Zero Hidrográfico (ZH)

The Zero Hidrográfico (ZH) is the chart datum used in Portugal.

ZH is located below the level of the Lowest Astronomical Tide (LAT).

ZH is the vertical reference for the soundings and depth contours represented in the nautical charts, as well as the tide height forecasts that are published in the Tide Tables of the Hydrographic Institute.

ZH Portugal Depth (Vertical CRS) - EPSG: 10349

For navigation safety purposes, hydrographic data must be reduced to ZH Portugal depth, in depths down to 200 m.

Depths are represented in metres (m) by positive values  below the ZH, and negative values above the ZH.

The relationship between ZH Portugal (depth) and vertical datum (height) depends on location and LAT. 

In Portugal,  for each location, the vertical datum (height), is established by DGT, and corresponds to an adopted mean sea level (MSL), in that location on a referred date.

For each geographic extension, the value of the correspondence between the ZH Portugal (depth) and the respective Vertical CRS (height) can be consulted HERE.

Similarly, for Lusophone African countries, there are several ZH's established in relation to the tide stations implemented.

For more information see "Tide Table" published annually by the Hydrographic Institute and available for download (free) at Loja do Navegante (http://loja.hidrografico.pt/).

The use of this EPSG code: 10349 - ZH Portugal Depth (Vertical CRS) is recommended by the Instituto Hidrográfico, for all hydrographic data that are related to ZH, in mainland Portugal and in the Azores and Madeira archipelagos.