Geometry of Bridge aggregations

[TDYCARHugh]

While implementing enhanced mappings from S-57 to S-101 2.0 we discovered some issues we felt should be considered.

The intent for adding a geometry to bridges and other aggregations was to simplify/support dynamic name placement on the ECDIS. The intent being that the ECDIS could place the name in the visible portion of the geometry and not have to do extra processing to construct the aggregated geometry.

We would like to draw attention to a statement about bridges in DCEG 6.6.1
"If it is required to encode the name of a bridge over navigable water, the Bridge should be encoded using geometry of type curve or surface, associated with all relevant components of the bridge using the association Bridge Aggregation. The extent of the geometry of the Bridge should utilize the geometry of all the components of the bridge so as to cover its full extent."

If followed literally this adds complexity and problems which were not intended or fully considered.

• making a geometry from all the components would require a union operation to avoid overlapping or self crossing geometry.
• edges of the spans and supporting features would need to be cut up and shared where they are used by the Bridge geometry.
• the resulting geometry of the Bridge will include additional complexity which is not really needed for the intended purpose of placing a bridge name.
• supporting structures sometimes support more than one bridge, such as parallel train and road bridges. Overlapping Bridge geometries in such a case will increase the chance of conflicting bridge names.
• There are also some cases where bridge spans might overlap such as on/off ramps that circle over/under each other.

Allowing producers to make the bridge geometry using only a portion of the full bridge aggregation would allow the producer to simplify the geometry needed to indicate where the name should be placed.

Typically it would make sense to build the bridge geometry from only the spans and perhaps only for spans which overlap water.

The paragraph as it is worded could be interpreted like this "encoded using geometry of type curve or surface, associated with all relevant components of the bridge" will just use spans for the bridge geometry since they are the only relevant components for the purpose of name placement. If the aggregation does not include the structures then the second sentence could still be met by using all the spans in the aggregation. We have chosen to make the Bridge geometry from the span features but include all the spans and supporting features in the aggregation.

We propose that the paragraph be reworded or clarified.

Consideration and clarification on this matter will also need to be propagated to S-65 conversion guidance, S-158 validation tests and S-164 test datasets.