The Shom's paper maps have changed, are you up to date?

A nautical chart is a wealth of information for those who know how to find it. From its cover to its back, a certain amount of information makes it much easier to use and avoid traps. However, the Shom maps have changed pace to modernize, it is an opportunity to recall or identify important indications.

Charly Fernbach Published on June 20, 2019

Reading a paper map and its useful information

The coverage of the chart gives us general information. The title of the card and its number will be read there, allowing it to be referenced in the Shom catalogue. We also see the scale of the representation.

On older generation maps, the front side shows, in the form of a drawing, the location of the map in its region.

On modern documents, this reproduction is found on the back of the map, as well as the date of printing and the geodetic representation system.

This is one of the major points in the evolution of our paper cards, they have changed their reference system. Previously governed by the French ED 50 system, they now comply with the international standard WGS 84, which corresponds to the protocol used by GPS.

On old maps, this information can be found by folding it out and reading the cartridge.

These two nautical charts of the shom are different. The one on the left has an updated 2007 edition and is in ED50. The one on the right has an updated draw of 2014 and uses the WGS84 geodetic system corresponding to GPS.

The cartridge of a nautical chart

This part of the chart contains valuable information that you should read before using it.
We find there:

the official name of the map: "From the island of Penfret to the Birvideaux plateau and around Lorient"
its scale 1: 50,000. The scale of the map corresponds to a multiplication to arrive at the real size.
Example: a scale of 1/50 000 => 1 cm on the map is equivalent to 50 000 cm or 500 metres on the ground.
the reference level of the probes on the card . It corresponds to the level of the lowest Seas of a tide (coef 120), it is the hydrographic zero
the reference level of altitudes on land. Reference: average level 3.1 m above hydrographic datum. The height of a hill is given from this level
The geodetic system used to report geographical positions. On older maps using the ED 50 system, the cartridge informs us of corrections to be made to the coordinates to match the WGS 84 system.
The beaconing system. In this case, we are subject to the rules of region A, which means that for us the red colour is on the port side when we enter a port.
The projection system used . On paper cards in general it is the Mercator projection
The origin of the measurements and readings used to draw up this map.