---
type: Rendering Constraint
title: Explicit raster replacement
description: Deterministic first-valid numerical inspection for explicitly grouped raster sources.
tags: [rendering, raster, composition, inspection]
visibility: public
---

# Meaning

An explicit raster replacement group selects one numerical contribution at a
point. Higher priority is tried first; an unavailable or NoData contribution
falls back to the next member. A valid zero and a negative value are data.
Values from different providers are not averaged. The chosen value retains
its actual Layer and provider attribution in hover and Details.

Matching fields or overlapping Layers alone never create a replacement group.
Independent Layers can intentionally show separate values for comparison.

# Ownership

Each participating [Map Connection](/presentation/map-connection.md) saves its
membership in its versioned query: a portable map-local group and member key,
semantic contract/version, numeric field and unit, priority, an inclusive
minimum and exclusive maximum zoom, and optional geographic bounds.
The database validates the selected Channel's field and unit and compatible
Data Product membership. The current cross-provider implementation accepts
static imagery with a separate, single-field numerical inspection plane.
Its imagery declares sampling `none`; encoded scalar and temporal data require
their own composition adapter, as PM2.5 already has.

Saved queries survive reload, copies and portable recipe import/export.
Catalog changes do not silently regroup existing maps. Per-Channel delivery,
acquisition, quotas and access remain independent. Group membership never
authorizes another Source or Channel.

# Rendering and inspection

The baseline raster stays below regional imagery. Provider alpha defines
pixel coverage, so transparent or failed regional tiles expose the baseline.
The contribution's saved zoom interval is shared by rendering and inspection;
optional bounds constrain raster tile coverage and candidate inspection.
Native Mapbox raster surfaces preserve globe, morph and Mercator projection.

Inspection tries eligible members in deterministic priority/member order.
Hidden and out-of-zoom members do not participate. A valid preferred result
does not request or show the baseline. NoData, provider errors and timeouts
advance to the fallback; cancellation stops the whole point request.

PM2.5 Auto uses the same first-valid resolver through its existing source-local
spatial-fallback declaration. Europe/Global modes, common forecast timestamps,
palette and legend stay source-local. This is distinct from the static
cross-provider connection group.

This bounded increment keeps separate provider controls. A single logical
Layer with one control across providers, temporal cross-provider matching and
mixed raster/vector compositions require additional contracts.