Book Review: Leaflet.js Essentials by Paul Crickard III

This is a review of Leaflet.js Essentials by Paul Crickard III.

Contents

• Chapter 1 introduces creating a basic map from scratch with leaflet, how to use map tiles, how to create popups, and how to enable geolocation. 
• Chapter 2 shows how to create GeoJSON data, how to style GeoJSON layers by type (LineString, Polygon, etc.), and how to filter to display only a subset of the data. 
• Chapter 3 introduces the leaflet.heat plugin for density heat maps and the heatmap.js library for intensity heat maps. It also teaches how to style and animate heat maps, how to create choropleths, and how to use buttons outside the map to change what's shown in the map. 
• Chapter 4 introduces how to create custom markers and marker shadows as well as several markers that are available online. We also learn how to cluster markers, animate markers, and create pie or bar chart markers. 
• Chapter 5 shows how to use many ESRI resources in Leaflet, including map tiles, shapefiles, and geocoding. 
• Chapter 6 introduces using Node.JS, Python, and C# with leaflet, including getting data via AJAX calls, using MongoDB to save created map points, and creating desktop applications with C#.

Review

This is a very well laid-out, detailed guide to Leaflet. It is intended for complete newbies to Leaflet, but it provides some very advanced information as well. The book claims you should have some JavaScript knowledge before beginning, but I'm not entirely sure this is necessary, since Leaflet maps rely on only very minimal JavaScript.

I enjoyed the breadth of this book. I think the book does a great job of covering the information a beginner would need. It also provides information on a wide variety of topics like good tips for designing for mobile, a very comprehensive list of map tiles (many of these I did not know about), and information on several JavaScript libraries and Leaflet plug-ins that allow for additional functionality within the maps.

On the other hand, there is one major criticism I have with this book, especially for a beginner audience. I'm not that happy with the included code samples or how they are organized. This may just be personal preference, but I prefer books that have the "before this section" code and the "after this section" code so you can start with the before and then double-check with the after if you made an error or didn't understand something. I feel like the provided code samples made it difficult to follow along with programming while reading.

All in all, I wouldn't hesitate to recommend this book to someone beginning to learn Leaflet. For more advanced Leaflet users, there are some gems that may or may not make the book a worthwhile purchase. I will say that I use Leaflet almost every day, and I did learn several things from reading Leaflet.js Essentials.

4 out of 5 stars

---

Note: The publisher asked me to review this book and provided my copy.

Add Icons to Layer Control in Leaflet

When you have multiple layers, it can help to add a legend to the map. This tutorial shows how to add icons to the layer control in Leaflet. This tutorial shows only marker layers, but you could create icons for polygon layers and use those as well.

Legend with marker icons

The following code sets up our map, layers, and the layer control. This is the standard way maps look in Leaflet, with no visual indication of which layer goes with which markers, without checking the boxes on and off.

var map = L.map('map', {
  center: [33.8, -84.4],
  zoom: 11,
    layers: [trainLayer, treeLayer]
});

var overlayMaps = {
  "Train": trainLayer,
  "Tree": treeLayer
};

L.control.layers(null, overlayMaps, {
  collapsed: false
}).addTo(map);

Default legend

In order to add a legend to the layer control, you can simply change the overlayMaps code to add any HTML you'd like. In this case, we'll add the map icon pngs with a small height so they will look okay in the layer control:

var overlayMaps = {
  "<img src='http://mollietaylor.com/skills/js/leaflet/train.png' height=24>Train": trainLayer,
  "<img src='http://mollietaylor.com/skills/js/leaflet/arbol.png' height=24>Tree": treeLayer
};

It's really that simple! I searched the documentation and the web, and I couldn't figure out how to do this, so I figured I'd just try adding the HTML. Turns out it looks great!

Full code available on CodePen. Map icons by Nicolas Mollet and Axel Rodriguez.

Changing Leaflet Slidemapper to a Side-by-Side View

The default setup in slidemapper is to have the slides either above or below the map. However, it is also possible to place the slides to one side of the map.

Slides beside map

CSS changes

We need to make a couple of changes to how the smapp-show and smapp-map classes are displayed. You'll want to play around with these some, but this is what worked well for me:

    .smapp-show { 
      width:50%; 
    }
    .smapp-show .slide { 
      overflow-y:visible; 
    }
    .smapp-map { 
      margin:10px; 
      width:48%; 
      height:95%; 
      position:absolute; 
      top:0; 
      right:0; 
    }

Slidemapper changes

On the slidemapper side of things, the only edit we need to make is changing the options of our slideshow. Specifically, we need to edit the slideHeight and mapHeight to something appropriate for having the slides beside the map. For example, I chose:

    $mySlideMap = $('#slideshow-container').slideMapper({
      slideHeight: 540,
      mapHeight: '98%'
    });

And that's all the changes you need to make to have a side-by-side slideshow in slidemapper! The full code is available as a gist.

