diff --git a/CHANGELOG.md b/CHANGELOG.md index f9f0f2c6..1a04968d 100644 --- a/CHANGELOG.md +++ b/CHANGELOG.md @@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0. ### Added - Summaries are allowed to specify JSON Schema in addition to ranges and sets of values. ([#1045](https://github.com/radiantearth/stac-spec/issues/1045)) +- Added `preview` relation type for interoperable thumbnails to best practices. ([#1090](https://github.com/radiantearth/stac-spec/issues/1090)) ### Changed @@ -17,6 +18,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0. - Updated examples for automatic collection creation from code and validation ([#1080](https://github.com/radiantearth/stac-spec/pull/1080) - Clarified that stac_extensions should also list extensions that are used in Collection summaries. ([#1077](https://github.com/radiantearth/stac-spec/issues/1077)) - The Stats Object for Summaries has been renamed to Range Object (no functional change). +- `changed`, `created` (common metadata) and temporal extents (collections): Timestamps must be always in UTC ([#1095](https://github.com/radiantearth/stac-spec/issues/1095)) ## [v1.0.0-rc.2] - 2021-03-30 diff --git a/best-practices.md b/best-practices.md index 77e01e6b..8ab6a3fb 100644 --- a/best-practices.md +++ b/best-practices.md @@ -45,6 +45,7 @@ - [Keep catalogs in sync with cloud notification and queue services](#keep-catalogs-in-sync-with-cloud-notification-and-queue-services) - [How to Differentiate STAC Files](#how-to-differentiate-stac-files) + This document makes a number of recommendations for creating real world SpatioTemporal Asset Catalogs. None of them are required to meet the core specification, but following these practices will make life easier for client tooling and for users. They come about from practical experience of implementors and introduce a bit more 'constraint' for @@ -71,8 +72,8 @@ So to enable all the great web tools (like [stacindex.org](http://stacindex.org) [Google Cloud Storage](https://cloud.google.com/storage/docs/cross-origin), or [Apache Server](https://enable-cors.org/server_apache.html). Many more are listed on [enable-cors.org](https://enable-cors.org/server.html). We recommend enabling CORS for all requests ('\*'), so that diverse online tools can access your data. If you aren't sure if your server has CORS enabled you can use -[test-cors.org](https://www.test-cors.org/). Enter the URL of your STAC root [Catalog](catalog-spec/catalog-spec.md) JSON -and make sure it gets a response. +[test-cors.org](https://www.test-cors.org/). Enter the URL of your STAC root [Catalog](catalog-spec/catalog-spec.md) or +[Collection](collection-spec/collection-spec.md) JSON and make sure it gets a response. ### STAC on the Web @@ -81,7 +82,7 @@ surprised that there is nothing about HTML in the entire specification. This is should be on web pages without ending up with very bad looking pages. But the importance of having web-accessible versions of every STAC Item is paramount. -The main recommendation is to have an HTML page for every single STAC Item and Catalog. They should be visually pleasing, +The main recommendation is to have an HTML page for every single STAC Item, Catalog and Collection. They should be visually pleasing, crawlable by search engines and ideally interactive. The current best practice is to use a tool in the STAC ecosystem called [STAC Browser](https://github.com/radiantearth/stac-browser/). It can crawl most any valid STAC implementation and generate unique web pages for each Item and Catalog (or Collection). While it has a default look and feel, the design can easily be @@ -293,7 +294,7 @@ following table lists some of the most common ones you may encounter or use. | `image/png` | Visual PNGs (e.g. thumbnails) | | `image/jpeg` | Visual JPEGs (e.g. thumbnails, oblique) | | `text/xml` or `application/xml` | XML metadata [RFC 7303](https://www.ietf.org/rfc/rfc7303.txt) | -| `application/json` | A JSON file (often metadata, or [labels](https://github.com/radiantearth/stac-spec/tree/master/extensions/label#labels-required)) | +| `application/json` | A JSON file (often metadata, or [labels](https://github.com/radiantearth/stac-spec/tree/master/extensions/label#labels-required)) | | `text/plain` | Plain text (often metadata) | | `application/geo+json` | [GeoJSON](https://geojson.org/) | | `application/geopackage+sqlite3` | [GeoPackage](https://www.geopackage.org/) | @@ -393,6 +394,10 @@ file that just has the bands needed for display ## Catalog & Collection Practices +*Note: This section uses the term 'Catalog' (with an uppercase C) to refer to the JSON entity specified in the +[Catalog spec](catalog-spec/catalog-spec.md), and 'catalog' (with a lowercase c) to refer to any full STAC implementation, +which can be any mix of Catalogs Collections and Items.