2. Lesson: Symbology — The Free Quantum GIS Training Manual 1.0 documentation

The symbology of a layer is its visual appearance on the map. The basic strength of GIS over other ways of representing data with spatial aspects is that with GIS, you have a dynamic visual representation of the data you’re working with. Therefore, the visual appearance of the map (which depends on the symbology of the individual layers) is very important. The end user of the maps you produce will need to be able to easily see what the map represents. Equally as important, you need to be able to explore the data as you’re working with it, and good symbology helps a lot. In other words, having proper symbology is not a luxury or just nice to have. In fact, it’s essential for you to use a GIS properly and produce maps and information that people will be able to use. To change a layer’s symbology, open its Layer Properties. Let’s begin by changing the color of the urban layer. This is good stuff so far, but there’s more to a layer’s symbology than just its color. Next we want to change the color of the farms (the rural layer), but we also want to eliminate the lines between the different farms so as to make the map less visually cluttered. Under the Style tab, you will see the same kind of dialog as before. This time, however, you’re doing more than just quickly changing the color. Sometimes you will find that a layer is not suitable for a given scale. For example, a dataset of all the continents may have low detail, and not be very accurate at street level. When that happens, you want to be able to hide the dataset at inappropriate scales. Test the effects of this by zooming in and out in your map, noting when the streets layer disappears and reappears. You can use your mouse wheel to zoom in increments. Alternatively, use the zoom tools to zoom to a window: Now that you know how to change simple symbology for layers, the next step is to create more complex symbology. QGIS allows you to do this using symbol layers. Now there’s a second symbol layer. Being a solid color, it will of course completely hide the previous kind of symbol. Plus, it has a Solid Line border style, which we don’t want. Clearly this symbol has to be changed. When symbol layers are rendered, they are also rendered in a sequence, similar to how the different map layers are rendered. This means that in some cases, having many symbol layers in one symbol can cause unexpected results. To prevent this from happening, you can enable symbol levels, which will control the order in which the different symbol layers are rendered. When you’re done, remember to save the symbol itself so as not to lose your work if you change the symbol again in the future. You can save your current symbol style by clicking the Save Style ... button under the Style tab of the Layer Properties dialog. Save your style under exercise_data/styles. You can load a previously saved style at any time by clicking the Load Style ... button. Before you change a style, keep in mind that any unsaved style you are replacing will be lost. The roads must be dark gray or black, with a thin yellow outline, and a dashed white line running in the middle to make them resemble a real road. Symbol levels also work for classified layers (i.e., layers having multiple symbols). Since we haven’t covered classification yet, you will work with some rudimentary preclassified data. In addition to setting fill colors and using predefined patterns, you can use different symbol layer types entirely. The only type we’ve been using up to now was the Simple Fill type. The more advanced symbol layer types allow you to customize your symbols even further. Each type of vector (point, line and polygon) has its own set of symbol layer types. First we will look at the types available for points. Once you have applied the style, take a look at its results on the map. As you can see, these symbols change direction along with the road but don’t always bend along with it. This is useful for some purposes, but not for others. If you prefer, you can change the symbol layer in question back to the way it was before. As a result, you have a textured symbol for the urban layer, with the added benefit that you can change the size, shape and distance of the individual dots that make up the texture. Changing the symbology for the different layers has transformed a collection of vector files into a legible map. Not only can you see what’s happening, it’s even nice to look at! Changing symbols for whole layers is useful, but the information contained within each layer is not yet available to someone reading these maps. What are the streets called? Which administrative regions do certain areas belong to? What are the relative surface areas of the farms? All of this information is still hidden. The next lesson will explain how to represent this data on your map. Source.


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Last Modified: April 24, 2016 @ 1:10 am