Differences Between the isa and ehorizon Layer Groups in OCM Maps

In the Optimized Client Map (OCM) developer guide, a Layer Group is a method of aggregating multiple layers of data needed for one or more main use cases. The OCM catalog represents layer groups as catalog layers. In the OCM catalog, layers (layer groups) contain bundled data tiles. Each catalog layer partition contains a tile subtree that only includes the layers associated with that layer group.

isa and ehorizon are both important OCM Layer Groups, but they have significant differences in terms of their optimization goals and included content.

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1. Detailed Overview of the ehorizon Layer Group
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The ehorizon Layer Group is intended to provide layers required for "Electronic Horizon" functionality.

Purpose and Dependencies: This layer group must be used alongside the routing layer group. Additionally, it can optionally be used with the adas layer group.
Content Characteristics: ehorizon typically includes foundational map data, ADAS-related attributes, and geometry information essential for predictive and situational awareness.

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2. Detailed Overview of the isa Layer Group
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The isa Layer Group consists of layers optimized for ISA (Intelligent Speed Assistance).

Objective: Its primary goal is to reduce the size of the Layer Group, particularly in scenarios where ADAS functionality is not required.
Data Source and Filtering: The isa layer group includes filtered copies of layers derived from the ehorizon and routing layer groups.
Content Exclusion: To minimize size, the isa layer group removes non-navigation layers, such as those provided exclusively for pedestrians.
Indexing and Consistency: Due to data filtering and removal, the segment indexing within isa layers may not align with non-isa layers. However, the schema remains the same, and the road segment's local_ids (local identifiers) will match.

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3. Summary of Core Differences
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| Feature | ehorizon Layer Group | isa Layer Group |
| --- | --- | --- |
| Primary Purpose | Supports Electronic Horizon functionality (can be used along with optional adas layer group). | Optimized specifically for ISA, focusing on size reduction. |
| Content Source | Contains comprehensive original and interoperability data layers. | Is a filtered copy of layers derived from ehorizon and routing layers. |
| Data Scope | Provides full data set needed for general eHorizon functions. | Removes non-navigation layers to decrease package size. |
| Segment Indexing | Standard indexing within its context. | Segment indexing is not aligned with non-isa layers (due to filtering). |
| Schema/Local ID | Standard schema structure. | Schema remains the same; road segment local_ids still match. |