The matn says that if no object can be planted or set upright, a line may be drawn on the ground: lengthwise in front; another reported form is crosswise like a crescent.
The course presents this because it is in the source. It does not claim that drawing a line has the same physical visibility as an actual post when a suitable Sutrah is available.
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The Sutrah and Passing in Front of a Worshipper
The matn says that if no object can be planted or set upright, a line may be drawn on the ground: lengthwise in front; another reported form is crosswise like a crescent.
The course presents this because it is in the source. It does not claim that drawing a line has the same physical visibility as an actual post when a suitable Sutrah is available.