References

• https://github.com/cavis/slidemapper

Link to Another Slide in Leaflet Slidemapper

The slidemapper plugin for Leaflet is very useful, but when building large slideshows, it can be annoying that there is no easy way to link to specific slides. For example, you might want to create a table of contents or link from the last slide back to the first. Fortunately, there's a way to move to specific slides, instead of just advancing or moving back by 1. We'll use jQuery to do this. We already have jQuery loaded for slidemapper.

Specify Where Links Will Go

After the $mySlideMap code in your html file, you can create a list of links that you will later add to the slides. Generally, this will be the last thing in the <script> tag at the end of the body.

For example, in the sample code from the slidemapper example, the <script> initially contains:

    $mySlideMap = $('#slideshow-container')
    .slideMapper();

    $mySlideMap.slideMapper('add', EXAMPLEDATA);

At the end of the script, let's add a link to the first slide and a link to the last slide. The jQuery we use allows any code with a specific class to be transformed into a link. We can combine that with the slidemapper move method to create links to specific slides.

    $('.toc').click(function() {
      $mySlideMap.slideMapper('move', 0, true);
    });

    $('.end').click(function() {
      $mySlideMap.slideMapper('move', EXAMPLEDATA.length - 1, true);
    });

Add the Links

Next, we need to actually include the links in the slides. The links references will go in the data file. In the example, the file is named data.js.

Here's a link to the last slide from the first slide:

  // intro marker
  {
    icon: 'other.png',
    marker: [42.516846, -70.898499],
    center: [40.423, -98.7372],
    html: '<table style="margin:0 40px; padding:10px"><tr>' +
            '<td><img src="http://placehold.it/300x180&text=Map+Stuff"/></td>' +
            '<td style="padding-left:10px">' +
              '<h1>SlideMapper FTW!</h1>' +
              '<p>This is a demo of the different sorts of slides you can setup using slidemapper.</p>' +
              '<p><a class="end">Skip to the end.</a></p>' +
            '</td>' +
          '</tr></table>',
    popup: 'So it begins!'
  },

And here's a link from the last slide to the first slide:

  // empty slide
  {
    html: '<div style="margin:0 40px; padding:20px 10px">' +
            '<div>' +
              '<h2>The End</h2>' +
              '<p>Goodbye.</p>' +
              '<p><a class="toc">Return to Table of Contents.</a></p>' +
            '</div>' +
          '</div>'
  }

Prettifying

Because of the unusual way these links are made using jQuery, we should probably add some css to make them look like links. I added the following css to the <head>, but you can add any css to the head or as a separate stylesheet.

  <style type="text/css">
    a {
      color: orange;
      cursor: pointer;
      text-decoration: underline;
    }
  </style>

Here's an example of a link from one slide to another

And now you have an easy way to navigate through a long slidemapper deck. You could even create a table of contents at the beginning and then link back to that from each slide.

The full code is available in a gist.

References

• http://stackoverflow.com/questions/950087/how-to-include-a-javascript-file-in-another-javascript-file
• https://github.com/cavis/slidemapper#methods

Center Map on Layer Change in Leaflet

In Leaflet, it can be helpful to change the bounds of the map when the user adds or changes the visible map layers.

The Basics

First, we'll start with the initial code including our map and polygon layers:

  var circle = L.circle([51.508, -0.11], 500, {
      color: 'red',
      fillColor: '#f03',
      fillOpacity: 0.5
  });

  var polygon = L.polygon([
      [51.509, -0.08],
      [51.503, -0.06],
      [51.51, -0.047]
  ]);

  var map = L.map('map', {
    center: [51.505, -0.09],
    zoom: 13
  });

  var overlayMaps = {
    "Circle": circle,
    "Polygon": polygon
  };

  L.tileLayer('http://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png', {
    attribution: '&copy; <a href="http://openstreetmap.org">OpenStreetMap</a> contributors, <a href="http://creativecommons.org/licenses/by-sa/2.0/">CC-BY-SA</a>'
  }).addTo(map);

  L.control.layers(overlayMaps, null, {
    collapsed: false
  }).addTo(map);

So far, most of this should be familiar from the Leaflet Quick Start Guide.

