Proposal for maps extension

[m-mohr]

No description provided.

[emmanuelmathot]

Thanks @m-mohr , this is a solid base and very close to what we discussed. Overall structure looks right to me. Some points in sub comments/suggestions

[emmanuelmathot]

Nothing prevents several maps from having the default role. Maps are an object, so there's no reliable "first" to fall back on either.

Suggested change

	| roles | \[string] | `default` marks the map that clients show by default. |
	| roles | \[string] | `default` marks the map that clients show by default. At most one map SHOULD have this role. If none has it, clients choose one. |

[emmanuelmathot]

If a map mixes a datacube asset (time) and a WMS (TIME), which key applies, and is matching case-sensitive? Keys should always be cube:dimensions names and clients map them to service parameters (WMS TIME/ELEVATION, WMTS dimension identifiers).

This also relates to stac-extensions/render#19 (fixing non-spatial dimensions of a datacube).

[emmanuelmathot]

Say how the parent is found, and that it only applies to Items.

Suggested change

	The keys and `id`s are looked up in the document first and then in the parent Collection.
	The keys and `id`s are looked up in the document first and then, for Items, in the parent Collection (found through the `collection` link).

[emmanuelmathot]

Worth stating the bounds explicitly. OpenLayers uses minResolution inclusive and maxResolution exclusive; matching it avoids clients disagreeing at exactly the boundary.

Suggested change

	Clients SHOULD hide the layer outside of this range.
	If both are provided, `min_resolution` must be lower than or equal to `max_resolution`.
	Clients SHOULD hide the layer outside of this range.
	`min_resolution` is inclusive and `max_resolution` is exclusive, i.e. the layer is shown if
	`min_resolution <= resolution < max_resolution`, as in OpenLayers.
	If both are provided, `min_resolution` must be lower than `max_resolution`.

[emmanuelmathot]

A map can have its own bbox. For a Collection with a global extent and a regional map, measuring at the entity's centre would convert at (0,0) instead of the region.

Suggested change

	The resolution is the **distance on the ground in meters per display (CSS) pixel**, measured at the center of the
	spatial extent of the STAC entity (`bbox` in Items, the first bounding box of the spatial extent in Collections).
	This makes the value independent of the projection of the map.
	The resolution is the **distance on the ground in meters per display (CSS) pixel**, measured at the center of the
	map's `bbox` if provided, otherwise of the spatial extent of the STAC entity
	(`bbox` in Items, the first bounding box of the spatial extent in Collections).
	This makes the value independent of the projection of the map.

[emmanuelmathot]

Suggested change

	| style | [Style Reference Object](#style-reference-object) | The portrayal that is applied to the data of an `asset` or `link` source. |
	| style | [Style Reference Object](#style-reference-object) | The stylesheet that is applied to the data of an `asset` or `link` source. |

[emmanuelmathot]

Keep render as a source only. A render always references its own data (assets is required in the Render Extension v2.1.0), so a render without data isn't valid, and using it as a style would mean its assets are silently replaced by the layer's source. Either an asset (optionally with a stylesheet) or a render, not both.

Suggested change

	| Field Name | Type | Description |
	| ---------- | ------------------- | ----------- |
	| link | string\|Link Object | The `id` of a stylesheet link in the document, or an embedded stylesheet link. |
	| render | string | The key of a render that is applied to the data of the layer's source. |
	Each Style Reference Object MUST provide exactly one of `link` and `render`.
	**link**: Stylesheet links use the relation type `stylesheet`, which is
	[registered by IANA](https://www.iana.org/assignments/link-relations/link-relations.xhtml).
	The `type` is REQUIRED and identifies the encoding of the stylesheet, preferably with the media types of
	[OGC API - Styles](https://docs.ogc.org/DRAFTS/20-009.html), e.g. `application/vnd.mapbox.style+json` or `application/vnd.ogc.sld+xml`.
	**render**: Only renders that don't reference any data themselves can be applied to another source.
	| Field Name | Type | Description |
	| ---------- | ------------------- | ----------- |
	| link | string\|Link Object | **REQUIRED**. The `id` of a stylesheet link in the document, or an embedded stylesheet link. |
	**link**: Stylesheet links use the relation type `stylesheet`, which is
	[registered by IANA](https://www.iana.org/assignments/link-relations/link-relations.xhtml).
	The `type` is REQUIRED and identifies the encoding of the stylesheet, preferably with the media types of
	[OGC API - Styles](https://docs.ogc.org/DRAFTS/20-009.html), e.g. `application/vnd.mapbox.style+json` or `application/vnd.ogc.sld+xml`.
	A render can't be used as a style. It always references its own data and is used as a layer source instead,
	see `render` in the [Layer Object](#layer-object).

[emmanuelmathot]

Drop the two render-as-style rows, following the change to the Style Reference Object.

Suggested change

	| `{"asset": "fields", "style": {"render": "fields"}}` | The asset, styled by a render. |
	| `{"render": "ndvi"}` | A render with its own data, rendered by the client or delivered by a web map link that references the render. |
	| `{"link": "topo-wms"}` | A service as it's delivered. |
	| `{"link": "fields-pmtiles", "style": {"link": "fields-outline"}}` | The data of a service, styled by a stylesheet. |
	| `{"link": "fields-pmtiles", "style": {"render": "fields"}}` | The data of a service, styled by a render. |
	| `{"render": "ndvi"}` | A render with its own data, rendered by the client or delivered by a web map link that references the render. |
	| `{"link": "topo-wms"}` | A service as it's delivered. |
	| `{"link": "fields-pmtiles", "style": {"link": "fields-outline"}}` | The data of a service, styled by a stylesheet. |

[emmanuelmathot]

Style is stylesheet-only, see the README comment on the Style Reference Object.

Suggested change

	"oneOf": [
	{
	"required": [
	"link"
	]
	},
	{
	"required": [
	"render"
	]
	}
	],
	"required": [
	"link"
	],

[emmanuelmathot]

Suggested change

	},
	"render": {
	"type": "string",
	"minLength": 1
	}
	}