Cross-checking positions on ECDIS

25 Jan 2011 Resource

How do we comply with requirement of cross checking the vessel's position being recorded on ECDIS?

Our vessel is now fully ECDIS compliant, i.e. we are sailing using electronic charts only, with no paper charts. At a recent vetting inspection, the inspector noted that there was no record of any cross checks on the vessel’s position being recorded on the ECDIS.

With paper charts the vessels position was plotted using GPS and cross checked with visual and radar ranges and bearings and recorded on the chart. There appears to be no provision for this process on our ECDIS. How can we comply with this requirement using ECDIS?


All ECDIS equipment fitted since about the beginning of 2009 have a facility for plotting manually-obtained bearing and distance lines of position (LOPs) to determine own ship’s position on the display. Some older equipment also have such a facility. The ECDIS can then be set to a mode that uses the LOP-derived position as the origin for  automated DR/EP position fixing, in place of using satellite derived position. In this case the initial LOP-derived  position is recorded, together with the LOPs used in its derivation. The recorded ship’s track will therefore also be based on DR/EP calculations until the equipment is switched back to satellite position mode. There is no IMO requirement for the LOPs to be recorded if the position is not used as the origin for DR/EP.

The NI strongly encourages ECDIS users to make frequent  use of the LOP facilities on ECDIS to check satellite derived position. However, the NI discourages the continual switching between normal and DR/EP modes of operation for the sole purpose of recording LOPs as it would greatly increase the possibility that DR/EP mode will be left  inadvertently switched on – with possibly dire consequences if the belief was that the ECDIS was in normal position mode, despite mandatory indications to the contrary on the display.

It therefore means that on most equipment, because it is not required by IMO, there is no good method of recording when and how satellite derived position has been checked. By contrast, on paper charts, the record of the LOPs and resultant position is normally kept until a subsequent voyage uses the same chart and so has become a useful record for inspectors and auditors to check that good practice is being maintained.

Another complication is that under many circumstances on ECDIS  it can be better to take very  frequent single LOPs (visual and/or radar) and ensure that when immediately plotted they pass very close to the current satellite-derived position. This practice does not actually  generate a secondary position but instead gives the possibility of very low latency and therefore high accuracy checks on the satellite derived position. However, a single LOP only gives partial confirmation.  Subsequent (single) LOPs  are needed to be taken at different bearings (generally on different objects), to confirm the continued accuracy of satellite positioning. These are ideally approximately 90°apart but frequent checking is generally preferable to waiting for near 90° opportunities.

If inspectors and auditors continue to need manual LOPs being recorded they will need to lobby IMO to amend the ECDIS Performance Standards. In which case IMO should also allow the recording of single LOPs. In practice, it would take many years for such a change to become widely available and so other checks by inspectors, such as quizzing OOWs on how manual position checks are made on the specific ECDIS, should perhaps become the immediate norm.

ECDIS users could perhaps insert a position-related Mariner’s Note when each LOP check has been made. It need only contain a brief description, perhaps as short as ‘LOP’. The frequency of these notes can then also be checked by inspectors and auditors. Of course, it would be easy to generate a note without actually performing any checks, perhaps necessitating some verbal questioning.