Next, we want to add a listener function that will zoom and re-center upon a change in the circle or polygon layer.

The .on method of map allows you to watch for an event to occur and then execute a function when it does. In this case, we want to wait for the event 'baselayerchange'. This way the map will automatically zoom and recenter when the user changes layers.

  map.on('baselayerchange', function(e) {
    console.log(e);
    map.fitBounds(e.layer);
  });

The map automatically zooms to the bounds of the shape when the layer is activated.

Options

There are a few options for exactly how the map is zoomed and/or re-centered and for what type of layers are affected.

If you want to do this with overlay layers instead of base layers, you can substitute 'overlayadd' for 'baselayerchange'. Using overlay layers is more common for drawing shapes, but treating your layers as base layers makes it easy to display only one at a time.

  map.on('overlayadd', function(e) {
    console.log(e);
    map.fitBounds(e.layer);
  });

fitBounds automatically zooms to the tightest zoom level where the whole shape is visible. If you don't want to use fitBounds (say you're centering on a new overlay layer and don't want to zoom all the way in), you can use setView or panTo instead. panTo animates as the view changes.

  map.on('overlayadd', function(e) {
    console.log(e);
    map.panTo(e.layer);
  });

And there you have a few different ways to center and/or zoom in when an overlay layer is added or when the baselayer is changed. I'd recommend you check out the documentation for fitBounds, setView, and panTo and play around with the options. The options for L.control.layers are also helpful. For example, you can set collapsed to false to encourage users to change the layers.

The full code is available in a gist.

References

• http://leafletjs.com/reference.html#map-fitbounds
• http://leafletjs.com/reference.html#control-layers-baselayerchange
• http://leafletjs.com/reference.html#map-overlayadd

Merge by City and State in R

Often, you'll need to merge two data frames based on multiple variables. For this example, we'll use the common case of needing to merge by city and state.

First, you need to read in both your data sets:

# import city coordinate data:
coords <- read.csv("cities-coords.csv",
  header = TRUE,
  sep = ",")

# import population data:
data <- read.csv("cities-data.csv",
  header = TRUE,
  sep = ",")

Next comes the merge. You can use by.x and by.y to declare which variables the merge will be based on. If the variables have exactly the same name in both data sets, you can use by instead of by.x and by.y.

x and y represent the two data sets you are merging, in that order.

You also want to state whether you want to include all data from either data set, using all or all.x and all.y. In this case, we want to make sure we hold onto all our city data, even data for the cities we do not have coordinates for.

# merge data & coords by city & state:
dataCoords <- merge(coords, data, 
  by.x = c("City", "State"),
  by.y = c("city", "state"),
  all.x = FALSE,
  all.y = TRUE)

Running that code shows what we would expect. Houston is included in the final data set even though there are no coordinates for it, while Dallas is not included since it has coordinates but no data:

            City State Latitude  Longitude year population
1        Chicago    IL 41.85003  -87.65005 2012    2714856
2       Columbus    GA 32.46098  -84.98771 2012     198413
3       Columbus    OH 39.96118  -82.99879 2012     809798
4       Columbus    OH 39.96118  -82.99879 2010     787033
5    Los Angeles    CA 34.05223 -118.24368 2012    3857799
6       New York    NY 40.71427  -74.00597 2012    8336697
7       New York    NY 40.71427  -74.00597 2010    8175133
8  San Francisco    CA 37.77823 -122.44250 2012     825863
9  San Francisco    CA 37.77823 -122.44250 2010     805235
10       Houston    TX       NA         NA 2012    2160821

Bonus

If you'd like to get a list of which cases got merged in but lack coordinate data, there's a simple line of code to do that:

> dataCoords[!complete.cases(dataCoords[,c(3,4)]),]
      City State Latitude Longitude year population
10 Houston    TX       NA        NA 2012    2160821

Also, you might want to tidy up the names of your variables, if they followed different conventions in their respective initial data sets:

> names(dataCoords) <- c("City", "State", "Latitude", "Longitude", "Year", "Population")
> dataCoords
            City State Latitude  Longitude Year Population
1        Chicago    IL 41.85003  -87.65005 2012    2714856
2       Columbus    GA 32.46098  -84.98771 2012     198413
3       Columbus    OH 39.96118  -82.99879 2012     809798
4       Columbus    OH 39.96118  -82.99879 2010     787033
5    Los Angeles    CA 34.05223 -118.24368 2012    3857799
6       New York    NY 40.71427  -74.00597 2012    8336697
7       New York    NY 40.71427  -74.00597 2010    8175133
8  San Francisco    CA 37.77823 -122.44250 2012     825863
9  San Francisco    CA 37.77823 -122.44250 2010     805235
10       Houston    TX       NA         NA 2012    2160821

The full sample code is available as a gist.

