<s>
A	O
digital	B-Application
elevation	I-Application
model	I-Application
(	O
DEM	O
)	O
or	O
digital	B-Application
surface	I-Application
model	I-Application
(	O
DSM	O
)	O
is	O
a	O
3D	O
computer	O
graphics	O
representation	O
of	O
elevation	O
data	O
to	O
represent	O
terrain	O
or	O
overlaying	O
objects	O
,	O
commonly	O
of	O
a	O
planet	O
,	O
moon	O
,	O
or	O
asteroid	O
.	O
</s>
<s>
DEMs	O
are	O
used	O
often	O
in	O
geographic	B-Application
information	I-Application
systems	I-Application
(	O
GIS	B-Application
)	O
,	O
and	O
are	O
the	O
most	O
common	O
basis	O
for	O
digitally	O
produced	O
relief	O
maps	O
.	O
</s>
<s>
A	O
digital	B-Application
terrain	I-Application
model	I-Application
(	O
DTM	O
)	O
represents	O
specifically	O
the	O
ground	O
surface	O
while	O
DEM	O
and	O
DSM	O
may	O
represent	O
tree	O
top	O
canopy	O
or	O
building	O
roofs	O
.	O
</s>
<s>
While	O
a	O
DSM	O
may	O
be	O
useful	O
for	O
landscape	O
modeling	O
,	O
city	O
modeling	O
and	O
visualization	B-Application
applications	O
,	O
a	O
DTM	O
is	O
often	O
required	O
for	O
flood	O
or	O
drainage	O
modeling	O
,	O
land-use	O
studies	O
,	O
geological	O
applications	O
,	O
and	O
other	O
applications	O
,	O
and	O
in	O
planetary	O
science	O
.	O
</s>
<s>
There	O
is	O
no	O
universal	O
usage	O
of	O
the	O
terms	O
digital	B-Application
elevation	I-Application
model	I-Application
(	O
DEM	O
)	O
,	O
digital	B-Application
terrain	I-Application
model	I-Application
(	O
DTM	O
)	O
and	O
digital	B-Application
surface	I-Application
model	I-Application
(	O
DSM	O
)	O
in	O
scientific	O
literature	O
.	O
</s>
<s>
In	O
most	O
cases	O
the	O
term	O
digital	B-Application
surface	I-Application
model	I-Application
represents	O
the	O
earth	O
's	O
surface	O
and	O
includes	O
all	O
objects	O
on	O
it	O
.	O
</s>
<s>
In	O
contrast	O
to	O
a	O
DSM	O
,	O
the	O
digital	B-Application
terrain	I-Application
model	I-Application
(	O
DTM	O
)	O
represents	O
the	O
bare	O
ground	O
surface	O
without	O
any	O
objects	O
like	O
plants	O
and	O
buildings	O
(	O
see	O
the	O
figure	O
on	O
the	O
right	O
)	O
.	O
</s>
<s>
define	O
the	O
DEM	O
as	O
a	O
subset	O
of	O
the	O
DTM	O
,	O
which	O
also	O
represents	O
other	O
morphological	O
elements	O
,	O
or	O
define	O
a	O
DEM	O
as	O
a	O
rectangular	O
grid	O
and	O
a	O
DTM	O
as	O
a	O
three-dimensional	O
model	O
(	O
TIN	B-Algorithm
)	O
.	O
</s>
<s>
Some	O
datasets	O
such	O
as	O
SRTM	B-Algorithm
or	O
the	O
ASTER	O
GDEM	O
are	O
originally	O
DSMs	O
,	O
although	O
in	O
forested	O
areas	O
,	O
SRTM	B-Algorithm
reaches	O
into	O
the	O
tree	O
canopy	O
giving	O
readings	O
somewhere	O
between	O
a	O
DSM	O
and	O
a	O
DTM	O
)	O
.	O
</s>
<s>
A	O
DEM	O
can	O
be	O
represented	O
as	O
a	O
raster	B-Algorithm
(	O
a	O
grid	O
of	O
squares	O
,	O
also	O
known	O
as	O
a	O
heightmap	B-Data_Structure
when	O
representing	O
elevation	O
)	O
or	O
as	O
a	O
vector-based	O
triangular	B-Algorithm
irregular	I-Algorithm
network	I-Algorithm
(	O
TIN	B-Algorithm
)	O
.	O
</s>
<s>
The	O
TIN	B-Algorithm
