the-scenes looks at the exhibition and other collections of the Leventhal Map Center, as well as attend a fancy opening reception.

More than that, it was an interesting opportunity to make something for a public space where all sorts of different people would see it. This meant some different design choices from our typical web maps. For example, we used a lot of animation with Animate.css. Almost everything touchable triggers an animated transition. Even when it’s idle, maps still move. The idea was to catch people’s eyes as they walk past, and draw them in to explore.

We found that there are special challenges to this kind of work. Put an interactive map in a room with a bunch of tourists for a few months, and you will learn all kinds of new ways in which your map can break. We recommend testing your map by placing a bored yet excitable 4-year-old in front of it.

The touchscreen feature ran simply as a web page in full screen mode. We thought of some precautions to prevent this from breaking—for example, when the screen keyboard appeared, we needed to remove or disable some keys so that someone couldn’t press Control-W and kill the whole thing—but didn’t fully anticipate the effects of impatient fingers banging on everything in sight, or what would happen when a lot of CSS transitions run for hours on end. So it broke occasionally, and I’d get nerve-wracking video messages from my friends.

While we easily fixed bugs, implementing a fix was tricky for the exhibition staff. In the exhibition, the interactives ran on local machines, not the web, to ensure connectivity. And in a public space, they couldn’t exactly have the computers sitting where visitors could reach them. So to date, this is probably our only project where deploying and update required a ladder.

Go explore zoominginonhistory.com, see some cool maps, and learn something about American history—no ladder required!

Zooming In On History - BPL
Interactive touchscreen feature for a map exhibition

Quick tip about Javascript architecture

Most of the work we do here at Axis involves coding Javascript interactive maps. Frequently, the map will have elements controlled by UI components and/or charts and/or timelines. Calling each of the map functions after a UI component gets interacted with (or chart/timeline/etc…), can get exponentially convoluted if you aren’t careful.

If you are fairly new to coding you may not be familiar that code can have design patterns. Just like there are cartographic principles that help with everything from layout to typography to color, there are coding principles that help with code clarity, maintainability, and efficiency.

A good book to refer to on code design patterns for Javascript is Learn JavaScript Design Patterns by Addy Osmani. The particular pattern I found recently useful is Publish/Subscribe (nested under the Observer pattern in the above book). There are a couple of ways to implement it, but since we use jQuery in most everything anyways, I’ll talk about jQuery’s implementation.

jQuery has the ability to register and listen for custom events. How this works in practice is fairly easy. First you need to sort out what events are actually going to happen. These aren’t events like ‘click’, but custom events like ‘yearchange’ or ‘dataupdated’ or ‘mapmove’. Once you have that set, whenever you create a component that is going to be affected by one of these custom events, you register it on the document.

function createMap() {
  //Create map code
  $(document).on('yearchange:', function(newyear) {
    //update map with new year information

Then on any and all UI elements that affect the year (menus, animation controls, etc…), all you need to do is call $(document).trigger('yearchange:', newyear). The great thing about this is that you can attach listeners on multiple different components (e.g. maybe the timeline and the chart and the map all get updated with a yearchange) and the UI element doesn’t need to know a thing about any of them.

Couple of notes:

  • There is a colon after the word yearchange. That is completely optional, but is a way to future-proof your custom event in the off chance that jQuery (or a jQuery plugin) decides to use the same event name. You could also achieve this by using slashes and creating a nesting effect (e.g. /yearchange, /yearchange/increase, /yearchange/decrease, etc…) which might be helpful to group events if you have a lot of them.
  • Why register on the document element? Because of the way Javascript handles bubbling through the DOM, you can always be sure that if the listener is on document, it’s going to be heard; plus it keeps separation of concerns (another code design thing). If you put the listener on the map object for example, then the UI element needs to know where the listener is (breaking the entire reason for this system which was that the UI didn’t need to know anything).

That’s it. If you have a several different components being updated or changed by several other components, consider the Pub/Sub pattern and if you are using jQuery already in your project, consider using custom events.

D3 web maps for static cartography production

We’re a mapping company that’s most at home on the web. So when a more traditional static cartography job came along, we even took that to the web.

