Chapter 13 Transform Your Map Data

In Chapter 7: Map Your Data, we introduced basic concepts about interactive web maps, which are made up of different data layers. When users explore an interactive map, they usually click on the upper layer, which often displays some combination of points, polylines, and polygons, on top of a seamless set of basemap tiles that are built from raster or vector data. Whether you create maps with drag-and-drop tools such Datawrapper or customize Leaflet map code templates, you may need to transform data to work with one of these types of map layers.

In this chapter, we will delve further into the topic of geospatial data and its different formats, such as GeoJSON, the open-standard format most commonly used in this book. You’ll learn how to find and extract geographic boundary files in this format from the crowd-sourced OpenStreetMap platform. Also, we’ll show how to convert or create your own top-level map layer data using the GeoJson.io tool, and how to edit these layers with spreadsheet data using the Mapshaper tool. Moreover, you’ll also learn how to georeference a high-quality static map image and transform it into interactive map tiles using the the Map Warper tool. All of these free, web-based geodata tools are easy to learn, and in many cases they replace the need for more costly or complex geographic information systems, such as the proprietary ArcGIS and the open-source QGIS desktop applications. Finally, we’ll conclude with strategies to bulk geocode large batches of address data, and to pivot points into polygon data, which enables you to display this information in choropleth maps. By the end of this chapter, you should feel much more confident in navigating the somewhat-overwhelming world of geospatial data.

Let’s start with a general overview of geospatial data, and introduce you to various file formats to ensure you are ready to create, use, and share map data.