References

• https://stat.ethz.ch/pipermail/r-help/2006-August/111120.html

gist

Change R code tabs to 2 spaces.

bold package names
italicize functions

http://hilite.me/
manni

for R posts, use: sessionInfo()

ggplot Fit Line and Lattice Fit Line in R

Let's add a fit line to a scatterplot!

Fit Line in Base Graphics

Here's how to do it in base graphics:

ols <- lm(Temp ~ Solar.R,
  data = airquality)

summary(ols)

plot(Temp ~ Solar.R,
  data = airquality)
abline(ols)

Fit line in base graphics in R

Fit Line in ggplot

And here's how to do it in ggplot:

library(ggplot2)
ggplot(data = airquality,
    aes(Solar.R, Temp)) + 
  geom_point(pch = 19) + 
  geom_abline(intercept = ols$coefficients[1],
    slope = ols$coefficients[2])

You can access the info from your regression results through ols$coefficients.

Edit: Thanks to an anonymous commenter, I have learned that you can simplify this by using geom_smooth.  This way you don't have to specify the intercept and slope of the fit line.

ggplot(data = airquality,
    aes(Solar.R, Temp)) + 
  geom_point(pch = 19) + 
  geom_smooth(method = lm,
    se = FALSE)

Fit line in ggplot in R

Fit Line in Lattice

In lattice, it's even easier. You don't even need to run a regression; you can just add to the type option.

library(lattice)

xyplot(Temp ~ Solar.R,
  data = airquality,
  type = c("p", "r"))

Fit Line in Lattice in R

The code is available in a gist.

References

• http://stackoverflow.com/questions/12972039/plotting-xyplot-with-regression-line-on-lattice-graphics

Compare Regression Results to a Specific Factor Level in R

Including a series of dummy variables in a regression in R is very simple. For example,

ols <- lm(weight ~ Time + Diet,
  data = ChickWeight)
summary(ols)

The above regression automatically includes a dummy variable for all but the first level of the factor of the Diet variable.

Call:
lm(formula = weight ~ Time + Diet, data = ChickWeight)

Residuals:
     Min       1Q   Median       3Q      Max 
-136.851  -17.151   -2.595   15.033  141.816 

Coefficients:
            Estimate Std. Error t value Pr(>|t|)    
(Intercept)  10.9244     3.3607   3.251  0.00122 ** 
Time          8.7505     0.2218  39.451  < 2e-16 ***
Diet2        16.1661     4.0858   3.957 8.56e-05 ***
Diet3        36.4994     4.0858   8.933  < 2e-16 ***
Diet4        30.2335     4.1075   7.361 6.39e-13 ***
---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

Residual standard error: 35.99 on 573 degrees of freedom
Multiple R-squared:  0.7453,  Adjusted R-squared:  0.7435 
F-statistic: 419.2 on 4 and 573 DF,  p-value: < 2.2e-16

This is great, and it's often what you want. But in this case, it's comparing each of the diets to Diet1. In some cases, you might want to compare to a specific diet that isn't the first in the factor list.

How can we choose which dummy to compare to? Fortunately, it's simple to compare to a specific dummy in R. We can just relevel the factor so the dummy we want to compare to is first.

ChickWeight$Diet <- relevel(ChickWeight$Diet,
  ref = 4)

olsRelevel <- lm(weight ~ Time + Diet,
  data = ChickWeight)
summary(olsRelevel)

The "ref" argument allows us to change the reference level of the factor variable. This means that when we perform regression analysis

You can use table or str to find the factor levels, if you don't already know them.

