Points Affecting the Appearance of Objects at Different Distances

From Military/Info Publishing

An object appears nearer:	An Object appears more distant:
When looking over water, or a large ravine or depression	When looking over rolling country
When the sun is behind the observer	When the sun is in front of the observer
When the air is clear, especially after a rain	When the air is not clear, due to fog, smoke, rain, etc.
When the background is in contrast with the color of the object	When the background is similar in color to that of the object.
On hot days, esepecially when the ground is moist, an object will appear more distant if observed from a kneeling of sitting position. This is due to refraction and heat radiation. Sometimes this refraction renders the object invisible.

The Tendency is to underestimate:

When using field glasses.

When trees are leafless--as is winter.

When the trees or branches are silhouetted against a clear skyline or a contrasting background.

From: Notes on Training Field Artillery Details by Captain Robert M.Danford and Captain Onorio Moretti. New Haven: Yale University Press (1917); page 148-149

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