¿How to handle polygon symbologies in Qgis?
The symbology of a layer is its visual appearance on the map. The basic strength of Geographic Information Systems (GIS) over other forms of spatial data representation is that, with GIS, you can get a dynamic visual representation of the data you are working with. You must learn to make good use of these tools, in order to generate efficient graphic information, understandable for the people who are going to access your work.
¡Let´s start!
First of all, you must take into account that the spatial information you use, in this case vector files (.shp) of polygon type, have a symbology with which you can experiment, depending on the type of information you want to disclose and the attributes or characteristics of the data you are going to process, so for this exercise you can work with the information you have, in this case, we will work with vector files.
1.¿How to categorize vector layers of type polygon?
As we already learned how to load shapefile layers in Qgis and edit their attribute table, be careful to work with a file with data containing information as ID, Name, Area, Perimeter, Length, etc, in order to better visualize the changes we will make in their symbolism Then load the layers of your interest.
In this case, we will work with a polygon type layer, which will be the representation of the land uses of Colombia.
This layer has attributes that will serve to “play” with its visual representation.
To access the Symbology tools, just double-click on the layer of interest and select the Symbology option.You can also visualize the window by right-clicking on the layer of interest/Properties/Symbology.
Next, a new window will be displayed in which you will first visualize the fill, color and design options of your layer. In this case, the crop layer is of polygon type, but something similar will be observed for point and line type layers.
At the top part, there are listed visualization options for the layer of interest, there you will find different ways to represent the information contained in the file, depending on what you need to represent graphically.
Within these options you will find the categorization or
used to represent the entities of a layer, using a symbol defined by you, as a user, whose appearance reflects the discrete values of a field or an expression. In the Value field you will be able to display a list in which the fields of the layer’s attribute table will appear, there you must select the field on the basis of which you want the symbolic categorization to be performed, choose the color representation or the format, then click on Classify, Apply and Accept.
- ¿How to add texts from layers?
In the previous step you learned how to categorize the graphical representation of a vector layer, based on its attributes, but what if you want the alphanumeric data to be displayed
on the map as well?, well this can be done in a simple way, by double clicking on the name of your layer and choosing the option Labels or right clicking on the layer/Properties/Labels.
Next, a new window will be displayed in which, similar to the previous one, at the top you can decide if you want to block the labels, not show them or add a label. As in our case we wish to add labels, in the Value field we must select on the basis of which attribute the data is required to be labeled (this again will depend on the information you have and the information you want to make known). At the bottom, the software allows you to select the type of font, style, size, color, shape, among other characteristics, which again will depend on you and your tutor’s indications. Finally, with everything set, select Apply and Accept.
Note: When you are going to manage symbologies in Qgis you must take into account to “play well” with colors, sizes, among other characteristics, to show the information in the indicated way, with the purpose of understanding what is in the map and what you want to show.