<s>
Viewshed	B-Application
analysis	I-Application
is	O
a	O
computational	O
algorithm	O
that	O
delineates	O
a	O
viewshed	O
,	O
the	O
area	O
that	O
is	O
visible	O
(	O
on	O
the	O
base	O
terrain	O
surface	O
)	O
from	O
a	O
given	O
location	O
.	O
</s>
<s>
It	O
is	O
a	O
common	O
part	O
of	O
the	O
terrain	O
analysis	O
toolset	O
found	O
in	O
of	O
most	O
geographic	B-Application
information	I-Application
system	I-Application
(	O
GIS	B-Application
)	O
software	O
.	O
</s>
<s>
The	O
analysis	O
uses	O
the	O
elevation	O
value	O
of	O
each	O
cell	O
of	O
the	O
digital	B-Application
elevation	I-Application
model	I-Application
(	O
DEM	B-Application
)	O
to	O
determine	O
visibility	O
to	O
or	O
from	O
a	O
particular	O
cell	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
viewshed	B-Application
analysis	I-Application
is	O
commonly	O
used	O
to	O
locate	O
communication	O
towers	O
or	O
determining	O
the	O
view	O
from	O
a	O
road	O
.	O
</s>
<s>
For	O
example	O
,	O
when	O
locating	O
a	O
landfill	B-Application
,	O
the	O
analysis	O
can	O
determine	O
from	O
where	O
the	O
landfill	B-Application
is	O
visible	O
to	O
keep	O
it	O
hidden	O
from	O
view	O
.	O
</s>
<s>
A	O
viewshed	B-Application
analysis	I-Application
can	O
be	O
performed	O
using	O
one	O
of	O
many	O
GIS	B-Application
programs	O
,	O
such	O
as	O
,	O
GRASS	B-Application
GIS	I-Application
(	O
r.los	O
,	O
r.viewshed	O
)	O
,	O
QGIS	B-Application
(	O
)	O
,	O
,	O
,	O
SAGA	B-Language
GIS	I-Language
(	O
Visibility	O
)	O
,	O
TNT	O
Mips	O
,	O
ArcMap	B-Application
,	O
Maptitude	B-Application
,	O
ERDAS	B-Application
IMAGINE	I-Application
.	O
</s>
<s>
A	O
viewshed	O
is	O
created	O
from	O
a	O
DEM	B-Application
by	O
using	O
an	O
algorithm	O
that	O
estimates	O
the	O
difference	O
of	O
elevation	O
from	O
one	O
cell	O
(	O
the	O
viewpoint	O
cell	O
)	O
to	O
the	O
next	O
(	O
the	O
target	O
cell	O
)	O
.	O
</s>
<s>
When	O
performing	O
a	O
viewshed	B-Application
analysis	I-Application
,	O
several	O
variables	O
can	O
be	O
used	O
to	O
limit	O
or	O
adjust	O
the	O
calculation	O
.	O
</s>
<s>
If	O
no	O
height	O
is	O
given	O
,	O
then	O
the	O
viewshed	B-Application
analysis	I-Application
uses	O
the	O
cell	O
value	O
of	O
the	O
DEM	B-Application
in	O
which	O
the	O
tower	O
is	O
located	O
.	O
</s>
<s>
The	O
viewshed	B-Application
analysis	I-Application
can	O
also	O
have	O
a	O
limited	O
viewing	O
angle	O
.	O
</s>
<s>
The	O
final	O
variable	O
used	O
in	O
the	O
viewshed	B-Application
analysis	I-Application
is	O
the	O
radius	O
value	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
the	O
radio	O
tower	O
,	O
if	O
the	O
radio	O
signal	O
has	O
a	O
limited	O
range	O
,	O
perhaps	O
10	O
miles	O
,	O
then	O
the	O
radius	O
variable	O
can	O
be	O
set	O
to	O
limit	O
the	O
viewshed	B-Application
analysis	I-Application
to	O
a	O
10-mile	O
radius	O
.	O
</s>
<s>
Besides	O
tower	O
placement	O
a	O
viewshed	B-Application
analysis	I-Application
can	O
be	O
used	O
for	O
other	O
applications	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
viewshed	B-Application
analysis	I-Application
could	O
estimate	O
the	O
impact	O
of	O
the	O
addition	O
of	O
a	O
large	O
building	O
.	O
</s>
<s>
The	O
viewshed	B-Application
analysis	I-Application
would	O
show	O
all	O
the	O
areas	O
from	O
which	O
the	O
building	O
could	O
be	O
seen	O
as	O
well	O
as	O
any	O
views	O
that	O
would	O
be	O
obscured	O
from	O
any	O
particular	O
location	O
.	O
</s>
<s>
An	O
example	O
of	O
using	O
a	O
viewshed	B-Application
analysis	I-Application
on	O
a	O
line	O
segment	O
is	O
from	O
the	O
Wyoming	O
State	O
Office	O
,	O
Bureau	O
of	O
Land	O
Management	O
,	O
in	O
which	O
the	O
office	O
used	O
a	O
viewshed	B-Application
analysis	I-Application
to	O
determine	O
the	O
visibility	O
from	O
National	O
Historic	O
Trails	O
across	O
Wyoming	O
.	O
</s>
<s>
In	O
another	O
example	O
the	O
United	O
States	O
Geological	O
Survey	O
(	O
USGS	O
)	O
used	O
a	O
viewshed	B-Application
analysis	I-Application
to	O
assist	O
NASA	O
's	O
Mars	B-Application
Exploration	O
Rover	O
(	O
MER	O
)	O
project	O
.	O
</s>
<s>
When	O
NASA	O
needed	O
to	O
find	O
appropriate	O
landing	O
spots	O
for	O
the	O
Mars	B-Application
rovers	O
,	O
they	O
turned	O
to	O
the	O
USGS	O
to	O
map	O
the	O
best	O
possible	O
sites	O
.	O
</s>
<s>
In	O
this	O
case	O
the	O
viewshed	O
indicates	O
the	O
areas	O
which	O
may	O
or	O
may	O
not	O
be	O
visible	O
by	O
the	O
Mars	B-Application
rovers	O
from	O
each	O
landing	O
site	O
(	O
MER	O
Landing	O
Site	O
Viewshed	B-Application
Analysis	I-Application
)	O
.	O
</s>
