<s>
The	O
geo	B-Application
URI	I-Application
scheme	I-Application
is	O
a	O
Uniform	B-Protocol
Resource	I-Protocol
Identifier	I-Protocol
(	O
URI	B-Protocol
)	O
scheme	O
defined	O
by	O
the	O
Internet	O
Engineering	O
Task	O
Force	O
's	O
RFC	B-Application
5870	I-Application
(	O
published	O
8	O
June	O
2010	O
)	O
as	O
:	O
</s>
<s>
a	O
Uniform	B-Protocol
Resource	I-Protocol
Identifier	I-Protocol
(	O
URI	B-Protocol
)	O
for	O
geographic	O
locations	O
using	O
the	O
'	O
geo	O
 '	O
scheme	O
name	O
.	O
</s>
<s>
A	O
'	O
geo	O
 '	O
URI	B-Protocol
identifies	O
a	O
physical	O
location	O
in	O
a	O
two	O
-	O
or	O
three-dimensional	O
coordinate	O
reference	O
system	O
in	O
a	O
compact	O
,	O
simple	O
,	O
human-readable	O
,	O
and	O
protocol-independent	O
way	O
.	O
</s>
<s>
The	O
current	O
revision	O
of	O
the	O
vCard	O
specification	O
supports	O
geo	B-Application
URIs	I-Application
in	O
a	O
vCard	O
's	O
"	O
GEO	O
"	O
property	O
,	O
and	O
the	O
GeoSMS	O
standard	O
uses	O
geo	B-Application
URIs	I-Application
for	O
geotagging	O
SMS	O
messages	O
.	O
</s>
<s>
Android	B-Application
based	O
devices	O
support	O
geo	B-Application
URIs	I-Application
,	O
although	O
that	O
implementation	O
is	O
based	O
on	O
a	O
draft	O
revision	O
of	O
the	O
specification	O
,	O
and	O
supports	O
a	O
different	O
set	O
of	O
URI	B-Protocol
parameters	O
and	O
query	O
strings	O
.	O
</s>
<s>
A	O
geo	B-Application
URI	I-Application
is	O
not	O
to	O
be	O
confused	O
with	O
the	O
former	O
website	O
of	O
GeoURL	B-Application
(	O
which	O
had	O
implemented	O
ICBM	O
addresses	O
)	O
.	O
</s>
<s>
A	O
simple	O
geo	B-Application
URI	I-Application
might	O
look	O
like	O
:	O
</s>
<s>
If	O
a	O
third	O
comma-separated	O
value	O
is	O
present	O
,	O
it	O
represents	O
altitude	B-Application
;	O
so	O
,	O
coordinates	O
of	O
a	O
3D	O
grid	O
.	O
</s>
<s>
Coordinates	O
in	O
the	O
Southern	O
and	O
Western	O
hemispheres	O
as	O
well	O
as	O
altitudes	B-Application
below	O
the	O
coordinate	O
reference	O
system	O
(	O
depths	O
)	O
are	O
signed	O
negative	O
with	O
a	O
leading	O
dash	O
.	O
</s>
<s>
The	O
geo	B-Application
URI	I-Application
also	O
allows	O
for	O
an	O
optional	O
"	O
uncertainty	O
"	O
value	O
,	O
separated	O
by	O
a	O
semicolon	O
,	O
representing	O
the	O
uncertainty	O
of	O
the	O
location	O
in	O
meters	O
,	O
and	O
is	O
described	O
using	O
the	O
"	O
u	O
"	O
URI	B-Protocol
parameter	O
.	O
</s>
<s>
A	O
geo	B-Application
URI	I-Application
with	O
an	O
uncertainty	O
parameter	O
looks	O
as	O
follows	O
:	O
</s>
<s>
A	O
geo	B-Application
