Grids--Geographic versus Projected

The grid for a DEM can be created in two ways: a geographic latitude-longitude grid, or a rectangular Universal Transverse Mercator (UTM) grid.  This are sometimes called unprojected (geographic) and projected.  This is different from the Horizontal datum; every data set also has a datum.

  Characteristics Advantages Disadvantages Typical DEMS
Geographic Grids Data sets a uniform size for specified quadrangle size

True north to the top of the map.

Current choice for both NGA and USGS.

No problems merging data sets in any direction. 

Not tied to any particular map projection, but a map with uniform lat/long spacing is generally a very poor choice.

Data spacing changes significantly from east to west, and insignificantly from north the south, within a data set.  MICRODEM can compute these differences for you.

Different spacing north-south and east-west.

Must be projected to get a reasonable map (done automatically, on-the-fly in MICRODEM)

Complicated distance and angle computations

NGA DTED

SRTM

USGS NED

Google Earth

UTM Grids

(SPCS similar)
Data sets vary in size in both the number of rows and columns size for specified quadrangle size. UTM grid north to the top of the map

Cartesian coordinate

Easy to compute distance, angles

Constant data spacing, both east-west and north-south and throughout a data set.

Merging problems at the edges of UTM zones every 6 or longitude.

If data sets are clipped to quad map sheets defined by geographic coordinates, the rectangular grid will have missing values.

Tied to projection

USGS Native and SDTS DEMs

A few DEMs uses other grids, such as the polar stereographic used in IBCAO or the Mercator grid used by Smith and Sandwell's (1997) Global Predicted Topography. MICRODEM handles the two types of DEM grid transparently to the user. The only obvious difference would be the stairsteps in the UTM grid on the geographic data sets like DTED.


Last revision 7/1/2015