1) Lack of Global coverage
Complications in obtaining worldwide chart coverage mainly due to the policy differences between the national Hydrographic authorities.
The lack of official ENC makes things just worse. Even if a number of Hydrographic Offices have started ambitious programs of ENC production, very few official electronic charts in S-57/3 format are available today. The result is that ECDIS users have to supplement ENC data with non-ENC electronic charts, such as
- Commercial databases of electronic charts, produced by organizations other than Hydrographic Offices
- Electronic charts produced by Hydrographic Offices, not compliant with S-57/3 (e.g. vector charts in formats other than S-57/3, raster charts, etc.).
- Required the use of the so-called dual-fuel ECDIS: a system fed with both ENC and non-ENC electronic charts.
- Since the ECDIS operates in non-equivalent mode when using non-ENC charts, S-52 and the IMO Performance Standards require that these are not mixed with the ENC. Therefore, they must be loaded in the ECDIS into a separate storage area and must remain clearly distinguishable from official charts even after compilation in the SENC.
2) Problem of Geodetic Datum shifts in unapproved charts
The issue of geodetic datum shifts is not a concern for ECDIS since all ENCs should be referenced to WGS84, but it does affect paper charts, RNCs and privately produced vector charts. It is therefore very important that shipping companies use only approved RNC charts in areas where ENC is not available.
The issue arises because the earth is not a perfect sphere. Over the centuries mapmakers have devised local models of the earth’s shape. Because the earth is not a regular shape the accuracy of each datum will vary as you get further from the specific location for which it was defined. Hundreds of different datums exist to define positions; examples are OSGB36, European 1950, Indian, etc.
Satellite systems require a global datum and GPS positions are based on the World Geodetic System 1984 (WGS-84) which is a model of the entire earth.
Problems arise when you are receiving positions based on one datum and plotting them on a chart which is based on a different datum.