URI	I-Application
may	O
,	O
for	O
example	O
,	O
be	O
included	O
on	O
a	O
web	O
page	O
,	O
as	O
HTML	B-Language
:	O
</s>
<s>
so	O
that	O
a	O
geo	O
URI-aware	O
user	B-Protocol
agent	I-Protocol
such	O
as	O
a	O
web	B-Application
browser	I-Application
could	O
launch	O
the	O
user	O
's	O
chosen	O
mapping	O
service	O
;	O
or	O
it	O
could	O
be	O
used	O
in	O
an	O
Atom	B-Application
feed	I-Application
or	O
other	O
XML	B-Protocol
file	O
.	O
</s>
<s>
The	O
only	O
justified	O
use	O
of	O
other	O
CRS	O
today	O
is	O
,	O
perhaps	O
,	O
to	O
preserve	O
projection	O
in	O
large-scale	O
maps	O
,	O
as	O
local	O
UTM	O
,	O
or	O
for	O
non-terrestrial	O
coordinates	O
such	O
as	O
those	O
on	O
the	O
Moon	O
or	O
Mars	B-Application
.	O
</s>
<s>
The	O
syntax	O
and	O
semantic	O
of	O
the	O
CRS	O
parameter	O
,	O
separated	O
by	O
a	O
semicolon	O
,	O
is	O
described	O
at	O
section	O
8.3	O
of	O
RFC5870	B-Application
.	O
</s>
<s>
A	O
geo	B-Application
URI	I-Application
for	O
a	O
hypothetical	O
lunar	O
CRS	O
created	O
in	O
2011	O
might	O
be	O
:	O
</s>
<s>
The	O
Geo	B-Application
URI	I-Application
scheme	I-Application
semantics	O
,	O
expressed	O
in	O
the	O
section	O
3.4	O
of	O
the	O
RFC5870	B-Application
,	O
is	O
not	O
explicit	O
about	O
some	O
mathematical	O
assumptions	O
,	O
so	O
it	O
is	O
open	O
to	O
interpretation	O
.	O
</s>
<s>
The	O
semantic	O
of	O
coord-c	O
for	O
WGS-84	O
is	O
altitude	B-Application
(	O
specifically	O
the	O
"	O
ground	O
elevation	O
"	O
,	O
relative	O
to	O
the	O
current	O
geoid	O
Earth	O
Gravitational	O
Model	O
attached	O
to	O
WGS84	O
)	O
,	O
and	O
the	O
concept	O
is	O
extended	O
for	O
other	O
coordinates	O
(	O
of	O
non-default	O
CRS	O
)	O
.	O
</s>
<s>
The	O
RFC	O
explains	O
that	O
"	O
...	O
undefined	O
<altitude>	O
MAY	O
assume	O
that	O
the	O
URI	B-Protocol
refers	O
to	O
the	O
respective	O
location	O
on	O
Earth	O
's	O
physical	O
surface.	O
"	O
</s>
<s>
However	O
,	O
"	O
...	O
an	O
<altitude>	O
value	O
of	O
0	O
MUST	O
NOT	O
be	O
mistaken	O
to	O
refer	O
to	O
'	O
ground	O
elevation	O
 '	O
"	O
.	O
</s>
<s>
In	O
other	O
words	O
,	O
when	O
an	O
altitude	B-Application
is	O
defined	O
,	O
the	O
measurement	O
is	O
done	O
relative	O
to	O
the	O
geoid	O
(	O
#5	O
;	O
black	O
line	O
in	O
the	O
image	O
)	O
,	O
a	O
surface	O
defined	O
by	O
Earth	O
's	O
gravity	O
approximating	O
the	O
mean	O
sea	O
level	O
.	O
</s>
<s>
When	O
it	O
is	O
undefined	O
,	O
the	O
elevation	O
is	O
assumed	O
to	O
be	O
the	O
altitude	B-Application
of	O
the	O
latitude-longitude	O
point	O
,	O
that	O
is	O
its	O
height	O
(	O