* + ### Static and Dynamic Catalogs As mentioned in the main [overview](overview.md), there are two main types of catalogs - static @@ -446,7 +451,7 @@ providers, and users could browse down to both. The leaf Items should just be li ### Catalog Layout -Creating a catalog involves a number of decisions as to what folder structure to use to represent sub-catalogs, items +Creating a catalog involves a number of decisions as to what folder structure to use to represent sub-catalogs, Items and assets, and how to name them. The specification leaves this totally open, and you can link things as you want. But it is recommended to be thoughtful about the organization of sub-catalogs, putting them into a structure that a person might reasonably browse (since they likely will with [STAC on the Web](#stac-on-the-web) recommendations). For example @@ -463,14 +468,14 @@ if you follow these recommendations. 1. Root documents (Catalogs / Collections) should be at the root of a directory tree containing the static catalog. 2. Catalogs should be named `catalog.json` and Collections should be named `collection.json`. 3. Items should be named `.json`. -4. Sub-Catalogs should be stored in subdirectories of their parent +4. Sub-Catalogs or sub-Collections should be stored in subdirectories of their parent (and only 1 subdirectory deeper than a document's parent, e.g. `.../sample/sub1/catalog.json`). -5. Items should be stored in subdirectories of their parent Catalog. -This means that each Item and its assets are contained in a unique subdirectory. -6. Limit the number of Items in a Catalog or sub-Catalog, grouping / partitioning as relevant to the dataset. +5. Items should be stored in subdirectories of their parent Catalog or Collection. + This means that each Item and its assets are contained in a unique subdirectory. +6. Limit the number of Items in a Catalog or Collection, grouping / partitioning as relevant to the dataset. 7. Use structural elements (Catalog and Collection) consistently across each 'level' of your hierarchy. For example, if levels 2 and 4 of the hierarchy only contain Collections, -don't add a Catalog at levels 2 and 4. + don't add a Catalog at levels 2 and 4. #### Dynamic Catalog Layout @@ -483,7 +488,7 @@ different sub-catalog organization structures. For example one catalog could div by providers, and users could browse down to both. The leaf Items should just be linked to in a single canonical location (or at least use a rel link that indicates the location of the canonical one). It is recommended that dynamic catalogs provide multiple 'views' to allow users to navigate in a way that makes sense to them, providing multiple 'sub-catalogs' -from the root Catalog that enable different paths to browse (country/state, date/time, constellation/satellite, etc). But the +from the root that enable different paths to browse (country/state, date/time, constellation/satellite, etc). But the canonical 'rel' link should be used to designate the primary location of the Item to search engine crawlers. #### Mixing STAC Versions @@ -608,9 +613,9 @@ implement it. #### Relative Published Catalog This is a self-contained catalog as described above, except it includes an absolute `self` link at -the root catalog, to identify its online location. This is designed so that a self-contained catalog (of either type, with its +the root to identify its online location. This is designed so that a self-contained catalog (of either type, with its assets or just metadata) can be 'published' online -by just adding one field (the self link) to its root catalog. All the other links should remain the same. The resulting catalog +by just adding one field (the self link) to its root (Catalog or Collection). All the other links should remain the same. The resulting catalog is no longer compliant with the self-contained catalog recommendations, but instead transforms into a 'relative published catalog'. With this, a client may resolve Item and sub-catalog self links by traversing parent and root links, but requires reading multiple sources to achieve this. @@ -627,13 +632,14 @@ with the `type` field) to communicate the structure and content of related entit Types](https://www.iana.org/assignments/link-relations/link-relations.xhtml) as much as possible. The following table describes a number of the common official relations that are used in production STAC implementations. -| Type | Description | -| ------------ | ------------------------------------------------------------ | +| Type | Description | +| --------- | ------------------------------------------------------------ | | alternate | It is recommended that STAC Items are also available as HTML, and should use this rel with `"type" : "text/html"` to tell clients where they can get a version of the Item or Collection to view in a browser. See [STAC on the Web in Best Practices](#stac-on-the-web) for more information. | | canonical | The URL of the [canonical](https://en.wikipedia.org/wiki/Canonical_link_element) version of the Item or Collection. API responses and copies of catalogs should use this to inform users that