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Let's Try For U & Me Have you ever wondered how maps are made? In this article, we will take our first step towards making maps from spatial data using Quantum Geographic Information System (QGIS). Q GIS is one of the most widely used open source desktop tools for map- making and basic GIS analysis. You can download it from http:// download.qgis.org. QGIS is useful for visualising and editing spatial data, and for querying its features. Features are the geographical objects in the layer, e.g., each individual airport is a feature in the airports layer. A feature will have attributes describing it, such as the geometry, name, category, etc. QGIS has a Map Composer, which allows you to add the essential map elements and get the map print- ready. QGIS comes with dozens of handy plug-ins, and a Python console, just in case developers want to interact with the data through the command interface. For this tutorial, we will be using version 1.7.3 with the Alaska dataset available at http:// download.osgeo.org/qgis/data/qgis_ sample_data.zip. Visualise shapefiles QGIS supports most common vector file formats—Shapefile, KML, GML, GPX, etc. The procedure to open the files is the same for all formats: 1. On the menu, click Add Vector Layer. Map-Making with QGIS Figure 1: Alaska and Airports shapefiles Figure 2: Layer styles Part—1 www.LinuxForU.com | LINUX FOR YOU | JUNE 2012 | 31

Map Making with QGIS

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Page 1: Map Making with QGIS

Let 's Try For U & Me

Have you ever wondered how maps are made? In this article, we will take our first step towards making maps from spatial data using Quantum Geographic Information System (QGIS).

QGIS is one of the most widely used open source desktop tools for map-

making and basic GIS analysis. You can download it from http://download.qgis.org. QGIS is useful for visualising and editing spatial data, and for querying its features. Features are the geographical objects in the layer, e.g., each individual airport is a feature in the airports layer. A feature will have attributes describing it, such as the geometry, name, category, etc.

QGIS has a Map Composer, which allows you to add the essential map elements and get the map print-ready. QGIS comes with dozens of handy plug-ins, and a Python console, just in case developers want to interact with the data through the command interface. For this tutorial, we will be using version 1.7.3 with the Alaska dataset available at http://download.osgeo.org/qgis/data/qgis_sample_data.zip.

Visualise shapefilesQGIS supports most common vector file formats—Shapefile, KML, GML, GPX, etc. The procedure to open the files is the same for all formats:1. On the menu, click Add Vector

Layer.

Map-Making with QGIS

Figure 1: Alaska and Airports shapefiles

Figure 2: Layer styles

Part—1

www.LinuxForU.com | LINUX For YoU | JUNE 2012 | 31

Page 2: Map Making with QGIS

For U & Me Let 's Try

The authors are researchers at the Center for Study of Science, Technology and Policy (CSTEP), Bengalore. They work in the fields of GIS and Agent-Based Simulation. At CSTEP they have built a web-based GIS framework to simulate and visualize disaster impact. They love working with open source tools and are active contributors to Openstreetmaps.

By: Sagar Arlekar & Niket Narang

Figure 3: Feature labels

Figure 4: Feature selection

2. Browse to the source (for our example, the qgis_sample_data/shapefiles directory).

3. Selectthefiles(inourcase,alaska.shp and airports.shp). Figure 1 shows the Map Canvas with the Alaska and Airports layers.

Layer stylesStylingallowsustosetfillcolours,patterns, borders, icons, etc, for the features in the layer:1. Right click the Airport layer in the

Layers panel and select Properties.2. Under the Style tab you can

change the symbol and colour, the size of the symbol, set transparency, and categorise the features (see Figure 2).

Layer labelsLabelsaretextidentifiersforfeaturesinthe layer. QGIS allows you to choose the attribute you want to show as the label, and set its style too (see Figure 3):1. Right click the Airport layer and

select Properties.2. Under the Labels tab, check mark

Display Labels.3. Select the Field Containing Label; this

is the text to be shown on the layer.4. Set the font size, colour, type, and

the text position with respect to the feature location.

Attributes tableAttributes describe the features in a layer. The attribute values can be edited and queried. To view attribute names and data, right click the Airport layer and choose Open Attribute Table.

To query features of your interest, addthetexttofilterthedataintheLook Fortextbox,andselectthedesiredfield.For instance: look for ‘Military’ in the field‘Cat’.ThenhitSearch. You will findthatallmilitaryairportsareselectedand highlighted in the Attribute Table, as in Figure 4, and on the map. For complex queries, try Advanced Search.

Selected features can be saved as anewshapefile:clickLayer > Save Selection as vector file. Select the

format as ESRI Shapefile and save.In Part II, we will learn how to

publish a map, plot custom data and perform geoprocessing.

32 | JUNE 2012 | LINUX For YoU | www.LinuxForU.com