GIS software offer many options in regards with the symbology of an object.
Here there are instructions about how to define some main contour lines (every 10 meters) by displaying them with thicker line width. This symbology will help to distinguish them from the other contours.
1. Right click on the .shp file of the contour lines.
2. Click Properties.
3. From the menu of the Properties window, click Symbology.
4. Click on Categories > Unique values.
5. Change the Value Field to Elevation.
6. Click on Add Values.
7. On the Add Values window, select the main elevations (in this example, every 10m) and click OK.
8. Select the new contour values that were just added in the Symbology window, then right click and select Group Values.
9. Select the symbol you want for these main contours as well as the other ones that are grouped and labeled as <all other values>.
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