they are direct copy of another STAC Item, using the canonical rel to refer back to the primary location. | | via | The URL of the source metadata that this STAC Item or Collection is created from. Used similarly to canonical, but refers back to a non-STAC record (Landsat MTL, Sentinel tileInfo.json, etc) | -| prev | Indicates that the link's context is a part of a series, and that the previous in the series is the link target. Typically used in STAC by API's, to return smaller groups of Items or Catalogs. | -| next | Indicates that the link's context is a part of a series, and that the next in the series is the link target. Typically used in STAC by API's, to return smaller groups of Items or Catalogs. | +| prev | Indicates that the link's context is a part of a series, and that the previous in the series is the link target. Typically used in STAC by API's, to return smaller groups of Items or Catalogs/Collections. | +| next | Indicates that the link's context is a part of a series, and that the next in the series is the link target. Typically used in STAC by API's, to return smaller groups of Items or Catalogs/Collections. | +| preview | Refers to a resource that serves as a preview (see [RFC 6903, sec. 3](https://tools.ietf.org/html/rfc6903#section-3)), usually a lower resolution thumbnail. In STAC this would usually be the same URL as the [thumbnail](#thumbnail) asset, but adding it as a link in addition enables OGC API clients that can't read assets to make use of it. It also adds support for thumbnails to STAC Catalogs as they can't list assets. | ### Versioning for Catalogs diff --git a/catalog-spec/catalog-spec.md b/catalog-spec/catalog-spec.md index 6cdd6d55..c30eb781 100644 --- a/catalog-spec/catalog-spec.md +++ b/catalog-spec/catalog-spec.md @@ -36,8 +36,9 @@ and fields to be compliant. This Catalog specification primarily defines a structure for information to be discoverable. Any use that is publishing a set of related spatiotemporal assets is strongly recommended to also use the STAC Collection specification to provide additional information about the set of Items -contained in a Catalog, in order to give contextual information to aid in discovery. Every STAC Collection is -also a valid STAC Catalog. +contained in a Catalog, in order to give contextual information to aid in discovery. +STAC Collections all have the same fields as STAC Catalogs, but with different allowed +values for `type` and `stac_extensions`. ## Catalog fields @@ -89,11 +90,11 @@ The following types are commonly used as `rel` types in the Link Object of a STA | ------- | ----------- | | self | STRONGLY RECOMMENDED. *Absolute* URL to the location that the Catalog file can be found online, if available. This is particularly useful when in a download package that includes metadata, so that the downstream user can know where the data has come from. | | root | STRONGLY RECOMMENDED. URL to the root STAC Catalog or [Collection](../collection-spec/README.md). Catalogs should include a link to their root, even if it's the root and points to itself. | -| parent | URL to the parent STAC Catalog or Collection. Non-root Catalogs should include a link to their parent. | -| child | URL to a child STAC Catalog or Collection. | +| parent | URL to the parent STAC entity (Catalog or Collection). Non-root Catalogs should include a link to their parent. | +| child | URL to a child STAC entity (Catalog or Collection). | | item | URL to a STAC Item. | -**Note:** A link to at least one `item` or `child` Catalog is **REQUIRED**. +**Note:** A link to at least one `item` or `child` (Catalog or Collection) is **REQUIRED**. There are additional `rel` types in the [Using Relation Types](../best-practices.md#using-relation-types) best practice, but as they are more typically used in Collections, as Catalogs tend to just be used to structure STAC organization, so tend to just use diff --git a/collection-spec/collection-spec.md b/collection-spec/collection-spec.md index fa14e58f..f237bc4a 100644 --- a/collection-spec/collection-spec.md +++ b/collection-spec/collection-spec.md @@ -198,7 +198,7 @@ Clients only interested in the overall extent will only need to access the first It is recommended to only use multiple temporal extents if a union of them would then include a large uncovered time span (e.g. only having data for the years 2000, 2010 and 2020). -Each inner array consists of exactly two dates and times. +Each inner array consists of exactly two dates and times, each in UTC. Each date and time MUST be formatted according to [RFC 3339, section 5.6](https://tools.ietf.org/html/rfc3339#section-5.6). The temporal reference system is the Gregorian calendar. @@ -253,9 +253,9 @@ This is done where there is not a clear official option, or where STAC uses an o | Type | Description | | ------- | ------------------------------------------------------------ | | self | STRONGLY RECOMMENDED. *Absolute* URL to the location that the Collection