After releveling the factor variable, we can simply perform the same regression again, and this time it will compare the results to the new reference level:

olsRelevel <- lm(weight ~ Time + Diet,
  data = ChickWeight)
summary(olsRelevel)

Call:
lm(formula = weight ~ Time + Diet, data = ChickWeight)

Residuals:
     Min       1Q   Median       3Q      Max 
-136.851  -17.151   -2.595   15.033  141.816 

Coefficients:
            Estimate Std. Error t value Pr(>|t|)    
(Intercept)  41.1578     4.0828  10.081  < 2e-16 ***
Time          8.7505     0.2218  39.451  < 2e-16 ***
Diet1       -30.2335     4.1075  -7.361 6.39e-13 ***
Diet2       -14.0674     4.6665  -3.015  0.00269 ** 
Diet3         6.2660     4.6665   1.343  0.17989    
---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

Residual standard error: 35.99 on 573 degrees of freedom
Multiple R-squared:  0.7453,  Adjusted R-squared:  0.7435 
F-statistic: 419.2 on 4 and 573 DF,  p-value: < 2.2e-16

Now we can choose any factor level as the reference for the series of dummies in the regression analysis. The code is available in a gist.

Reference

• http://stackoverflow.com/a/3872213/2926101

LWIMW3: Trail Magic

I just finished my submission for Look What I Made Weekend 3. Look What I Made Weekend (LWIMW) is a chance for people to create something over the course of 48 hours. The concept is based on Ludum Dare and other game jams, but for LWIMW you don't have to make a game. Instead, you are free to pursue any creative endeavor and show off your results at the end.

NB: The content below is mostly a reprint of my submission at LWIMW.

My project is an interactive website that will be part of the companion site to a book my friend Scott Thigpen is writing.

I didn't quite start from scratch on this project. This image shows the progress I had made before the weekend. You can also view it on the web.

pre-LWIMW

The other images show the current status after the weekend.

I made a lot of progress this weekend. I added GPS routes to the map, added a table of contents, added marker clustering, improved the graphic design (CSS and basemap), and added about 25% of the final content.

post-LWIMW

post-LWIMW

The only essential things I have left to do are adding the rest of the content, working with my friend on palette and graphics, and tweaking some small things.

You can view the current state of the Trail Magic site here.

Append Layer to overlayMaps in Leaflet

What if we want to create a layer based on geolocation, but have the layer only be added to the map once geolocation occurs? I didn't find this example in any Leaflet tutorials, but it's pretty simple with some basic JavaScript

I'll start with some code that should look familiar from the Leaflet Quick Start Guide, but with each item as its own layer.

  var marker = L.marker([51.5, -0.09]);

  var circle = L.circle([51.508, -0.11], 500, {
      color: 'red',
      fillColor: '#f03',
      fillOpacity: 0.5
  });

  var polygon = L.polygon([
      [51.509, -0.08],
      [51.503, -0.06],
      [51.51, -0.047]
  ]);

  var map = L.map('map', {
    center: [51.505, -0.09],
    zoom: 13,
    layers: [marker, circle, polygon]
  });

  var overlayMaps = {
    "Marker": marker,
    "Circle": circle,
    "Polygon": polygon
  };
  L.tileLayer('http://tile.cloudmade.com/[API-KEY]/29889/256/{z}/{x}/{y}.png', {
    attribution: 'Map data &copy; <a href="http://openstreetmap.org">OpenStreetMap</a> contributors, <a href="http://creativecommons.org/licenses/by-sa/2.0/">CC-BY-SA</a>, Imagery © <a href="http://cloudmade.com">CloudMade</a>',
    maxZoom: 18
  }).addTo(map);

Next let's add the code that will display the three original layers if geolocation does not occur. In this case, we do not want the geolocated layer to show.

  function onLocationError(e) {
    alert(e.message);

    L.control.layers(null, overlayMaps).addTo(map);
  }

  map.on('locationerror', onLocationError);

You can see in the below screenshot that only the three layers are displayed.

With Geolocation Off

Finally, we need to add code that will display all the layers (including the geolocation layer) if geolocation does occur. We simply need to add the "yourLocation" layer to overlayMaps and then display the layers as normal.

  function onLocationFound(e) {
    yourLocation = L.marker(e.latlng);

    overlayMaps["You"] = yourLocation;

    L.control.layers(null, overlayMaps).addTo(map);
  }

  map.on('locationfound', onLocationFound);

The screenshots below show what happens once geolocation has occurred.

With Geolocation On

Showing Geolocation Layer

 The full code is available in a gist.

References

• http://stackoverflow.com/questions/695050/how-do-i-add-a-property-to-a-javascript-object-using-a-variable-as-the-name

Add and Remove Leaflet Circle on Click

Similar to the popup example in the Leaflet tutorial, you might want to allow a user to add a circle centered on the point they click.

