skip navigation link

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.


blue line
Return to Report Introduction