file can be found online, if available. This is particularly useful when in a download package that includes metadata, so that the downstream user can know where the data has come from. | -| root | URL to the root STAC Catalog or Collection. Collections should include a link to their root, even if it's the root and points to itself. | -| parent | URL to the parent STAC Catalog or Collection. Non-root Collections should include a link to their parent. | -| child | URL to a child STAC Catalog or Collection. | +| root | URL to the root STAC entity (Catalog or Collection). Collections should include a link to their root, even if it's the root and points to itself. | +| parent | URL to the parent STAC entity (Catalog or Collection). Non-root Collections should include a link to their parent. | +| child | URL to a child STAC entity (Catalog or Collection). | | item | URL to a STAC Item. All Items linked from a Collection MUST refer back to its Collection with the [`collection` relation type](../item-spec/item-spec.md#relation-types). | | license | The license URL(s) for the Collection SHOULD be specified if the `license` field is set to `proprietary` or `various`. If there is no public license URL available, it is RECOMMENDED to put the license text in a separate file and link to this file. | | derived_from | URL to a STAC Collection that was used as input data in the creation of this Collection. See the note in [STAC Item](../item-spec/item-spec.md#derived_from) for more info. | diff --git a/collection-spec/json-schema/collection.json b/collection-spec/json-schema/collection.json index 422385f2..19bb9611 100644 --- a/collection-spec/json-schema/collection.json +++ b/collection-spec/json-schema/collection.json @@ -153,7 +153,8 @@ "string", "null" ], - "format": "date-time" + "format": "date-time", + "pattern": "(\\+00:00|Z)$" } } } diff --git a/item-spec/common-metadata.md b/item-spec/common-metadata.md index 07df2234..b9596c46 100644 --- a/item-spec/common-metadata.md +++ b/item-spec/common-metadata.md @@ -19,8 +19,8 @@ Various *examples* are available in the folder [`examples`](../examples/). *JSON Schemas* can be found in the folder [`json-schema`](json-schema/). By default, these fields are only included and validated against in the core [Item schema](json-schema/item.json). -Implementation of any of the fields is not required, -if the specifications allowing these fields to be used don't say differently. + +Implementation of any of the fields is not required, unless explicitly required by a specification using the field. For example, `datetime` is required in STAC Items. ## Basics @@ -43,13 +43,13 @@ Fields to provide additional temporal information such as ranges with a start an | Field Name | Type | Description | | ---------- | ------------ | ----------- | | datetime | string\|null | See the [Item Spec Fields](item-spec.md#properties-object) for more information. | -| created | string | Creation date and time of the corresponding data (see below). | -| updated | string | Date and time the corresponding data (see below) was updated last. | +| created | string | Creation date and time of the corresponding data (see below), in UTC. | +| updated | string | Date and time the corresponding data (see below) was updated last, in UTC. | All timestamps MUST be formatted according to [RFC 3339, section 5.6](https://tools.ietf.org/html/rfc3339#section-5.6). **created** and **updated** have different meaning depending on where they are used. -If those fields are available in the Item `properties`, it's referencing to the creation and update times of the metadata. +If those fields are available in the Item `properties`, they identify the creation and update times of the metadata. Having those fields in the Item `assets` refers to the creation and update times of the actual data linked to in the Asset Object. *NOTE: There are more date and time related fields available in the [Timestamps @@ -58,12 +58,12 @@ extension](https://github.com/stac-extensions/timestamps), which is not an offic ### Date and Time Range While a STAC Item can have a nominal datetime describing the capture, these properties allow an Item to have a range -of capture dates and times. An example of this is the [MODIS 16 day vegetation index product.](https://lpdaac.usgs.gov/products/mod13q1v006/). -The datetime property in a STAC Item and these fields are not mutually exclusive. +of capture dates and times. An example of this is the [MODIS 16 day vegetation index product](https://lpdaac.usgs.gov/products/mod13q1v006/). -**Important:** Using one of the fields REQUIRES to include the other field as well to enable a user to search STAC records by the provided times. +**Important:** Using one of the fields REQUIRES inclusion of the other field as well to enable a user to search STAC records by the provided times. So if you use `start_datetime` you need to add `end_datetime` and vice-versa. Both fields are also REQUIRED if the `datetime` field is set to `null`. +The datetime property in a STAC Item and these fields are not mutually exclusive. | Field Name | Type | Description | | -------------- | ------ | ------------------------------------------------------------ | @@ -126,11 +126,11 @@ The provider's role(s) can be one or more of the following elements: - *producer*: The producer of the data is the provider that initially captured and processed the source data, e.g. ESA for Sentinel-2 data. - *processor*: A processor is any provider who processed data to a derived product. - *host*: The host is the actual provider offering the data on their storage. - There should be no more than one host, specified as last element of the list. + There should be no more than one host, specified as the last element of the provider list. ## Instrument -Adds metadata specifying a platform and instrument used in a data collection mission. These fields will often be combined +Adds metadata specifying a platform and instrument used in a data collection mission. These fields will often be combined with domain-specific extensions that describe the actual data, such as the `eo` or `sar` extensions. - [JSON Schema](json-schema/instrument.json) @@ -147,10 +147,10 @@ with domain-specific extensions that describe the actual data, such as the `eo` #### platform -The unique name of the specific platform the instrument is attached to. For satellites this would -be the name of the satellite, whereas for drones this would be a unique name for the drone. Examples include -`landsat-8` (Landsat-8), `sentinel-2a` and `sentinel-2b` (Sentinel-2), `terra` and `aqua` (part of NASA EOS, -carrying the MODIS instruments), `mycorp-uav-034` (hypothetical drone name), and `worldview02` +The unique name of the specific platform the instrument is attached to. For satellites this would +be the name of the satellite, whereas for drones this would be a unique name for the drone. Examples include +`landsat-8` (Landsat-8), `sentinel-2a` and `sentinel-2b` (Sentinel-2), `terra` and `aqua` (part of NASA EOS, +carrying the MODIS instruments), `mycorp-uav-034` (hypothetical drone name), and `worldview02` (Maxar/DigitalGlobe WorldView-2). #### instruments @@ -162,15 +162,15 @@ specified as `['oli', 'tirs']`. Other instrument examples include `msi` (Sentine #### constellation -The name of a logical collection of one or more platforms that have similar payloads and have -their orbits arranged in a way to increase the temporal resolution of acquisitions of data with similar geometric and -radiometric characteristics. This field allows users to search for related data sets without the need to specify which -specific platform the data came from, for example, from either of the Sentinel-2 satellites. Examples include `landsat-8` +The name of a logical collection of one or more platforms that have similar payloads and have +their orbits arranged in a way to increase the temporal resolution of acquisitions of data with similar geometric and +radiometric characteristics. This field allows users to search for related data sets without the need to specify which +specific platform the data came from, for example, from either of the Sentinel-2 satellites. Examples include `landsat-8` (Landsat-8, a constellation consisting of a single platform), `sentinel-2` -([Sentinel-2](https://www.esa.int/Our_Activities/Observing_the_Earth/Copernicus/Sentinel-2/Satellite_constellation)), +([Sentinel-2](https://www.esa.int/Our_Activities/Observing_the_Earth/Copernicus/Sentinel-2/Satellite_constellation)), `rapideye` (operated by Planet Labs), and `modis` (NASA EOS satellites Aqua and Terra). In the case of `modis`, this -is technically referring to a pair of sensors on two different satellites, whose data is combined into a series of -related products. Additionally, the Aqua satellite is technically part of the A-Train constellation and Terra is not +is technically referring to a pair of sensors on two different satellites, whose data is combined into a series of +related products. Additionally, the Aqua satellite is technically part of the A-Train constellation and Terra is not part of a constellation, but these are combined to form the logical collection referred to as MODIS. #### mission @@ -184,10 +184,10 @@ data collection. The nominal Ground Sample Distance for the data, as measured in meters on the ground. There are many definitions of GSD. The value of this field should be related to the spatial resolution at the sensor, rather than the pixel size of images after orthorectification, pansharpening, or scaling. -The GSD of a sensor can vary depending on off-nadir and wavelength, so it is at the discretion of the implementer -to decide which value most accurately represents the GSD. For example, Landsat8 optical and short-wave IR bands -are all 30 meters, but the panchromatic band is 15 meters. The -`gsd` should be 30 meters in this case because that is nominal spatial resolution at the sensor. The