# Creating a map The topics that we will cover in this chapter include: 1. Creating a map 1. Define GeoJSON 1. Use a projection 1. Generate a `` using a projection and the GeoJSON data 1. Create a rectangular overlay based on coordinates entered into a form 1. Draw a rectangle on the map and display its map coordinates 1. Draw a polygon on the map and display its map coordinates In this section we'll generate `` elements from GeoJSON data that will draw a map of the world ## Define GeoJSON GeoJSON is just JSON data that has specific properties that are assigned specific data types. Here's an example: ```javascript { "type": "Feature", "geometry": { "type": "Point", "coordinates": [125.6, 10.1] }, "properties": { "name": "Dinagat Islands" } } ``` In this example, we have one `Feature` who's geometry is a `Point` with the coordinates `[125.6, 10.1]`. It has "Dinagat Islands" as its name. Each `Feature` follows this general structure: ```javascript { "type": STRING, "geometry": { "type": STRING, "coordinates": ARRAY }, "properties": OBJECT } ``` We can also have a `Feature Collection` which is many `Features` grouped together in a `features` array: ```javascript { "type": "FeatureCollection", "features": [ { "type": "Feature", "geometry": { "type": "Point", "coordinates": [102.0, 0.5] }, "properties": { "prop0": "value0" } }, { "type": "Feature", "geometry": { "type": "LineString", "coordinates": [ [102.0, 0.0], [103.0, 1.0], [104.0, 0.0], [105.0, 1.0] ] }, "properties": { "prop0": "value0", "prop1": 0.0 } }, { "type": "Feature", "geometry": { "type": "Polygon", "coordinates": [ [ [100.0, 0.0], [101.0, 0.0], [101.0, 1.0], [100.0, 1.0], [100.0, 0.0] ] ] }, "properties": { "prop0": "value0", "prop1": { "this": "that" } } } ] } ``` This basically follows the form: ```javascript { "type": "FeatureCollection", "features": ARRAY } ``` The `features` property is an array of `feature` objects which we've defined previously. ### Set up the HTML Let's set up a basic D3 page: ```html ``` The only thing different from the setup that we've used in previous chapters is this line: ```html ``` This just loads an external javascript file which sets our GeoJSON data to a variable. Here's what the beginning of it looks like: ```javascript var map_json = { type: "FeatureCollection", features: [ { type: "Feature", id: "AFG", properties: { name: "Afghanistan" }, geometry: { type: "Polygon", coordinates: [ //lots of coordinates ] } } // lots of other countries ] } ``` Note that the `map_json` variable is just a JavaScript object that adheres to the GeoJSON structure (it adds an `id` property which is optional). This is very important. If the object didn't adhere to the GeoJSON structure, D3 would not work as it should. In production, you would probably make an AJAX call to get this data, or at the very least, create your own geoJSON file similar to the one being hosted on rawgit.com. The setup above was created to make learning easier by decreasing the complexity associated with AJAX. ## Use a projection Now let's start our `app.js` file: ```javascript const width = 960; const height = 490; d3.select('svg') .attr('width', width) .attr('height', height); ``` At the bottom of `app.js` let's add: ```javascript const worldProjection = d3.geoEquirectangular(); ``` This generates a projection, which governs how we're going to display a round world on a flat screen. There's lots of different types of projections we can use: https://github.com/d3/d3-geo/blob/master/README.md#azimuthal-projections The line above tells D3 to create an equirectangular projection (https://github.com/d3/d3-geo/blob/master/README.md#geoEquirectangular) ## Generate a `` using a projection and the GeoJSON data Now that we have our projection, we're going to generate `` elements for each data element in the `map_json.features` array. Then we set the fill of each element to `#099`. Add this at the end of app.js: ```javascript d3.select('svg').selectAll('path') .data(map_json.features) .enter() .append('path') .attr('fill', '#099'); ``` Here's what it should look like at the moment if we open index.html in Chrome and view the elements tab in the developer tools: ![](https://i.imgur.com/ljSlk4s.png) We created the `path` elements, but they each need a `d` attribute which will determine how they're going to drawn (i.e. their shape). We want something like: ```javascript d3.selectAll('path').attr('d', (datum, index)=>{ //somehow use datum to generate the value for the 'd' attributes }); ``` Writing the kind of code described in the comment above would be very difficult. Luckily, D3 can generate that entire function for us. All we need to do is specify the projection that we created earlier. At the bottom of `app.js` add the following: ```javascript const dAttributeFunction = d3.geoPath() .projection(worldProjection); d3.selectAll('path').attr('d', dAttributeFunction); ``` `geoPath()` generates the function that we'll use for the `d` attribute, and `projection(worldProjection)` tells it to use the `worldProjection` var created earlier so that the `path` elements appear as an equirectangular projection like this (This is helpful because we can use different projections to view a round world on a flat screen in different ways): ![](https://i.imgur.com/hX7hOoB.png) ## Create a rectangular overlay