<s>
An	O
orthophoto	B-General_Concept
,	O
orthophotograph	B-General_Concept
,	O
orthoimage	B-General_Concept
or	O
orthoimagery	B-General_Concept
is	O
an	O
aerial	O
photograph	O
or	O
satellite	O
imagery	O
geometrically	O
corrected	O
(	O
"	O
orthorectified	B-General_Concept
"	O
)	O
such	O
that	O
the	O
scale	O
is	O
uniform	O
:	O
the	O
photo	O
or	O
image	O
follows	O
a	O
given	O
map	B-Application
projection	I-Application
.	O
</s>
<s>
Unlike	O
an	O
uncorrected	O
aerial	O
photograph	O
,	O
an	O
orthophoto	B-General_Concept
can	O
be	O
used	O
to	O
measure	O
true	O
distances	O
,	O
because	O
it	O
is	O
an	O
accurate	O
representation	O
of	O
the	O
Earth	O
's	O
surface	O
,	O
having	O
been	O
adjusted	O
for	O
topographic	O
relief	O
,	O
lens	O
distortion	O
,	O
and	O
camera	O
tilt	O
.	O
</s>
<s>
Orthophotographs	B-General_Concept
are	O
commonly	O
used	O
in	O
geographic	B-Application
information	I-Application
systems	I-Application
(	O
GIS	B-Application
)	O
as	O
a	O
"	O
map	O
accurate	O
"	O
background	O
image	O
.	O
</s>
<s>
An	O
orthorectified	B-General_Concept
image	O
differs	O
from	O
"	O
rubber	O
sheeted	O
"	O
rectifications	O
as	O
the	O
latter	O
may	O
accurately	O
locate	O
a	O
number	O
of	O
points	O
on	O
each	O
image	O
but	O
"	O
stretch	O
"	O
the	O
area	O
between	O
so	O
scale	O
may	O
not	O
be	O
uniform	O
across	O
the	O
image	O
.	O
</s>
<s>
A	O
digital	B-Application
elevation	I-Application
model	I-Application
(	O
DEM	O
)	O
is	O
required	O
to	O
create	O
an	O
accurate	O
orthophoto	B-General_Concept
as	O
distortions	O
in	O
the	O
image	O
due	O
to	O
the	O
varying	O
distance	O
between	O
the	O
camera/sensor	O
and	O
different	O
points	O
on	O
the	O
ground	O
need	O
to	O
be	O
corrected	O
.	O
</s>
<s>
An	O
orthoimage	B-General_Concept
and	O
a	O
"	O
rubber	O
sheeted	O
"	O
image	O
can	O
both	O
be	O
said	O
to	O
have	O
been	O
"	O
georeferenced	O
"	O
;	O
however	O
,	O
the	O
overall	O
accuracy	O
of	O
the	O
rectification	O
varies	O
.	O
</s>
<s>
Software	O
can	O
display	O
the	O
orthophoto	B-General_Concept
and	O
allow	O
an	O
operator	O
to	O
digitize	O
or	O
place	O
linework	O
,	O
text	O
annotations	O
or	O
geographic	O
symbols	O
(	O
such	O
as	O
hospitals	O
,	O
schools	O
,	O
and	O
fire	O
stations	O
)	O
.	O
</s>
<s>
Some	O
software	O
can	O
process	O
the	O
orthophoto	B-General_Concept
and	O
produce	O
the	O
linework	O
automatically	O
.	O
</s>
<s>
Production	O
of	O
orthophotos	B-General_Concept
was	O
historically	O
achieved	O
using	O
mechanical	O
devices	O
.	O
</s>
<s>
An	O
orthophotomosaic	O
is	O
a	O
raster	B-Algorithm
image	I-Algorithm
made	O
by	O
merging	O
orthophotos	B-General_Concept
—	O
aerial	O
or	O
satellite	O
photographs	O
which	O
have	O
been	O
transformed	O
to	O
correct	O
for	O
perspective	O
so	O
that	O
they	O
appear	O
to	O
have	O
been	O
taken	O
from	O
vertically	O
above	O
at	O
an	O
infinite	O
distance	O
.	O
</s>
<s>
Google	B-Application
Earth	I-Application
images	O
are	O
of	O
this	O
type	O
.	O
</s>