Planet +The GSD of a sensor can vary depending on geometry (off-nadir / grazing angle) and wavelength, so it is at the +discretion of the implementer to decide which value most accurately represents the GSD. For example, Landsat8 +optical and short-wave IR bands are all 30 meters, but the panchromatic band is 15 meters. The +`gsd` should be 30 meters in this case because that is the nominal spatial resolution at the sensor. The Planet PlanetScope Ortho Tile Product has an `gsd` of 3.7 (or 4 if rounding), even though the pixel size of the images is 3.125. For example, one might choose for WorldView-2 the Multispectral 20° off-nadir value of 2.07 and for WorldView-3 the Multispectral 20° off-nadir value of 1.38. diff --git a/item-spec/item-spec.md b/item-spec/item-spec.md index 5f609c52..0ea52c7a 100644 --- a/item-spec/item-spec.md +++ b/item-spec/item-spec.md @@ -84,7 +84,7 @@ It is important that an Item identifier is unique within a Collection, and that [Collection identifier](../collection-spec/collection-spec.md#id) in turn is unique globally. Then the two can be combined to give a globally unique identifier. Items are *[strongly recommended](#collections)* to have Collections, and not having one makes it more difficult to be used in the wider STAC ecosystem. -If an Item does not have a Collection, then the Item identifier should be unique within its root Catalog. +If an Item does not have a Collection, then the Item identifier should be unique within its root Catalog or root Collection. As most geospatial assets are already uniquely defined by some identification scheme from the data provider it is recommended to simply use that ID. @@ -127,7 +127,7 @@ resources below. | Field Name | Type | Description | | ---------- | ------------ | ------------------------------------------------------------ | -| datetime | string\|null | **REQUIRED.** The searchable date and time of the assets, in UTC. It is formatted according to [RFC 3339, section 5.6](https://tools.ietf.org/html/rfc3339#section-5.6). `null` is allowed, but requires `start_datetime` and `end_datetime` from [common metadata](common-metadata.md#date-and-time-range) to be set. | +| datetime | string\|null | **REQUIRED.** The searchable date and time of the assets, which must be in UTC. It is formatted according to [RFC 3339, section 5.6](https://tools.ietf.org/html/rfc3339#section-5.6). `null` is allowed, but requires `start_datetime` and `end_datetime` from [common metadata](common-metadata.md#date-and-time-range) to be set. | #### datetime @@ -136,7 +136,8 @@ or representative time in the case of assets that are combined together. Though complex thing to capture, for this purpose keep in mind the STAC spec is primarily searching for data, so use whatever single date and time is most useful for a user to search for. STAC content extensions may further specify the meaning of the main `datetime` field, and many will also add more -datetime fields. +datetime fields. **All times in STAC metadata should be in [Coordinated Universal +Time](https://en.wikipedia.org/wiki/Coordinated_Universal_Time) (UTC).** If there's clearly no meaningful single 'nominal' time, it is allowed to use `null` instead. In this case it is **required** to specify a temporal interval with the fields `start_datetime` and `end_datetime` from [common metadata](common-metadata.md#date-and-time-range). For example, if @@ -192,8 +193,8 @@ This happens where there is not a clear official option, or where STAC uses an o | Type | Description | | ------------ | ------------------------------------------------------------ | | self | STRONGLY RECOMMENDED. *Absolute* URL to the Item if it is available at a public URL. This is particularly useful when in a download package that includes metadata, so that the downstream user can know where the data has come from. | -| root | URL to the root STAC Catalog or Collection. | -| parent | URL to the parent STAC Catalog or Collection. | +| root | URL to the root STAC entity (Catalog or Collection). | +| parent | URL to the parent STAC entity (Catalog or Collection). | | collection | STRONGLY RECOMMENDED. URL to a Collection. *Absolute* URLs should be used whenever possible. The referenced Collection is STRONGLY RECOMMENDED to implement the same STAC version as the Item. A link with this `rel` type is *required* if the `collection` field in properties is present. | | derived_from | URL to a STAC Item that was used as input data in the creation of this Item. | diff --git a/item-spec/json-schema/datetime.json b/item-spec/json-schema/datetime.json index 3a1480db..fa540a32 100644 --- a/item-spec/json-schema/datetime.json +++ b/item-spec/json-schema/datetime.json @@ -20,29 +20,34 @@ "title": "Date and Time", "description": "The searchable date/time of the assets, in UTC (Formatted in RFC 3339) ", "type": ["string", "null"], - "format": "date-time" + "format": "date-time", + "pattern": "(\\+00:00|Z)$" }, "start_datetime": { "title": "Start