based on coordinates entered into a form First, let's create a form for the user to enter lat/lng coordinates for the top/left and bottom/right portions of the box: ```html ``` - Create a submit handler for the form - Prevent its default behavior of submitting so we stay on the page - Remove any rectangles that may have previously been drawn ```js d3.select('form').on('submit', (event) => { //set up a submit handler on the form event.preventDefault(); // stop the form from submitting }) ``` At the bottom of the submit handler, retrieve what the user entered in the inputs for the top left coordinate: ```js const lat1 = d3.select('input:first-child').property('value'); //get first latitude input const lng1 = d3.select('input:nth-child(2)').property('value'); //get first longitude input ``` Now we'll use `worldProjection()` like our previous scales to convert a data point into pixel coordinates. It takes as a parameter, an array containing a latitude value and a longitude value (`[lat,lng]`). It returns an array containing x and y coordinates (`[x,y]`). Place the following at the bottom of your submit handler: ```js const location1 = worldProjection([lat1, lng1]); //convert these into pixel coordinates const x1 = location1[0]; //reassign for ease of reading const y1 = location1[1]; ``` Do the same for the second for bottom right coordinate inputs (keep in mind, in the html the `
` tag is the 3rd child of its parent, so the 3rd and 4th inputs are actually `nth-child(4)` and `nth-child(5)`: ```js const lat2 = d3.select('input:nth-child(4)').property('value'); //get second latitude input (the
is :nth-child(3) const lng2 = d3.select('input:nth-child(5)').property('value'); //get second longitude input const location2 = worldProjection([lat2, lng2]); //convert these into pixel coordinates const x2 = location2[0]; //reassign for eas of reading const y2 = location2[1]; ``` Lastly, create the rectangle by appending it to the SVG, filling it with black, assigning the top left x/y coordinates and computing the height/width using the `x2` and `y2` coordinates: ```js d3.select('svg').append('rect') // append a rectangle to the svg .attr('fill', '#000') //make it black .attr('x', x1) //assign top left x location .attr('y', y1) //assign top left y location .attr('width', x2-x1) //compute width from bottom right x coordinate .attr('height', y2-y1) //compute height from bottom right y coordinate ``` The finished submit handler: ```js d3.select('form').on('submit', (event) => { //set up a submit handler on the form event.preventDefault(); // stop the form from submitting d3.selectAll('rect').remove(); //remove any previously drawn rectangles const lat1 = d3.select('input:first-child').property('value'); //get first latitude input const lng1 = d3.select('input:nth-child(2)').property('value'); //get first longitude input const location1 = worldProjection([lat1, lng1]); //convert these into pixel coordinates const x1 = location1[0]; //reassign for ease of reading const y1 = location1[1]; const lat2 = d3.select('input:nth-child(4)').property('value'); //get second latitude input (the
is :nth-child(3) const lng2 = d3.select('input:nth-child(5)').property('value'); //get second longitude input const location2 = worldProjection([lat2, lng2]); //convert these into pixel coordinates const x2 = location2[0]; //reassign for eas of reading const y2 = location2[1]; d3.select('svg').append('rect') // append a rectangle to the svg .attr('fill', '#000') //make it black .attr('x', x1) //assign top left x location .attr('y', y1) //assign top left y location .attr('width', x2-x1) //compute width from bottom right x coordinate .attr('height', y2-y1) //compute height from bottom right y coordinate }); ``` ## Draw a rectangle on the map and display its map coordinates First create a table in our HTML. Once the user draws the rectangle on the map, we'll have the app fill the data in here appropriately: ```html

lat	lng

``` Now set up a `drag` behavior and attach it to the SVG: ```js const dragBehavior = d3.drag() //set up the behavior .on('start', (event) => { //create the start handler }) .on('drag', (event) => { //create the drag handler }) d3.select('svg').call(dragBehavior); //atach dragBehavior to the svg ``` Next, let's use `worldProjection.invert()` to take visual x/y coordinates and convert them into lat/lng coordinates. Has parameters, it takes an array containing x and y pixel coordinates `[x,y]`, and it returns and array containing lat and lang coordinates `[lat,lng]`. We'll use `worldProjection.invert([event.x, event.y])[0]` to get the latitude and `worldProjection.invert([event.x, event.y])[1]` to get the longitude. We'll then insert those values into the cells in `tr:nth-child(2)` (`nth-child(1)` is the header `tr`). At the bottom of the start handler, add the following: ```js d3.select('table tr:nth-child(2) td:first-child') //select the top left cell of the table. The table header is tr:nth-child(1) //use worldProject to convert the x/y pixel coordinates of the mouse click into lat/lng map coordinates //this takes as params [x,y] and returns [lat,lng] //insert the first element of the map coords array (lat) into the html of the table cell .html(worldProjection.invert([event.x, event.y])[0]) d3.select('table tr:nth-child(2) td:last-child') //select the