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Inventories of CO2 emissions from international shipping 2007–2012 67
Figure 48: Activity estimate quality assurance (2012)
Greater detail of the derivation of parameters from the LRIT data sets and their application in this comparative
analysis is given in Annex 3, along with analysis for 2010 and 2011.
Activity estimates and derived parameters (speed and draught)
In addition to the analysis carried out using LRIT data, a further quality analysis of the bottom-up method’s
estimate of activity (time in mode, speed estimation, draught estimation and distance covered) can be obtained
using noon report data. Noon report data record information daily, including average speed during the period
of the report and distance travelled. Noon reports also record the date and time a voyage begins and ends.
This information was aggregated over quarters, compared with the same data calculated using the bottom-up
model, and aggregated to the same quarter of each year.
The results for 2012 are presented in Figure 49 and Figure 50. The red line represents an ideal match (equal
values) between the bottom-up and noon report outputs, the solid black line the best fit through the data and
the dotted black lines the 95% confidence bounds on the best fit. The “x” symbols represent individual ships,
coloured according to the ship type category listed in the legend. The plots include all results, with no outliers
removed.
The activity estimation of days at sea and at port can be seen to have some scatter. This scatter is related
to the fact that for some of the time the ship is not observed and an extrapolation algorithm is used to
estimate activity. For any one ship, the reliability of that extrapolation is low. However, overall, the distribution
is approximately even and does not represent a significant degree of bias, as the best-fit line shows. The
reliability of the estimate of at-port and at-sea days appears consistent regardless of ship type.
The quality of the estimation of ship speed when at sea is higher than the quality of the port- and sea-time
estimation. The best-fit line shows close alignment with the red equilibrium line, albeit with a trend towards
underestimating the speeds of the larger container ships. The confidence bounds are closely aligned to the
best-fit line.
The draught observation shows the lowest quality of fit. The observed scatter implies a bias for the bottom-up
method to slightly overestimate draught. The agreement for ship types with low draught variability (e.g.
container ships) is good. This implies that the overall poor reliability is likely to be due to infrequent updating
of the draught data reported to the AIS receiver.
In earlier years (see Annex 3 for the data), similar relative quality assurance between the variables plotted can
be obtained; however, the absolute quality reduces for the earlier years, particularly 2007, 2008 and 2009.
This can be seen by comparing the 2012 results with Figure 51, even accounting for the fact that in 2009 there
are fewer ships in the noon reports data set. Days at sea and at-sea speed have significantly more scatter and
therefore wider confidence bounds than the equivalent plots in 2012. With the exception of some outlier data
in 2009, the speed agreement is moderate. However, the days-at-sea agreement implies that there is some