Changes in this quantity are associated with changes in radar reflectivity in a form of interferometry whose control variables are “persistent scatterers.” Measurements of this quantity standardized to the GTOPO30 (“G-topo-thirty”) system have since been improved by the ASTER instrument. Techniques like IfSAR (“IF-sar”) and InSAR (“IN-sar”) are used to obtain image representations of this quantity called DEMs (-5[1])(“D-E-M’s”). Temperature gradients relative to this quantity in dry and moist air parcels define the adiabatic lapse rate. High values of latitude and this quantity form the “tree line” of a habitat. (10[1])Constant (10[3])values of this quantity are represented by contour lines on topographic maps. For 10 points, what quantity is defined as zero at sea level? ■END■ (10[1])