The ongoing project involves producing a set of four maps for each of thirty-three different countries, showing GDP along with flood and earthquake risk. To make 132 maps, clearly we need to set up a good, easily-repeatable workflow. Because the map themes and symbologies are the same for each country, essentially we need a set of templates into which we can throw some data and get an appropriately styled map that we can export for refinement in Illustrator, and finally layout in InDesign.

Ordinarily that’s where GIS software like QGIS or ArcGIS comes in. But using GIS can be a slow, repetitive task. In the interest of creating a faster process suited to our purpose specifically, we thought it would be easier to develop our own web-based tool to handle the parts of the workflow normally handled by GIS. Fortunately, it’s easy to do such a thing with D3.

Here’s a shot of the tool. It looks rough, certainly, but it does its job: quickly get a country map as close as possible to final form, ready to be dropped into our Illustrator and InDesign templates.

D3 tool for static maps

In contrast to GIS, there are only a couple of moving parts here. All we have to do is select a country, adjust the map position and scale, and choose the map theme. Everything else is automated. For me, someone who usually accomplishes cartography via web development, it was a lot easier and faster to build this simple customized tool than to figure out how to get GIS to do exactly what we want. It’s also easy to keep up with any change requests, which usually only mean a few lines of code.

Below is a bit more about how this thing works, which leads to some tips we’ve learned about using a D3 web map in a static cartography workflow.


Data here are in two forms: topojson for the map layers (boundaries, cities, lakes, etc.) and CSV for the numeric data. All are loaded at runtime, where the CSV is joined to Admin 1 (province-level) geodata. We avoided pre-joining the CSV to the shape data because that would make updates more complicated, i.e., saving a new topojson file for any little data change.

Page setup

It’s important to get sizes and positioning exactly right so that the exported map drops perfectly into our templates without the need for any transformations. Our maps are A4 size and we’re working in millimeters, but of course in the browser we need pixel dimensions. It works out to be 841.89 x 595.28 pixels. Illustrator converts measurements at 72 pixels per inch, which is a lower resolution than modern screens, and thus the map appears suspiciously small in the browser—but don’t worry, it all turns out right in the end!

It’s also important to note the lat/lon bounds of the map in case outside geodata needs to match. In this case we generate a hillshade separately using GDAL. The extent values listed above the map—easily derived from D3’s projection.invert() method—plug right into the GDAL commands, producing a hillshade image that will fit perfectly on this map.

Map layout

D3 really shines with projecting and drawing geographic data, and this tool takes full advantage of that. It goes through the standard routine of drawing GeoJSON data to SVG paths, putting each data layer into a named <g> element that will later show up as a layer in Illustrator. We use the Mercator projection (sorry, cartographic hard-liners, it was by request) and use D3’s geo centroid() and bounds() functions to center the map on the country of interest and scale it approximately to fit. After that we can manually adjust position or scale to get the layout just right, fitting the country into some guides we added on top of the map.

Importantly, in addition to the standard map-drawing code, we use the projection’s clipExtent() method to clip everything to the page extent. Otherwise, when imported to Illustrator, we’d have a whole world’s worth of extraneous paths outside the artboard. While setting the clipExtent is enough to chop off polygons and anything drawn directly with the path generator, hiding labels takes a little extra effort. We have to check manually whether they’re outside the clip extent and hide them if so.

  .each( function(d){
    var c = path.centroid(d);
    if ( !isNaN(c[0]) && !isNaN(c[1]) ){
          x: c[0],
          y: c[1],
          opacity: 1
    } else {

Thematic symbols

D3’s scale functions really come in handy for these maps’ thematic layers. For the most part, we can create scales that have a constant range (such as the purple color scheme or the proportional symbol size) and set the domain to according to a particular country’s data if needed. GDP data (the choropleth map) is classified using the Jenks algorithm in Simple Statistics.

For the most part we leave legends to the Illustrator end of things. Values of the choropleth class breaks are simply listed in the containing HTML page, and then entered into the template in Illustrator. We do draw a simple proportional symbol legend because while the maximum size is constant, the size of good sample values (such as 1 or 5) changes from map to map. Flood and earthquake maps use bar chart symbols that, like the choropleth map, simply get some explanation in text.

Flood symbols

Mapping via code provides a big advantage if you’re looking for complex symbols. D3 in particular makes it easy to draw symbols tied to data—with almost total freedom of form.


