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An essential qualification for rapid and accurate sketcher is the ability to estimate distances with less than 10 per cent error up to 600 yards, and within 15 per cent error up to a mile. This can be acquired by constant practice in making estimates of various distances and then verifying the estimate by accurate measurement. Estimates of distance should be make in yards, and 100 yards should be definitely fixed in mind as a reference unit. In all estimation of distance the sketcher should bear in mind the effect of conditions of ground and light on estimates.
| Objects appear nearer than they really are: |
| When the sun is behind the observer and the object is in the bright light. |
| When seen over a body of water, snow, or level plain. |
| When below the observer. |
| When in high altitudes and very clear atmosphere. |
| In the above, cases add to the normal estimate. |
| Objects seem farther away than they really are: |
| When up a steep hill from the observer. |
| In poor light such as fog. |
| When seen across undulating ground. |
| In the above cases subtract from the normal estimate |
| Objects are distinguishable to average eyes at the following distances: |
| 9 to 12 miles, church spires |
| 5 to 7 miles, windmills |
| 2 to 2-1/2 miles, chimneys |
| 2,000 yards, trunks of large trees |
| 600 yards, individuals of a column |
| 500 yards, individual panes of glass in windows |
| 400 yards, arms and legs of dismounted men |
But visual acuity differs and the sketcher should learn for himself at what distances objects can be seen by him and their appearance at different ranges, by noting objects on the ground and scaling their distances from a good map.
Telegraph and telephone poles are usually set at fixed distances along any one line, so that the sketcher, by pacing one interval or by dividing a known distance by the number of poles contained, may secure their distance apart and make accurate measurements as far as the poles are visible.
From: United States Army. Field Service Pocket Book (1917); page 98-99
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