To create index contours in QGIS, you'll configure the symbology settings for your raster layer. Here's how:
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Open the Layer Properties: Right-click on your raster layer in the Layers panel and select "Properties."
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Navigate to Symbology: In the Layer Properties window, go to the "Symbology" tab.
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Set Render Type to Contours: In the "Render type" dropdown menu, select "Contours."
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Configure Contour Interval: Specify the desired interval between contour lines in the "Interval" box. This is the spacing between each standard contour line.
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Set Index Contour Interval: In the "Index contour interval" box, define the interval at which you want to display index contours. Index contours are generally bolder or otherwise visually distinct from regular contours, making them easier to read. A common practice is to set this to a multiple of the standard contour interval (e.g., 5 times the standard interval).
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Optional Settings:
- Contour Expression: You can define a specific expression if you want to only generate contours for a specific range of values.
- Offset: This can vertically shift the contour layer.
- Color: Adjust the color of the regular and index contours for better visibility.
- Line Thickness/Style: Customize the appearance (thickness, style) of both regular and index contour lines to differentiate them. Index contours are typically thicker than regular contours.
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Apply and OK: Click "Apply" to see the changes in the map canvas. Adjust the settings as needed until you achieve the desired result. Click "OK" to close the Layer Properties window.
Example:
Let's say you want to display contours at 10-meter intervals and highlight every 50 meters with an index contour:
- Interval: 10
- Index contour interval: 50
QGIS will then generate contour lines every 10 meters, and every 50 meters (50, 100, 150, etc.) a bolder, more prominent index contour will be displayed.
