Using QGIS (1) – Introduction

September 9th, 2007

We’ve briefly mentioned QGIS before but it is worth a more in-depth look now. Some of our upcoming projects will require downloading and using GIS data so QGIS is a great program for viewing the data. In their own words,

Quantum GIS (QGIS) is a user friendly Open Source Geographic Information System (GIS) that runs on Linux, Unix, Mac OSX, and Windows. QGIS supports vector, raster, and database formats. QGIS is licensed under the GNU General Public License. QGIS lets you browse and create map data on your computer. It supports many common spatial data formats (e.g. ESRI ShapeFile, geotiff).

Let’s quickly describe how to install QGIS, download data, and make a basic map. This post is primarily for those who have not used GIS software or data before.

QGIS Installation

Download the current release (v0.8.1) of QGIS here and run the installation program. They recommend installing it to a file path without spaces (e.g. C:\QGIS) to enable GRASS functionality which we may use later on.

Running QGIS opens a blank map:

QGIS Blank Map

Data Download

We now need to download some GIS data to add to our map. Probably the most common vector data format is a Shapefile and there are lots of places on the web to download them for free. In the US, many state governments provide Shapefiles for geographic information such as roads, county boundaries, lakes, etc. Do a search for your state name and GIS (e.g. “Michigan GIS”, “California GIS”) or browse www.gispilot.com for links.

Once you’ve found a Shapefile, download and unzip it to a location on your hard drive. You’ll immediately notice that the Shapefile is actually composed of several (3 or more) files. For example, we went to the Rhode Island GIS web site and downloaded the “Roads – All” Shapefile. After unzipping, there were 7 files named ROAD with different extensions – ROADS.shp, ROADS.shx, ROADS.dbf, etc. These files collectively are referred to as a “Shapefile”.

Mapping

Back in QGIS, go to Layer…Add a Vector Layer or just press the V key. Navigate to the location of the downloaded Shapefile and notice that only 1 file name appears. In our example, “ROADS.shp” is shown so double-click on it to add it to the map. The roads are drawn in the map and the name appears in the legend on the left-hand side:

QGIS Roads Map

Use the Map Navigation toolbar to explore the data:

QGIS Map Navigation Toolbar

Zoom in and use the Identify button to select a road feature.

QGIS Identify Tool

Upon selecting a feature, an identify window appears showing database information (“attributes”) about the road.

QGIS Identify Result

In our example, the road segment identified is named ATWOOD AVE and is located in PROVIDENCE county. Most of the other attributes are numbers so often additional research is needed to determine what they represent. Some GIS data will provide metadata documenting the attribute values. In the next post, we will use those attributes to classify the roads into groups and color them differently.

The next post will also introduce some more GIS data concepts that we will then incorporate in our custom mapping applications.

Continue to part 2