DEM	O
dataset	O
is	O
also	O
referred	O
to	O
as	O
a	O
primary	O
(	O
measured	O
)	O
DEM	O
,	O
whereas	O
the	O
Raster	B-Algorithm
DEM	O
is	O
referred	O
to	O
as	O
a	O
secondary	O
(	O
computed	O
)	O
DEM	O
.	O
</s>
<s>
The	O
DEM	O
could	O
be	O
acquired	O
through	O
techniques	O
such	O
as	O
photogrammetry	B-Application
,	O
lidar	O
,	O
IfSAR	O
or	O
InSAR	O
,	O
land	O
surveying	O
,	O
etc	O
.	O
</s>
<s>
DEMs	O
are	O
commonly	O
built	O
using	O
data	O
collected	O
using	O
remote	B-Application
sensing	I-Application
techniques	O
,	O
but	O
they	O
may	O
also	O
be	O
built	O
from	O
land	O
surveying	O
.	O
</s>
<s>
The	O
digital	B-Application
elevation	I-Application
model	I-Application
itself	O
consists	O
of	O
a	O
matrix	O
of	O
numbers	O
,	O
but	O
the	O
data	O
from	O
a	O
DEM	O
is	O
often	O
rendered	O
in	O
visual	O
form	O
to	O
make	O
it	O
understandable	O
to	O
humans	O
.	O
</s>
<s>
This	O
visualization	B-Application
may	O
be	O
in	O
the	O
form	O
of	O
a	O
contoured	O
topographic	O
map	O
,	O
or	O
could	O
use	O
shading	O
and	O
false	B-Application
color	I-Application
assignment	O
(	O
or	O
"	O
pseudo-color	B-Application
"	O
)	O
to	O
render	O
elevations	O
as	O
colors	O
(	O
for	O
example	O
,	O
using	O
green	O
for	O
the	O
lowest	O
elevations	O
,	O
shading	O
to	O
red	O
,	O
with	O
white	O
for	O
the	O
highest	O
elevation	O
.	O
</s>
<s>
Visualizations	B-Application
are	O
sometimes	O
also	O
done	O
as	O
oblique	O
views	O
,	O
reconstructing	O
a	O
synthetic	O
visual	O
image	O
of	O
the	O
terrain	O
as	O
it	O
would	O
appear	O
looking	O
down	O
at	O
an	O
angle	O
.	O
</s>
<s>
In	O
these	O
oblique	O
visualizations	B-Application
,	O
elevations	O
are	O
sometimes	O
scaled	O
using	O
"	O
vertical	O
exaggeration	O
"	O
in	O
order	O
to	O
make	O
subtle	O
elevation	O
differences	O
more	O
noticeable	O
.	O
</s>
<s>
Mappers	O
may	O
prepare	O
digital	B-Application
elevation	I-Application
models	I-Application
in	O
a	O
number	O
of	O
ways	O
,	O
but	O
they	O
frequently	O
use	O
remote	B-Application
sensing	I-Application
rather	O
than	O
direct	O
survey	O
data	O
.	O
</s>
<s>
Older	O
methods	O
of	O
generating	O
DEMs	O
often	O
involve	O
interpolating	B-Algorithm
digital	O
contour	O
maps	O
that	O
may	O
have	O
been	O
produced	O
by	O
direct	O
survey	O
of	O
the	O
land	O
surface	O
.	O
</s>
<s>
Note	O
that	O
contour	O
line	O
data	O
or	O
any	O
other	O
sampled	O
elevation	O
datasets	O
(	O
by	O
GPS	O
or	O
ground	O
survey	O
)	O
are	O
not	O
DEMs	O
,	O
but	O
may	O
be	O
considered	O
digital	B-Application
terrain	I-Application
models	I-Application
.	O
</s>
<s>
One	O
powerful	O
technique	O
for	O
generating	O
digital	B-Application
elevation	I-Application
models	I-Application
is	O
interferometric	O
synthetic	O
aperture	O
radar	B-Application
where	O
two	O
passes	O
of	O
a	O
radar	B-Application
satellite	O
(	O
such	O
as	O
RADARSAT-1	O
or	O
TerraSAR-X	O
or	O
Cosmo	O
SkyMed	O
)	O
,	O
or	O
a	O
single	O
pass	O
if	O
the	O
satellite	O
is	O
equipped	O
with	O
two	O
