  The efficacy of coronal mass ejection (CME) observations as a key input to
space weather forecasting is explored by comparing on and off Sun-Earth line
observations from the ESA/NASA SOHO and NASA STEREO spacecraft. A comparison is
made of CME catalogues based on L1 coronagraph imagery and off Sun-Earth line
coronagraph and heliospheric imager (HI) observations, for the year 2011.
Analysis reveals inconsistencies in the identification of a number of
potentially Earth-directed CMEs. The catalogues reflect our ability to identify
and characterise CMEs, so any discrepancies can impact our prediction of
Earth-directed CMEs. We show that 15 CMEs, which were observed by STEREO, that
had estimated directions compatible with Earth-directed events, had no
identified halo/partial halo counterpart listed in the L1 coronagraph CME
catalogue. In-situ data confirms that for 9 of these there was a consistent L1
Interplanetary CME (ICME). The number of such "discrepant" events is
significant compared to the number of ICMEs recorded at L1 in 2011, stressing
the need to address space weather monitoring capabilities, particularly with
the inclusion of off Sun-Earth line observation. While the study provides
evidence that some halo CMEs are simply not visible in near-Earth coronagraph
imagery, there is evidence that some halo CMEs viewed from L1 are compromised
by preceding CME remnants or the presence of multiple-CMEs. This underlines (1)
the value of multiple vantage point CME observation, and (2) the benefit of off
Sun-Earth line platform heliospheric imaging, and coronagraph imaging, for the
efficient identification and tracking of Earth-directed events.
