Mastering Contour Lines and Labels in QGIS: A Step-by-Step Guide
What is a contour line and why is it important?
A contour line is a line on a map that connects points of equal elevation, indicating the shape and relief of the terrain. Contour lines are commonly used in GIS (Geographic Information System) applications to represent elevation data on a map.
Contour lines provide important information about the topography of an area, allowing users to understand the shape, size, and elevation of landforms such as hills, mountains, valleys, and plateaus. They also provide a visual representation of the slope and steepness of the terrain, which can be useful for a variety of applications such as land use planning, environmental modeling, and geologic mapping.
In QGIS, contour lines can be generated from digital elevation models (DEMs) or other elevation data sources. By creating contour lines, users can more easily visualize and analyze elevation data, and make informed decisions based on this information. Overall, contour lines are an important tool for understanding the shape and structure of the earth’s surface, and are an essential component of many GIS applications.
Step-by-step instructions on how to use QGIS to create contour lines and labels
- Open QGIS and add the elevation data you want to use to create the contour lines.
- Once the data is loaded, go to the “Raster” menu and select “Extraction” and then “Contour”.
- In the dialog box that appears, select the input layer as your elevation data, choose the contour interval you want (e.g., 20), and select the output format as “ESRI Shapefile”. You can also specify a file name and location to save the output shapefile.
- Click “Run” and wait for the contour lines to be created. Once the process is complete, the new shapefile will be added to the Layers panel.
- To label, the contour lines, right-click on the contour layer in the Layers panel and select “Properties”.
- In the “Layer Properties” dialog box, go to the “Labels” tab and check the box next to “Label this layer”.
- In the “Label with” dropdown menu, select “Elevation” to label the contour lines with their elevation values.
- You can adjust the font size, color, and placement of the labels in the “Text style” section. You may also want to enable a buffer around the label to make it easier to read.
- Click “Apply” and then “OK” to save the changes and display the labeled contour lines on the map.
- If you want to create index contours (i.e., thicker lines with labels at specific elevations), you can use the field calculator command. Open the attribute table of the contour layer and create a new column called “Index” using the expression:
if("elevation" % 100 = 0, 1, NULL)
This will create a new column with a value of 1 for every contour line that is a multiple of 100, and a value of NULL for all other lines.
- To style the index contours differently, right-click on the contour layer in the Layers panel and select “Properties”. Go to the “Symbology” tab and choose “Categorized” as the symbol type.
- In the “Value” column, select the “Index” field you just created. Then, select a thicker line symbol and a different color for the index contours.
- To label only the index contours, go to the “Labels” tab in the Layer Properties dialog box and select the “Label this layer” checkbox.
- In the “Label with” dropdown menu, select the “Index” field. Adjust the font size and placement of the labels as desired.
- Click “Apply” and then “OK” to save the changes and display the labeled index contours on the map.
That’s it! These steps should guide you through the process of creating contour lines and labels in QGIS.
Choosing the right contour interval for your data
Choosing the right contour interval depends on the scale of your map, the level of detail you want to display, and the range of elevations in your data. Here are some general guidelines to help you choose the right contour interval for your data:
- Consider the scale of your map: The larger the scale of your map, the smaller the contour interval should be. For example, if you are creating a map of a small area such as a park, you may want to use a contour interval of 5 feet or less. However, if you are creating a map of a large region such as a state, you may want to use a contour interval of 50 feet or more.
- Consider the level of detail you want to display: If you want to show a lot of detail in your map, you may want to use a smaller contour interval. However, if you only need to show general elevation patterns, you may be able to use a larger contour interval.
- Use standard intervals: Some standard contour intervals that are commonly used include 10, 20, 50, and 100 feet. Using a standard interval can make it easier for viewers to read and interpret the map.
Ultimately, the contour interval you choose will depend on your specific data and map requirements. It may take some trial and error to find the right interval that displays the necessary information while still being clear and easy to read.
Exporting the contour lines and labels to a file or image format in QGIS. Here are the steps to follow:
- Ensure that your contour lines and labels are displayed as you want them to appear in the final output.
- Go to “Project” > “Import/Export” > “Export Map to Image” or “Export Map to PDF”. This will open the “Export as Image” or “Export as PDF” dialog box.
- In the “Export as Image” or “Export as PDF” dialog box, choose the file format you want to use, and set the desired resolution and output size.
- Under the “Items to Export” section, make sure that “Map Canvas” is selected, and that the “Draw annotations, graphic texts, and pictures” option is checked.
- Click “Save” to export the contour lines and labels to the selected file format.
Note that if you want to export only the contour lines and labels without the map background, you can use the “Export” button in the “Layer Styling” panel for the contour lines layer. This will export only the contour lines and labels as a vector file in the format of your choice.