6.A. Grid Spacing and Resolution
For any application, the horizontal grid spacing (which limits the resolution) and the vertical accuracy of GLOBE must be considered. The 30 arc-second grid spacing equates to about 1 kilometer, although that number decreases in the East/West (longitudinal) direction as latitude increases. The table below lists the approximate distance covered by 30 arc-seconds at different latitudes. Thus, at high latitudes there is an unavoidable redundancy of data in order to keep the 30 arc-second spacing consistent for the global data set. This is particularly true for the geographic version of Antarctica where the ground distance for 30 arc-seconds of longitude converges to zero at the South Pole.
Latitude Ground distance (meters) (degrees) E/W N/S ----------------------------------------- Equator 928 921 10 914 922 20 872 923 30 804 924 40 712 925 50 598 927 60 465 929 70 318 930 74 256 930 78 193 930 82 133 931 86 64 931 89 16 931 90 0 931
Resolution is defined as the minimum distance between two objects that can be separated in the image. Many people mistakenly equate "resolution" to "pixel" or grid cell size, when resolution is actually approximately 2.83 times grid cell size. Thus, the numbers above should be multiplied by 2.83 to get an estimate of horizontal resolution.
Users should maintain this distinction between grid spacing and resolution. Even though the global data set has a consistent 30 arc-second grid spacing, not all topographic features will be resolved at that spacing. The level of detail of the source data determines whether the 30 arc-second sampling interval is truly appropriate for resolving the important topographic features represented in the source.
The variation in ground dimensions for one 30 arc-second cell should be especially considered for any application that measures area of, or distance across, a group of cells. Derivative products, such as slope maps, drainage basin areas, and stream channel length, will be more reliable if they are calculated from a DEM that has been first projected from geographic coordinates to an equal area projection, so that each cell, regardless of latitude, represents the same ground dimensions and area as every other cell.
Certainly, a 30 arc-second grid spacing is appropriate for the areas derived from higher resolution DEMs (DTED, Japan-GSI, Italy-SGN, and the New Zealand DEM), and 30 arc-seconds has been shown to be suitable as the cell spacing for grids derived from DCW hypsography (Hutchinson, 1996; Shih and Chiu, 1996). However, coverage of DCW contours is not complete, and there are areas for which elevations were interpolated based only on very sparse DCW point data and/or distant contours.
Small areas of this nature are located in Africa, South America, and islands of southeast Asia, while Australia (the G.O.O.D. version from DCW) contains larger such areas. Australia (the B.A.D. version from AUSLIG sources) also has variable source point distribution, though distribution tends to be higher in areas of higher relief, tending to lead to higher horizontal resolution where needed. The quality of the contours from the Antarctic Digital Database for the interior of Antarctica does not realistically support a 30 arc-second (or even 1-kilometer) grid spacing, although such data are provided for completeness and consistency of the global product.