Luckily, there’s not a lot to explain here. SVG Crowbar, created by New York Times Graphics people specifically with D3 in mind, does a great job of exporting the SVG map to a layered file ready to use with Illustrator. We just have to make sure our various map layers are properly grouped and have id attributes, and we end up with a nice and neat file.

Map layers

A command line

Finally, a nice thing about doing the basic map work in the browser is that it’s easy for a web developer to do advanced things by entering code right in the console, without having to be familiar with more specialized code that might be used in GIS or other tools. We didn’t do much of that here, other than as a shortcut to set zoom levels, but theoretically we could invoke, change, or extend any other functions of the page to do things that might be needed in unique cases.

Adjusting the map in the console

Coder-friendly mapping

In closing, it merits noting that this kind of workflow might be good for us (and people like us) because JavaScript is much of what we do on a day-to-day basis. Everything that I’ve said is “easy” with D3 is, well, very difficult if you’re unfamiliar with this kind of coding. A good philosophy and habit is to use the right tool for the job, but it’s also wise, for expediency’s sake, not to dismiss just using the tool that you know. Sometimes the right tool is the easy tool.

Cartography with an actual blindfold

Last month, Andy gave a talk at the OpenVis Conference entitled Blindfolded Cartography. Essentially, things to look out for when designing maps (especially interactive ones) so that when real data comes in, the map/design/page doesn’t get wonky. Things like too long text, missing data, skewed data, etc… The last week or so, we’ve been working on adding accessibility features to one of our projects and wanted to share a few thoughts and lessons on Cartography with an actual blindfold.

First off, what are accessibility features and why should you care? Accessibility features are features and general design patterns that allow people with disabilities to view and interact with your content. This can range from simply allowing keyboard navigation all the way up to screen readers. Now, why should you care? We can get fairly pedantic about whether a font size should be 16pt or 18pt, should this light brown text be #f2e1cb or #f8f1e7, should this div have a margin of 20px or 22px? If we as developers take that much care about things that a lot of users aren’t going to consciously notice, shouldn’t we take at least some care with things that some of the users are really really going to notice?

So what categories of accessibility should you be aware of? There are 4 areas that I’m going to talk about; I’m sure there are more, but these cover the majority of cases that I’m currently aware of. Color, keyboard navigation, ARIA, and Screen Readers. Also, I should note, I am not an expert in this. What follows are the notes of a developer brand new to the area of accessibility.


Color can cover a whole host of issues, but the main areas to watch out for are color blindness and low contrast. About 10% of all males have some form of color blindness, of which the most prevalent is red/green (protanopia) color blindness. That’s a big percentage of your users. Simple fix – don’t put red and green right next to each other. As a mapping company trained in classical cartography techniques, this one is pretty much embedded in our blood and bones. Other versions of color blindness do exist though. It’s up to you to decide how many versions you want to design for (since the more you cover, the less options you have for design). A good site to figure out which colors you can use is Colorlab.

Magenta vs Gray for Red/Green color blind people Magenta vs Gray for Red/Green color blind people

A color tool for Cartography specifically and which has colorblind options is ColorBrewer (which Axis Maps hosts).

Color brewer example image Color Brewer

The other major category related to vision is contrast. Many users have a harder time distinguishing low contrast color pairings. For example, #ccc color text on a #999 background. You can check contrast between colors using the Color Contrast Check tool.

Contrast checker image - #999 vs #ccc Contrast Checker - #999 vs #ccc

Testing: The best way to test for color blindness issues is to have a someone who is actually color blind look at the site (and since 1 out 10 men are, there is a good chance you know someone who is). Barring that, run the various colors through Colorlab and the Color Contrast Check tool.


There are two types of people who use the keyboard only; the visually impaired and those with a disability making use of a mouse impossible (for those of you wondering, as I did, how you can use a keyboard but not a mouse, many alternate input devices emulate a keyboard but not a mouse – e.g. Sip and Puff devices).

