Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 217)

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Acquiring GPS Signals
Poor GPS Coverage
Under the following conditions, the GPS satellite signal will not be properly
acquired. As a result, the geotag information may not be recorded or
inaccurate geotag information may be recorded.
Indoors, underground, in tunnels or forests, between buildings, or in
valleys.
Near high-voltage power lines or mobile phones operating on the 1.5
GHz band.
When the camera is left inside a bag, etc.
When traveling a long distance.
When traveling through different environments.
Since GPS satellites move as time passes, satellite movement can
interfere with geotagging and cause missing or inaccurate geotag
information even in conditions other than the above. Additionally, the
geotag information may also include the route traveled even if the
camera was used at just one location.
The battery level may be low when you start using the camera due to the
effect of the GPS function. If necessary, recharge the battery or prepare
a charged, spare battery (sold separately).
The camera can receive GPS signals even in the vertical orientation.