These are terrain elements such as roads, railroads, shorelines, streams and utilities.
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To add these features at your airport select the Vector Line Tool which is located as shown here.
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Draw your line with as many vertex points as required and complete it by double clicking. This will open the property dialog and you may select the type and specifics of your vector line. Click OK to save this.
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You may move or delete vector lines as required. Also you may add or remove vertex points in an existing vector line.