Structure of USGS DEMS

DEMs are stored as a series of north-south elevation profiles. The profiles progress from west to east, and within each profile the points start from the south and follow sequentially to the north. Thus the first data point is at the southwest corner of the data set, and the last point at the northeast point of the data set.

USGS created a 1:100,000 DEM, but it never became as popular or widespread as the 24K or 250K.  SDTS replaced the original ASCII formats but was never very popular despite mandates that it be used. The current USGS push is to use 3DEP/NED, which uses a variety of formats.

 USGS DEM 
Spacing 
Columns 
Rows 
File Size 
Profiles
 
1:24,000
UTM--10 m & 30 m 
varies 
varies 
variable 
clipped
This is no longer available, and unless you have very good reasons, do not use any old data you might find.
1:100,000 
Geographic--2 arc sec 
constant 
constant 
constant 
filled
Never very common.
1:250,000 
Geographic--3 arc sec 
constant 
constant 
constant 
filled
Original source was NGA-predecessor agency, and there is little good reason to use this data.  SRTM is much better.

N3DEP/ED

1", 1/3", and 1/9"

variable

variable

variable

filled

 
1 m 1 m       filled Derived from lidar, with increasing coverage.

See also:


Last revision 11/2/2017