top right cell of the table //insert the second element of the map coords array (lng) into the html of the table cell .html(worldProjection.invert([event.x, event.y])[1]) ``` Now we want to begin drawing the rectangle. We'll start with a rectangle of 0 height and 0 width, at the point the user clicked. Add the following at the bottom of the start hander: ```js d3.selectAll('rect').remove(); //remove any previously drawn rectangles d3.select('svg').append('rect') //draw a rectangle of 0 height/width at the point where the drag started .attr('x', event.x) .attr('y', event.y); ``` When the user drags on the SVG, we want to update the rectangle's height/width and add lat/lng coordinates for the bottom right of the square in the table. First, let's take care of the table. Add the following to the `drag` handler (this is almost identical to the `start` handler, except we use `tr:nth-child(3)`): ```js d3.select('table tr:nth-child(3) td:first-child') //select the bottom left cell of the table //insert the first element of the map coords array (lat) into the html of the table cell .html(worldProjection.invert([event.x, event.y])[0]) d3.select('table tr:nth-child(3) td:last-child') //select the bottom right cell of the table //insert the second element of the map coords array (lng) into the html of the table cell .html(worldProjection.invert([event.x, event.y])[1]) ``` To update the rectangle's width/height, we need to calculate it based on the current event's x/y position and the rectangle's starting x/y position. Please add the following at the bottom of the `drag` handler: ```js //computer width of the box based on current x value and current rect's x value d3.select('rect').attr('width', event.x-d3.select('rect').attr('x')) //computer height of the box based on current y value and current rect's y value d3.select('rect').attr('height', event.y-d3.select('rect').attr('y')) ``` The finished drag behavior: ```js const dragBehavior = d3.drag() //set up the behavior .on('start', (event) => { //create the start handler d3.select('table tr:nth-child(2) td:first-child') //select the top left cell of the table. The table header is tr:nth-child(1) //use worldProject to convert the x/y pixel coordinates of the mouse click into lat/lng map coordinates //this takes as params [x,y] and returns [lat,lng] //insert the first element of the map coords array (lat) into the html of the table cell .html(worldProjection.invert([event.x, event.y])[0]) d3.select('table tr:nth-child(2) td:last-child') //select the top right cell of the table //insert the second element of the map coords array (lng) into the html of the table cell .html(worldProjection.invert([event.x, event.y])[1]) d3.selectAll('rect').remove(); //remove any previously drawn rectangles d3.select('svg').append('rect') //draw a rectangle of 0 height/width at the point where the drag started .attr('x', event.x) .attr('y', event.y); }) .on('drag', (event) => { //create the drag handler d3.select('table tr:nth-child(3) td:first-child') //select the bottom left cell of the table //insert the first element of the map coords array (lat) into the html of the table cell .html(worldProjection.invert([event.x, event.y])[0]) d3.select('table tr:nth-child(3) td:last-child') //select the bottom right cell of the table //insert the second element of the map coords array (lng) into the html of the table cell .html(worldProjection.invert([event.x, event.y])[1]) //computer width of the box based on current x value and current rect's x value d3.select('rect').attr('width', event.x-d3.select('rect').attr('x')) //computer height of the box based on current y value and current rect's y value d3.select('rect').attr('height', event.y-d3.select('rect').attr('y')) }) d3.select('svg').call(dragBehavior); //atach dragBehavior to the svg ``` ## Draw a polygon on the map and display its map coordinates ```html

lat	lng

``` ```js const width = 960; const height = 490; d3.select('svg') .attr('width', width) .attr('height', height); const worldProjection = d3.geoEquirectangular(); const dAttributeFunction = d3.geoPath() .projection(worldProjection); d3.select('svg').selectAll('path') .data(map_json.features) .enter() .append('path') .attr('fill', '#099') .attr('d', dAttributeFunction); let points = []; d3.select('svg').on('click', (event)=>{ points.push([event.x, event.y]) const path = d3.path(); path.moveTo(points[0][0], points[0][1]) for(let i = 1; i < points.length; i++){ path.lineTo(points[i][0], points[i][1]) } path.closePath() d3.select('#polygon').remove() d3.select('svg') .append('path') .attr('id', 'polygon') .attr('fill', 'black') .attr('d', path) }) d3.select('body').on('keydown', (event)=>{ d3.select('tbody').html(''); const coords = points.map(point => worldProjection.invert(point)) for(coord of coords){ const row = d3.select('tbody') .append('tr') row.append('td') .html(coord[0]) row.append('td') .html(coord[1]) } points = []; }) ``` ## Conclusion In this section we've covered how to use D3 to create a projection and render GeoJSON data as a map, and we've learned about using different projects to visualize the world. This can be helpful when displaying populations or perhaps average rainfall of various regions. Congratulations! You've made it to the end of this book. Now go off and create amazing visualizations.