There are a couple of key points to keep in mind when designing for keyboard users. Most browsers are already pretty good at keyboard navigation, IF the HTML is organized in a logical order. While this isn’t a problem when writing HTML for the most part, where you can run into issues is with adding pieces via Javascript. It’s really easy to add a bunch of divs with JS and as long as they look in the right place, ignore the fact that they are not keyboard navigable. One major gotcha, div is not keyboard navigable by default. You have to add the attribute tabindex="0" to make it navigable. a links and button elements are keyboard navigable by default so those don’t need an extra tabindex attribute. Best practice is to make all clickable areas that go somewhere a links and all clickable areas that trigger functionality button elements. Unless absolutely necessary, don’t use div for buttons/links.

tabindex=0 simply adds the element to the keyboard navigation index. Sometimes, especially with adding elements via Javascript, the default tab order ends up not really being a logical one. In these cases, you can override the tab order by changing the tabindex to any positive numerical value. A tabindex=1 will come before tabindex=2 and so on. This is not considered good practice, but in production, sometimes this ends up being the only way without a complete rewrite of the code.

Testing: Keyboard navigation is really easy to test. Just use the site with only the keyboard. Can you access all the functionality? Can you get to all the links and do they work by pressing space or enter? Are all the buttons/divs/links/etc… in a clear and logical tab order?


ARIA stands for “Accessible Rich Internet Applications” and is designed to let screen readers and other assistance software help make interactive pages usable by all users. ARIA tags are usually element attributes such as aria-hidden and role. Like other element attributes such as id and src, there are attributes for pretty much every element and they apply differently. I’ll mention a few that are good to know about:

  • aria-hidden - makes a particular element hidden from screen readers. This is useful if, for example, you have a description that is truncated for display reasons, but is expandable through mouse interaction.


Two divs. One with a class of hidden making it all but invisible to the naked eye, yet still there. The other one is the normal div that everybody sees, but with an aria-hidden="true" attribute to hide it from screen readers so they don’t repeat themselves.

<div class="full-description hidden">Blah blah, this is the full description and is not truncated with ellipsis</div>
<div class="description">Blah blah...</div>


Hidden class that reduces the element to 1px by 1px. The clip and overflow: hidden hides everything that goes out of the 1px by 1px box.

  position: absolute !important;
  margin: 0 !important;
  padding: 0 !important;
  height: 1px;
  width: 1px;
  clip: rect(1px 1px 1px 1px); /* IE6, IE7 */
  clip: rect(1px, 1px, 1px, 1px);
  overflow: hidden;
  • As seen above, the use of the .hidden class is very important. This allows screen readers to “see” something, even though people can’t see it.
  • Using a hidden class is different than using display: none or visible: hidden. Changing the display to none or the visibility to hidden takes the element out of the screen reader’s view.
  • role - ARIA-role w3 page - this one can get complicated as there are four different categories that can be used, but the idea is that you can tell the screen reader what the element is for. The four categories are:
    1. Abstract Roles – Do not use.
    2. Widget Roles – Define what the element is to be used for. This is helpful when, for example, you have div operating as a button. Most of the time, you should just mark it up as a button, but if you can’t, then you can assign role="button" and it acts as a button to screen readers.
    3. Document Structure – Defines what structure the element has. This is so the screen reader can take special care with the pronunciation and navigation of that element. For example, the math role tells the screen reader that mathematical formulas are in coming and to take appropriate action (such as saying “eh-ex-squared” instead of “ax-squared” when ax2 is encountered).
    4. Landmark Roles – Define the different areas of the document (such as navigation, main, search, etc…) so the screen reader can jump between them easily.
  • alt tags – This is pretty well known, but I’ll reiterate. Put descriptive alt tags on all images so that the screen reader knows what the picture is about. Make sure it’s short though, no need to wax eloquent on all the little details of the logo.
  • links – This is probably the one I’m most guilty of. Don’t have links where the only linked text is “Click here.” In context, this is fine. But out of context this doesn’t make any sense. And many screen reader users navigate by pulling up a list of links in which only the text of the actual link is available, so the links very well may be spoken out of context. Don’t say “click here”, say “Link to Annual Report PDF”. Oh, and don’t leave the full link in visible text or the screen reader will read “double-u, double-u, double-u, dot …”.

Testing: Testing ARIA tags is mostly about testing things with a screen reader. The testing part of the Screen Reader section below has more information.

Screen Readers

And now to the most well known accessibility genre, the screen reader. Screen Readers work by reading off the text from a web page. How they do this is different for each one and can devolve into a similar situation as the early 2000’s with web browsers – each one its own special flower. There is some standardization and it is getting better, but you will still run into situations where different screen readers read the same markup differently (especially when Javascript gets involved).

