Cumulative elevation gain
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In running, cycling, and mountaineering, cumulative elevation gain refers to the sum of every gain in elevation throughout an entire trip. It is sometimes also known as cumulative gain or elevation gain, or often in the context of mountain travel, simply gain. Elevation losses are not counted in this measure. Cumulative elevation gain, along with round-trip distance, is arguably the most important value used in quantifying the strenuousness of a trip. This is because hiking 10 miles (16 km) on flat land (zero elevation gain) is significantly easier than hiking up a large mountain with a round-trip distance of 10 miles (16 km). It is much harder to ascend vertically, or to increase elevation, than to walk on flat land because doing so also requires that the hiker increase his/her gravitational potential energy.
In the simplest case of a trip where hikers only travel up on their way to a single summit, the cumulative elevation gain is simply given by the difference in the summit elevation and the starting elevation. For example, if one were to start hiking at a trailhead with elevation 1,000 feet (300 m), and hike up to a summit of 5,000 feet (1,500 m), the cumulative elevation gain would just be 5000 ft - 1000 ft = 4000 ft. The loss of elevation on the descent is not relevant, because only increases in elevation are considered in this measure.
However, when climbing a mountain with some "ups-and-downs", or traversing several mountains, you must take into account every "up" along the whole route. This even means that the (usually small) uphills on the descent must be counted. For example, consider a mountain whose summit was 5,000 feet (1,500 m) in elevation, but somewhere on the way up, the trail went back down 250 feet (76 m). If starting at an elevation of 1,000 feet (300 m), one would gain 4,250 feet (1,300 m) on the way up (not 4000, because 250 is lost and has to be "regained"). The 250 feet on the way down is not counted.
If one were to hike over five hills of 100 vertical feet each, the cumulative elevation gain would be 5 x (100 ft) = 500 ft. Only the uphill sections are counted, not the downhills.
This concept makes travel on mountains which have more "ups-and-downs", or are generally more rugged, significantly more strenuous.