Third IMO Greenhouse Gas Study 2014

164 Third IMO GHG Study 2014
Following the merging of the T-AIS and S-AIS data sources, the data is resampled to hourly estimators for each
variable of interest: speed, draught and region. An aggregation time period of one hour was selected. The
uncertainty within this hourly estimate for speed estimated is dealt with later, in Annex 3.
Each field is resampled uniquely:
• Speed: The estimate of the speed at any time period is calculated as a time-weighted average of
reported speed within that period. The weighting is the elapsed time to the next reported message.
Figure 5 shows an example of this.
• Draught: The draught is taken as the maximum reported draught within that period. As per IMO
number, the draught is only reported in the static message, which appears less frequently. Thus, the
effect of error in estimation is low. Moreover, the draught does not have the range of uncertainty that
speed has across the hour and is typically only altered at the beginning of new voyages.
• Region classification: It is possible that a ship can be located in more than one region within the
resampling period, if the ship crosses a region boundary within that period. In order to rationalize the
data, the region of the ship is taken as the first reported region in that time period. As the regions are
large and the region indicator is only used to understand the global geographical coverage of the data
sets, it was assumed that this approach would not bias the overall coverage results as the number of
ships crossing boundaries at each hourly interval is small in comparison with those located wholly
within a region for the full hour duration.
• Organization flag: This is simply a coverage flag used to note from what data source the ship was
picked up. For the resampling of this variable, the first recorded activity report in the hour was taken
as the hourly resampled value. This variable is shown in the plot in Figure 6.
Figure 5: Top plot shows the reported speed for an indicative ship between 15 January 2011
and 31 January 2011, each message with an opacity of 50% so that density is apparent.
The lower plot shows the same ship with the speeds resampled. A reliability of 0 indicates that there
was no activity report for that resampled hour
On completion of the resampling, the data sets are matched to their respective ship technical characteristics
and the data anonymized by removing the IMO and MMSI codes.