# ENC Accuracy

Zones of Confidence (ZOC) define the accuracy of underwater feature depiction on ENCs in accordance with the IHO S-57 standard. CATZOC applies to both the horizontal coordinates of underwater objects and the depth measurement error.

![](images/ZOC_en.png)

The ZOC category is specified in ENC metadata and is used to assess the reliability of chart data during voyage planning and execution.

A single ENC cell may contain ZOCs of different categories.

![](images/CATZOC.png)

```{important}
When planning and monitoring a route, take into account the ZOC category of the navigation areas. Categories A1/A2 provide the highest accuracy of bathymetric data.

To automatically account for **CATZOC** and automatically calculate **XTD**, enable CATZOC consideration in the {doc}`route settings </sections/routes/route-creation/index>` before laying the route.
```

```{note}
The data is provided in accordance with **IHO Publication S-57** (International Hydrographic Organization Standard for Digital Hydrographic Data) and **IHO Publication S-67**. Mariners' Guide to Accuracy of Depth Information in Electronic Navigational Charts (ENC).
```

## ENC Underwater Feature Accuracy
| ZOC Category | Position Accuracy | Depth Accuracy |
|:------------:|:-----------------:|:---------------|
| **A1** | ± 5 m + 5% of depth | ± 0.5 m + 1% of depth<br><br>**Examples:**<br>Depth 10 m: ± 0.6 m<br>Depth 30 m: ± 0.8 m<br>Depth 100 m: ± 1.5 m<br>Depth 1000 m: ± 10.5 m |
| **A2** | ± 20 m | ± 1.0 m + 2% of depth<br><br>**Examples:**<br>Depth 10 m: ± 1.2 m<br>Depth 30 m: ± 1.6 m<br>Depth 100 m: ± 3.0 m<br>Depth 1000 m: ± 21.0 m |
| **B** | ± 50 m | ± 1.0 m + 2% of depth<br><br>**Examples:**<br>Depth 10 m: ± 1.2 m<br>Depth 30 m: ± 1.6 m<br>Depth 100 m: ± 3.0 m<br>Depth 1000 m: ± 21.0 m |
| **C** | ± 500 m | ± 2.0 m + 5% of depth<br><br>**Examples:**<br>Depth 10 m: ± 2.5 m<br>Depth 30 m: ± 3.5 m<br>Depth 100 m: ± 7.0 m<br>Depth 1000 m: ± 52.0 m |
| **D** | Greater than ZOC C | Greater than ZOC C |
| **U** | Unassessed — bathymetric data quality not evaluated | Errors not defined |

```{warning}
Categories **D** and **U** indicate poor quality or unassessed bathymetric data. Extra caution and the use of additional navigational aids are required in such areas.
```

![](images/ZOC_ACC_en.png)
