top marker altitude/height above ground level

Altitude:
ALT, angular distance above the horizon (especially of a celestial object);
altitude or height is defined based on the context in which it is used (aviation, geometry, geographical survey, sport, and more). As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object;
http://en.wikipedia.org/wiki/Altitude
Height in space of an object or point relative to sea level or ground level.
http://www.aerospaceguide.net/glossary/index.html

Ground level:
the height of the ground on which something stands

"top marker" image examples:
http://www.flickr.com/photos/arif_pasha/4048512753/
http://www.panoramio.com/photo/3154125
http://www.flickr.com/photos/blizzard84/6624123527/
http://www.pauldebois.com/?cat=4
http://www.flickr.com/photos/brockeshawn/448998047/
http://www.alibaba.com/product-gs/599563079/spin_top_marker....


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Note added at 11 hrs (2013-05-03 09:19:33 GMT)
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Marker:
some conspicuous object used to distinguish or mark something; "the buoys were markers for the channel";
An object used to mark a location; Someone or something that marks;
http://en.wiktionary.org/wiki/marker


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Note added at 1 day22 hrs (2013-05-04 20:42:24 GMT)
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Top:
the highest or extreme point of something (usually a mountain or hill); The highest or uppermost part of anything