The four main ones that are relevant to interactive web sites are JAWS, NVDA, VoiceOver, and ChromeVox. JAWS, the most widely used screen reader, is a commercially licensed piece of software for Windows, especially popular in academia and government. NVDA is an open-source program for Windows. VoiceOver belongs to Apple and is bundled with OSX. ChromeVox is a Google created plugin for Chrome that is really helpful for developers (though because of its integration with Chrome, it does create some headaches as seen below).

Each screen reader sees the world slightly differently. This can create some head bashing moments as the notes below make clear. They were filed as apart of a Github bug report for the accessibility upgrade I was working on this week. The list was a long list of images where each one, on click, would open up to a carousel widget to view more images from that spot. The carousel widget had a left and right HTML button that were supposed to be navigable by keyboard arrows, mouse clicks on display arrows, and keyboard “enter” when the focus was on a display arrow.

Tabbing through list

  • Chrome - reads each section when the section is focused, reads header
  • FF - reads all the sections when the entire list is focused (i.e. on normal tabbing through), reads each section when focused, does not read header when tabbed to, but does read header when shift-tabbed to
  • IE - reads each section when the section is focused, reads header


  • Chrome - does not read the arrow aria-label, reads the title and description of each carousel image after interacting with the arrow
  • FF - reads the arrow aria-label, when the focus is shifted to a new arrow (i.e. at the beginning or end when the current arrow is hidden) reads the new arrow, otherwise, doesn’t read anything on interaction
  • IE - same as FF

Screenshot of the carousel  with the problem arrows Screenshot of the carousel with the problem arrows

For the life of me, I couldn’t get Chrome to read the aria-label on the arrows, until I took the ChromeVox tutorial and realized that screen readers don’t just use tab to navigate. They use the arrow keys, the plus/minus keys, and a whole host of other shortcut keys. Tab is just for interactive elements (and apparently Chrome/ChromeVox was not reading the aria-label for interactive elements). This is by design. Keyboard only users just need tab to move through the interactable elements. They can read the non-interactive elements already. Screen Readers use other keys to move through the non-interactable elements such as paragraph blocks.

Which brings me to the final point, Screen Readers are complicated pieces of software that work in ways that are very different conceptually than how the majority of users see and interact with a web site. The best way to test your site is to get someone who knows how to use a screen reader properly, to test it. Failing that, make sure that you are at least somewhat versed in how your chosen screen reader actually works. It’s more than just tabs.

Testing: ChromeVox is a free plugin for Chrome that is really easy to setup and use. Make sure that you go through the tutorial first though as there are facets of screen reader navigation that are not necessarily intuitive for people used to mouse navigation. VoiceOver is included with OSX so that should be easy enough to include in testing if you are using a Mac. NVDA is free software as well, though for Windows. By using one or all of these, you should run into any bugs that will affect screen readers. Things to look for: can you hear all the content? Is anything behind an “interactive wall” that is not accessible with keyboard/screen reader? Do all the images have alt tags so you know what is going on without seeing the actual image?


Accessibility concerns affect a small but significant portion of users. If things are designed with them in mind, that can only be a good thing. Many of the above suggestions, with the possible exception of catering to color blindness, shouldn’t change a single thing on how your page looks, feels, and interacts. But by following them, more people will use and enjoy whatever content is there.

And if you really want to get a handle on how this stuff works in practice, do as Mercator did and use an actual blindfold:

Blindfolded Mercator

Blindfolded Cartography

This is an adaptation of a talk I gave at the 2015 OpenVis Conference in Boston, expanded in a few spots and abbreviated in others. You can see slides and, ugh, video from the talk.

At Axis Maps, a rough napkin sketch of our projects often looks like this:

Map viewer sketch

We're tasked with designing and building interactive maps of data that, for one reason or another, we can't yet see. Sometimes the client doesn't have data ready soon enough. Sometimes we expect data to change or be added after our work is done. Sometimes it's just too vast for us to lay eyes on all of it. Whatever the reason, this reality of web mapping is a significant departure from what we knew back in cartography school.

Traditional cartography allows—encourages, perhaps—perfectionism. We were taught attention to detail, and to find and tell the story in our data. Crafting beautiful, effective maps and telling stories with them is one thing when you can obsess over every detail; it's quite another when you have to do it as though blindfolded. (And I don't mean sea monsters and mapping unknowns; I mean mapping real things that are known but somehow inaccessible to the cartographer.)


