Differences Between the isa and ehorizon Layer Groups in OCM Maps

Summary
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The Optimized Client Map (OCM) catalog organizes map content into Layer Groups, each designed to support specific use cases. The ehorizon and isa layer groups provide different subsets of map content optimized for Electronic Horizon and Intelligent Speed Assistance (ISA) applications respectively.

Applies To
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Product: Optimized Client Map (OCM)
Layer Groups: ehorizon, isa
Use Cases: Electronic Horizon, Intelligent Speed Assistance (ISA)

Customer Symptoms
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Customers may want to:

Understand the difference between the ehorizon and isa layer groups.
Determine which layer group is appropriate for a particular application.
Understand the content, dependencies, and limitations of each layer group.
Understand how indexing and content filtering affect ISA use cases.

Explanation
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In the OCM Developer Guide, a Layer Group is a collection of layers that supports one or more related use cases. Layer Groups are represented in the catalog as catalog layers containing bundled data tiles. Each catalog layer partition contains a tile subtree that includes only the layers associated with that Layer Group.

The ehorizon Layer Group is designed to support Electronic Horizon functionality.

Must be used together with the routing Layer Group.
Can optionally be combined with the adas Layer Group.
Contains foundational map content, geometry information, and ADAS-related attributes required for Electronic Horizon use cases.

The isa Layer Group is optimized for Intelligent Speed Assistance (ISA) use cases.

Provides a reduced content set to minimize data size.
Contains filtered copies of selected layers originating from the ehorizon and routingLayer Groups.
Excludes non-navigation content, such as layers intended exclusively for pedestrian use cases.
Uses the same schema as other Layer Groups, while preserving road segment local_ids.
Segment indexing may differ because content filtering removes some segments from the data set.

Expected Behavior
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Applications requiring complete Electronic Horizon functionality should use the ehorizon Layer Group together with the required dependencies.

Applications focused on Intelligent Speed Assistance can use the isa Layer Group to reduce data volume while maintaining compatibility with the standard OCM schema.

Impact
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Understanding the differences between the Layer Groups helps:

Select the correct content package for a specific use case.
Reduce storage and bandwidth requirements for ISA implementations.
Ensure required dependencies are included for Electronic Horizon functionality.
Avoid confusion regarding indexing differences and content availability.

Coverage
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The information applies to OCM Layer Groups available in HERE map data products and is not limited to a specific geographic region.

Recommendations
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Use ehorizon when complete Electronic Horizon data is required.
Use isa when implementing Intelligent Speed Assistance and minimizing data size is important.
Review Layer Group dependencies before deployment.
* Do not rely on segment indexing consistency between ISA and non-ISA content sets.

Additional Information
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For detailed technical information about Layer Groups, schema definitions, and supported use cases, refer to the OCM Developer Guide.

Keywords / Tags
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OCM, Optimized Client Map, eHorizon, ehorizon, ISA, Intelligent Speed Assistance, Layer Group, Routing Layer Group, ADAS, Electronic Horizon, local_ids, Segment Indexing, Navigation Content, Map Data, Developer Guide