Date and Time", "description": "The searchable start date/time of the assets, in UTC (Formatted in RFC 3339) ", "type": "string", - "format": "date-time" + "format": "date-time", + "pattern": "(\\+00:00|Z)$" }, "end_datetime": { "title": "End Date and Time", "description": "The searchable end date/time of the assets, in UTC (Formatted in RFC 3339) ", "type": "string", - "format": "date-time" + "format": "date-time", + "pattern": "(\\+00:00|Z)$" }, "created": { "title": "Creation Time", "type": "string", - "format": "date-time" + "format": "date-time", + "pattern": "(\\+00:00|Z)$" }, "updated": { "title": "Last Update Time", "type": "string", - "format": "date-time" + "format": "date-time", + "pattern": "(\\+00:00|Z)$" } } } \ No newline at end of file diff --git a/overview.md b/overview.md index b498f7b5..306daf5f 100644 --- a/overview.md +++ b/overview.md @@ -53,10 +53,11 @@ A Catalog is a very simple construct - it just provides links to Items or to oth The closest analog is a folder in a file structure, it is the container for Items, but it can also hold other containers (folders / catalogs). -The Collection specification shares some fields with the catalog spec but has a number of additional fields: +The Collection entity shares most fields with the Catalog entity but has a number of additional fields: license, extent (spatial and temporal), providers, keywords and summaries. Every Item in a Collection links back to their Collection, so clients can easily find fields like the license. Thus every Item implicitly -shares the fields described in their parent Collection. +shares the fields described in their parent Collection. Collection entities can be used just like Catalog +entities to provide structure, as they provide all the same options for linking and organizing. But what *should* go in a Collection, versus just in a Catalog? A Collection will generally consist of a set of assets that are defined with the same properties and share higher level metadata. In the @@ -78,27 +79,30 @@ provide multiple grouping paths, serving as a sort of faceted search. The second case is used when one wants to represent diverse data in a single place. If an organization has an internal catalog with Landsat 8, Sentinel 2, NAIP data and several commercial imagery providers -then they'd have a root catalog that would link to a number of different Collections. +then they'd have a root Catalog that would link to a number of different Collections. So in conclusion it's best to use Collections for what you want user to find as starting point, and then -catalogs are just for structuring and grouping the data. Future work includes a mechanism to actually +Catalogs are just for structuring and grouping the data. Future work includes a mechanism to actually search Collection-level data, hopefully in concert with other specifications. ## Catalog Overview +*NOTE: The below examples all say Catalog, but those can all be Collections as well, as it has all the fields necessary to +serve as a Catalog* + There are two required element types of a Catalog: Catalog and Item. A STAC Catalog points to [STAC Items](item-spec/README.md), or to other STAC catalogs. It provides a simple linking structure that can be used recursively so that many Items can be included in a single Catalog, organized however the implementor desires. -STAC makes no formal distinction between a "root" catalog and the "child" catalogs. A root catalog -is simply the top-most catalog -- it has no parent. A nested catalog structure is useful (and +STAC makes no formal distinction between a "root" Catalog and the "child" Catalogs. A root Catalog +is simply the top-most Catalog or Collection -- it has no parent. A nested catalog structure is useful (and recommended) for breaking up massive numbers of catalog Items into logical groupings. For example, it might make sense to organize a catalog by date (year, month, day), or geography (continent, country, state/prov). See the [Catalog Layout](best-practices.md#catalog-layout) best practices section for more. -A simple Catalog structure might look like this: +A simple STAC structure might look like this: - catalog (root) - catalog @@ -164,8 +168,8 @@ each Item and Catalog, as well as ways to achieve that. ## Collection Overview -A STAC Collection extends the core fields of the Catalog construct to provide additional metadata to describe the set of Items it -contains. The required fields are fairly +A STAC Collection includes the core fields of the Catalog entity and also provides additional metadata to describe +the set of Items it contains. The required fields are fairly minimal - it includes the 4 required Catalog fields (id, description, stac_version and links), and adds license and extents. But there are a number of other common fields defined in the spec, and more common fields are also defined in [STAC extensions](extensions/). These serve as basic metadata, and ideally Collections also link to