As always in cartography, this is all about making compromises. We make design choices that we know are not are not ideal for most cases, but are at least acceptable for all cases. What follows is a list of some areas where we've found the "blindfold" particularly challenging, and some compromises that we or others have found helpful. And an occasional shrug where we haven't found good answers yet. Much of this is about design, some of it is about code (JavaScript), some of it isn't even about maps, and none of it is perfect.

Data classification

John Nelson has a succinctly handy explanation and demonstration of why data classification decisions are important on choropleth maps. One way of dividing data into bins can produce a starkly different map from another way, potentially in a functionally and aesthetically bad way.

Data classification example by John Nelson Data classification example by John Nelson

We want to devise a classification scheme around several questions: Will the map be useful and look good? Are the class breaks meaningful? Are they understandable? Are they pretty numbers? As usual, this isn't too difficult when dealing with a single data distribution. But we need satisfactory answers to those questions for any data set, without our manual intervention. And real data... well, real data are always messy.

Data in designs vs data in reality

Two common solutions are Jenks optimal breaks, where class breaks are calculated to optimize both grouping and spacing, and quantiles, in which each bin contains the same number of values. Both offer some assurances of visible spatial patterns, but both have drawbacks. For example, optimal breaks can be hard to understand, and quantiles can inappropriately group or split values.

Optimal breaks and quartiles

One compromise we've used can be described in two parts:

1. Unevenly distributed percentile breaks. Quantiles are friendly, readable, and somewhat reliable, so we often err toward them. Instead of dividing data into evenly sized classes, though, we might break down some of the classes further to separate or highlight extremes or important breakpoints, for example the top 5% in the histogram below.

A compromise classification

2. Use percentiles of unique values, not all values. Datasets often contain many duplicate values; when quantiles are calculated properly, a single value could span several classes. (A common example is having a whole bunch of zero values.) To avoid weird map legends and to better bring out geographic patterns, we might base our classification on the distribution of only unique values, avoiding duplicates.

// basic example
function getValueAtPercentile( data, percentile ){
  return data[ parseInt( (data.length - 1) * percentile ) ];
// using underscore.js
var dataArray = _.sortBy( [ 0, 0, 12, 23, 2, 5, 0, 5, 19, 0, 0, 0, 33, 9, 25, 0 ], Number );
var uniques = _.uniq( dataArray, true );
getValueAtPercentile( dataArray, .25 ); // = 0
getValueAtPercentile( uniques, .25 ); //  = 5


Expect holes in the data. Assume that sometimes data will be bad or missing. Catch these missing values in code, and, importantly, design for them. "No data" is data too.

Andy Kirk gave a talk called "The Design of Nothing: Null, Zero, Blank" at last OpenVis in 2014, covering (among other things) some no-data scenarios. It's worth a look:

There are two sides to handling missing data. On one side is code that won't break if a null value is thrown at it. Basically this means a lot of error handling and bailing out of functions if they're passed null values. But it also requires some understanding of the data format. A missing value could come through in a variety of ways.

// no data might be...
// &c. &c.

Be careful to avoid the JavaScript gotcha of equating zero with "no data"—usually not the same thing in reality.

// a tempting way to catch "no data" in javascript
if ( data ){
  // yay, we have data!

// but watch out!
var data = 0; // zero is real data
if ( data ){
  // zero won't get us in here :(

The other side is design. One common scenario is missing data in a choropleth map. We like using texture to indicate missing data. It's distinct, not easily confused with colored values; and it's explicit, keeping "no data" as part of the story instead of hiding it. If you're working with D3, check out Textures.js for easy texturing.

choropleth textures

Another common case is gaps in time series data. Again, it's good to be explicit. Interpolation can be misleading. For example, I like the dashed line in the iOS Health app. It explicitly indicates a gap in data without losing continuity.

iOS health - gaps in data

On maps, one thing we've tried is using a special symbol to indicate a gap in data. Below is a frame from an interactive animated map where proportional symbols remain on the map even when there's no data for the current year, showing the most recent available data but with a different, hollow symbol.