antennas	O
(	O
like	O
the	O
SRTM	B-Algorithm
instrumentation	O
)	O
,	O
collect	O
sufficient	O
data	O
to	O
generate	O
a	O
digital	B-Application
elevation	I-Application
map	I-Application
tens	O
of	O
kilometers	O
on	O
a	O
side	O
with	O
a	O
resolution	O
of	O
around	O
ten	O
meters	O
.	O
</s>
<s>
Other	O
kinds	O
of	O
stereoscopic	B-Algorithm
pairs	O
can	O
be	O
employed	O
using	O
the	O
digital	O
image	O
correlation	O
method	O
,	O
where	O
two	O
optical	O
images	O
are	O
acquired	O
with	O
different	O
angles	O
taken	O
from	O
the	O
same	O
pass	O
of	O
an	O
airplane	O
or	O
an	O
Earth	O
Observation	O
Satellite	O
(	O
such	O
as	O
the	O
HRS	O
instrument	O
of	O
SPOT5	O
or	O
the	O
VNIR	O
band	O
of	O
ASTER	O
)	O
.	O
</s>
<s>
The	O
SPOT	O
1	O
satellite	O
(	O
1986	O
)	O
provided	O
the	O
first	O
usable	O
elevation	O
data	O
for	O
a	O
sizeable	O
portion	O
of	O
the	O
planet	O
's	O
landmass	O
,	O
using	O
two-pass	O
stereoscopic	B-Algorithm
correlation	O
.	O
</s>
<s>
Later	O
,	O
further	O
data	O
were	O
provided	O
by	O
the	O
European	B-Algorithm
Remote-Sensing	I-Algorithm
Satellite	I-Algorithm
(	O
ERS	O
,	O
1991	O
)	O
using	O
the	O
same	O
method	O
,	O
the	O
Shuttle	B-Algorithm
Radar	I-Algorithm
Topography	I-Algorithm
Mission	I-Algorithm
(	O
SRTM	B-Algorithm
,	O
2000	O
)	O
using	O
single-pass	O
SAR	O
and	O
the	O
Advanced	O
Spaceborne	O
Thermal	O
Emission	O
and	O
Reflection	O
Radiometer	O
(	O
ASTER	O
,	O
2000	O
)	O
instrumentation	O
on	O
the	O
Terra	O
satellite	O
using	O
double-pass	O
stereo	O
pairs	O
.	O
</s>
<s>
upright	O
=	O
1.2	O
|thumb|MOLA	O
digital	B-Application
elevation	I-Application
model	I-Application
showing	O
the	O
two	O
hemispheres	O
of	O
Mars	B-Application
.	O
</s>
<s>
A	O
tool	O
of	O
increasing	O
value	O
in	O
planetary	O
science	O
has	O
been	O
use	O
of	O
orbital	O
altimetry	O
used	O
to	O
make	O
digital	B-Application
elevation	I-Application
map	I-Application
of	O
planets	O
.	O
</s>
<s>
A	O
primary	O
tool	O
for	O
this	O
is	O
laser	O
altimetry	O
but	O
radar	B-Application
altimetry	O
is	O
also	O
used	O
.	O
</s>
<s>
Planetary	O
digital	B-Application
elevation	I-Application
maps	I-Application
made	O
using	O
laser	O
altimetry	O
include	O
the	O
Mars	B-Application
Orbiter	O
Laser	O
Altimeter	O
(	O
MOLA	O
)	O
mapping	O
of	O
Mars	B-Application
,	O
the	O
Lunar	O
Orbital	O
Laser	O
Altimeter	O
(	O
LOLA	O
)	O
and	O
Lunar	O
Altimeter	O
(	O
LALT	O
)	O
mapping	O
of	O
the	O
Moon	O
,	O
and	O
the	O
Mercury	O
Laser	O
Altimeter	O
(	O
MLA	O
)	O
mapping	O
of	O
Mercury	O
.	O
</s>
<s>
The	O
quality	O
of	O
a	O
DEM	O
is	O
a	O
measure	O
of	O
how	O
accurate	O
elevation	O
is	O
at	O
each	O
pixel	B-Algorithm
(	O
absolute	O
accuracy	O
)	O
and	O
how	O
accurately	O
is	O
the	O
morphology	O
presented	O
(	O
relative	O
accuracy	O
)	O
.	O
</s>
<s>
grid	O
resolution	O
or	O
pixel	B-Algorithm
size	O
;	O
</s>
<s>
interpolation	B-Algorithm
algorithm	O
;	O
</s>
<s>
Rendering	O
of	O
3D	B-Application
visualizations	I-Application
.	O
</s>
