C.L. Dass

This guide will teach you, step-by-step, how to join your data to a shapefile so that you can quickly start making interesting maps. We will be using free, open-source software and public domain files to make it easy for anyone to follow along. At the end, you will have a shapefile with data that you can then import into TileMill.

Before we begin you will need the following:

If you are relatively new to GIS or don’t yet have any shapefiles to work with, here are a couple of places to start:

A shapefile will come in the form of a group of several similarly named files with different extensions. These shapefiles will normally include some geographic attribute data such as names and id codes, but are mostly data-less. For the purposes of this guide we will be using a shapefile of the United States and joining it with population data.

In order to join the two, you must ensure that both your data and your shapefile share a field or column with common values, called a key. This is often a name or ID code. In your data table, these identifiers must be unique, meaning one row per name or ID. Joining works by adding fields from your data table to the shapefile’s attribute table based on matching values found in the key columns.

In this example, the key in our data table will be geoid. As you will soon see, there is a corresponding field in our shapefile. It would also be possible to use the name field here, but it is often risky to use place names due to the possibility of typos or spelling discrepancies.

Once your data is looking good you will want to save it as a .dbf file. This is the format that shapefile’s use to store data. Open your data in LibreOffice Calc and save it in .dbf format.

CSV format can also be used, but there is an extra step that must be done before joining. See the section at the bottom of this guide for more on this.

CSV files can be joined in the very same way. This is a commonly used format that can be read by many applications, including Quantum GIS. However, QGIS interprets all CSV columns as strings by default, which is a problem if you intend to use numbers to conditionally style your map.

As a solution you can create an accompanying .csvt file which allows you to specify column types for your CSV file. To create one open up a new document in any text editor.

The .csvt file contains only one line. Column types are in quotes and separated by commas. The order should be the same as the column order in your CSV.

So if your CSV columns looks like this: geoid,name,population where the geoid and name contain strings, and the population field contains numbers, the content of your .csvt would look like this: "String","String","Real"

Save the file with the same name as your CSV but with the .csvt extension and keep it in the same folder. That’s it. You do not ever have to load the .csvt file, it remains associated with your CSV in the background.

A network protocol is a set of established rules that dictate how to format, transmit and receive data so that computer network devices -- from servers and