Gaps in time series data on a map


Text in an interactive map setting is more than just labels. Marty Elmer has written nicely about prose on maps, and how it's ubiquitous yet overlooked. Text, too, can be an unknown part of map data—one that can ruin otherwise beautiful layouts.

The big lesson from experience here is to restrict the space allotted to text. If you've designed for a short sentence, assume you'll be fed a novel, so make sure text doesn't overflow all over everything. CSS properties like max-height and overflow:auto can help. For shorter labels, truncation and abbreviation (such as via text-overflow:ellipsis may be useful, but also take advantage of what you do know about the data to be clever about this. For example, in the chart below we knew that some labels would be for congressional districts. If we simply chopped off those labels after a few characters, they'd all be the same, so instead we put the ellipsis in the middle and retained the district number at the end.

Label abbreviations

One last thing: mind your number formatting! It's easy to forget to design label formats for things like singular versus plural or different orders of magnitude, in which case you end up with some funny-looking labels.

Bad number formatting

The Lorem Ipsum Map

All of this, broadly, is about the challenge of designing around missing or fake data. The "lorem ipsum map" is a phrase used by Rob Roth and Mark Harrower (2008), as a caution against designing user interfaces around a placeholder map. In studying the usability of a map we made for the UW-Madison Lakeshore Nature Preserve, they found some negative reactions to the cold, metallic UI (which we designed first), in contrast to the green and fuzzy map (which we didn't add until later). It's hard to offer solid advice about this, other than to suggest trying to understand at least the basic nature of the unknown data and what it might look like. ¯\_(ツ)_/¯

Instead of designing around a blank spot, however, we often generate fake data. That of course can be a challenge itself, because how do we design somewhat realistic fake data when we don't know what the real data look like? Carolyn Fish has a good term for this goal—"smart dummy data"—and an interesting anecdote about challenges she faced trying to make OpenStreetMap data stand in for totally inaccessible (to her) classified data.

All that said, fake data can be an excellent test of code. In some ways, the more unrealistic the better. If the code can handle ridiculous data scenarios, it can probably handle the real data.

The Human Touch

Big picture time. The ultimate goal, really, is to approximate good, human design when we're forced to do it through rules and algorithms applied to unknown data. I'm going to keep pointing to Daniel Huffman to explain why this matters, at least until he makes good on his promises to follow up on four-year-old articles.

To illustrate how enormous a task handmade design can be in the web cartography age, consider The Essential Geography of the United States of America by David Imus. You probably saw this map a few years ago when a Slate writer declared it "The Greatest Paper Map of the United States You'll Ever See." Indeed, it's a lovely map with tons of manual details.

The Essential Geography of the United States of America

The map is 1:4,000,000 scale, and according to the Slate article, it took nearly 6,000 hours to complete. 1:4,000,000 is approximately zoom level 7 in the standard slippy map scheme. Just for kicks, let's extrapolate and say David Imus wanted to design a fully multiscale web map (zoom levels 0 to 18) with the same attention to detail. How long would it take him? I'll spare you the details of my calculation: it's 2,803 years and 222 days. And that's only the United States. Im-freaking-possible, in other words. That's why we need algorithms.

Big Design

This gets into what I want to call "big design." So-called "big data" isn't just a computational challenge; it's a design problem too. How do we accomplish good human design when there's simply too much data for a human to see?

The problem is perhaps unique to cartography, where at some level we want to see almost every piece of data we have, not aggregates and distillations. For most of us that means a map based on OpenStreetMap data down to the highest detail levels. With 2 million users mapping all over the world, quality and consistency can't be 100% guaranteed. Our challenge is to come up with design rules that ensure the map looks pretty good everywhere, at all scales. Yikes.

Nicki Dlugash of Mapbox spoke to this very topic at NACIS 2014. To summarize her advice:

  1. Prioritize areas you care about for your specific use case.
  2. Prioritize the most typical or common examples of a feature.
  3. Look for the most atypical examples of a feature to develop a good compromise.

To help you along the way, there are things taginfo, where you can explore OSM tags and get a sense of things like geographic variation; or things like this view in Mapbox Studio where you can inspect many places at once.

Mapbox Studio "places" view

And of course the nice thing about OpenStreetMap is that you can always fix the data if you find problems!

Good Luck

Well, I hope that assortment of tips and problems has been useful. Good luck as you stumble through the dark!

Blindfolded Mercator