or	O
negative	O
depth	O
)	O
relative	O
to	O
the	O
geoid	O
(	O
i.e.	O
</s>
<s>
A	O
point	O
with	O
a	O
measure	O
"	O
altitude	B-Application
=	O
0	O
"	O
is	O
,	O
however	O
,	O
not	O
to	O
be	O
confused	O
with	O
an	O
undefined	O
value	O
:	O
it	O
refers	O
to	O
an	O
altitude	B-Application
of	O
0	O
meters	O
above	O
the	O
geoid	O
.	O
</s>
<s>
The	O
use	O
of	O
a	O
geoid	O
stands	O
in	O
contrast	O
to	O
GeoJSON	B-General_Concept
,	O
which	O
uses	O
direct	O
ellipsoid	O
height	O
.	O
</s>
<s>
Geo	B-Application
URI	I-Application
is	O
not	O
about	O
exact	O
abstract	O
positions	O
,	O
strictly	O
it	O
is	O
a	O
location	O
estimate	O
,	O
and	O
we	O
can	O
interpret	O
it	O
(	O
from	O
RFC5870	B-Application
and	O
RFC5491	O
)	O
as	O
the	O
approximate	O
physical	O
position	O
of	O
an	O
object	O
in	O
the	O
Earth	O
's	O
surface	O
.	O
</s>
<s>
The	O
RFC	B-Application
5870	I-Application
does	O
not	O
formalize	O
the	O
use	O
of	O
the	O
"	O
uncertainty	O
"	O
term	O
.	O
</s>
<s>
So	O
,	O
in	O
a	O
coarse-statistical	O
or	O
any	O
non-statistical	O
numerical	B-General_Concept
analysis	I-General_Concept
,	O
the	O
GeoURI	B-Application
uncertainty	O
is	O
a	O
condition	B-Algorithm
number	I-Algorithm
.	O
</s>
<s>
Putting	O
all	O
together	O
,	O
adopting	O
these	O
clues	O
,	O
the	O
usual	O
statistical	O
assumptions	O
,	O
and	O
the	O
explicit	O
definitions	O
of	O
the	O
RFC	O
,	O
we	O
obtain	O
the	O
Geo	B-Application
URI	I-Application
's	O
uncertainty	O
mathematical	O
properties	O
:	O
</s>
<s>
"	O
The	O
single	O
uncertainty	O
value	O
is	O
applied	O
to	O
all	O
dimensions	O
given	O
in	O
the	O
URI	B-Protocol
"	O
(	O
section	O
3.4.3	O
)	O
.	O
</s>
<s>
adopting	O
standard	B-General_Concept
error	I-General_Concept
model	I-General_Concept
:	O
the	O
model	O
of	O
the	O
most	O
common	O
descriptive	B-General_Concept
statistics	I-General_Concept
modeling	I-General_Concept
.	O
</s>
<s>
the	O
colony	O
is	O
a	O
3D	O
object	O
at	O
the	O
(	O
exactly	O
)	O
the	O
Terrain	O
surface	O
,	O
so	O
at	O
precise	O
altitude	B-Application
(	O
approximated	O
to	O
a	O
zero	O
uncertainty	O
measure	O
)	O
.	O
</s>
<s>
Some	O
vendors	O
,	O
such	O
as	O
Android	B-Application
OS	I-Application
,	O
have	O
adopted	O
extensions	O
to	O
the	O
"	O
geo	O
"	O
URI	B-Protocol
scheme	O
:	O
</s>
<s>
Android	B-Application
adopts	O
an	O
unconventional	O
approach	O
to	O
displaying	O
the	O
points	O
:	O
it	O
shows	O
the	O
map	O
for	O
,	O
but	O
does	O
not	O
display	O
a	O
map	O
pin	O
,	O
when	O
a	O
location	O
is	O
given	O
in	O
the	O
standard	O
way	O
.	O
</s>
