Elevation: Definitions: Elevation - height above some arbitrary datum; on USGS maps this datum is mean sea level (
https://www.powershow.com/view1/19e336-ZDc1Z/Elevation_Definitions_Elevation_-_height_above_some_arbitrary_datum_on_USGS_maps_this_datum_is_mean_sea_level_powerpoint_ppt_presentation
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