After defining your map, you first need to declare the variable you'll be using:

  var clickCircle;

While popups automatically disappear on the next click, circles do not. The following code removes the former circle from the leaflet map before drawing the new circle:

  function onMapClick(e) {
    if (clickCircle != undefined) {
      map.removeLayer(clickCircle);
    };

Next, we want our function to draw the new circle (NB: "1609 * 3" is the radius of the circle. This makes the radius equal 3 miles):

    clickCircle = L.circle(e.latlng, 1609 * 3, {
      color: '#f07300',
      fillOpacity: 0,
      opacity: 0.5
    }).addTo(map);
  }

And finally, we need to add an event listener so that the function will be run when a user clicks on the map:

  map.on('click', onMapClick);

The full code is available in a gist.

References

• http://leafletjs.com/examples/quick-start.html
• http://stackoverflow.com/questions/15507737/adding-removing-l-control-from-leaflet-js-map
• http://stackoverflow.com/questions/11272552/remove-polygon-from-the-map

Embed Audio in Leaflet Pop-up

Here's how to embed audio in a leaflet map. This example will also show you how to embed most other HTML in a leaflet map, or how to embed audio in an HTML file.

In the html file:

  function onEachAudio(feature, layer) {
    layer.bindPopup(feature.properties.name + "<br>" + feature.properties.html);
  };

  new L.GeoJSON.AJAX("audio.geojson", {
    onEachFeature: onEachAudio,
    pointToLayer: function(feature, latlng) {
      return L.marker(latlng, {icon: audioIcon});
    }
  }).addTo(map);

And here's the geojson format where you'll include your audio HTML and the coordinates where you want to view each file:

{
  "type": "FeatureCollection",
  "features": [
  {
    "type": "Feature",
    "properties": {
      "name": "<a href='http://www.freesound.org/people/genghis%20attenborough/sounds/212798/'>Deep basement</a>",
      "html": "<p><audio width='300' height='32' src='http://www.freesound.org/data/previews/212/212798_205108-lq.mp3' controls='controls'><br />Your browser does not support the audio element.<br /></audio></p>"
    },
    "geometry": {
      "type": "Point",
      "coordinates": [-100,34]
    }
  },{
    "type": "Feature",
    "properties": {
      "name": "<a href='http://www.freesound.org/people/John%20Sipos/sounds/125696/'>Atlantis docks then lands.</a>",
      "html": "<p><audio width='300' height='32' src='http://www.freesound.org/data/previews/125/125696_593024-lq.mp3' controls='controls'><br />Your browser does not support the audio element.<br /></audio></p>"
    },
    "geometry": {
      "type": "Point",
      "coordinates": [-84,40]
    }
  }
  ]
}

Just substitute in the address of your audio files for the sample files above in the "html" property of the geojson features.

Example Leaflet Map with Audio

The full code is available in a gist.

Import JSON Data from an External File in Leaflet

If your JSON or GeoJSON data is long, you might want to store it in a separate file to make your code more readable or to reduce repetition and allow you to access the same data file from multiple pages. You can do that with jQuery.

First, load jQuery 1.10.2 or the newest version of jQuery (2.0 onward does not support Internet Explorer 6, 7, or 8) in the header:

<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js"></script>

Next, in your leaflet script at the bottom of the body, you'll need to add all your other code and your basemap, like normal, for example:

  var map = L.map('map').setView([38.0740, -55], 3);

  function onEachFeature(feature, layer) {
    layer.bindPopup(feature.properties.City + ", " + feature.properties.State + ", " + feature.properties.Country);
  }

Then, you can just use jQuery to get the JSON or GeoJSON file. A simple version of this would be:

  $.getJSON("cities.geoJSON", function (cities) {
    L.geoJson(cities).addTo(map);
  })

But you can also add more options to your L.geoJson code, just like you would if using data stored in the HTML file. For example:

  $.getJSON("cities.geoJSON", function (cities) {
    L.geoJson(cities, {
      onEachFeature: onEachFeature,
      pointToLayer: function (feature, latlng) {
        switch (feature.properties.Remember) {
          case '1': return L.marker(latlng, {icon: visitedIcon});
          case '?': return L.marker(latlng, {icon: uncertainIcon});
          case '0': return L.marker(latlng, {icon: uncertainIcon});
        }
      }
    }).addTo(map);
  })

So just a couple lines of jQuery allow you to store your JSON or GeoJSON data in a separate file.

The example code is available in a gist and the example is viewable here.

Further Reading

• http://lyzidiamond.com/posts/external-geojson-and-leaflet-the-other-way/