In your portal, you work with geographic data through layers. Layers, also called web layers, are logical collections of geographic data that are used to create maps and scenes; they are also the basis for geographic analysis. For example, a building layer could represent a collection of campus buildings and include attributes that describe each building's properties, such as the name of the building, what type of building it is, the size of the building, and other potential attributes. Additional examples of layers include earthquake epicenters, historical traffic patterns, terrain, 3D buildings, and parcels.
Data sources
The fundamental types of data that can be displayed in a map are features and imagery. Different types of layers have different capabilities. For example, publishing data in a CSV file creates a feature layer. Feature layers give you the ability to query and edit features in client applications and manage access to the actual data.
The data used in layers comes from a variety of sources, including Portal for ArcGIS hosted services, ArcGIS Server services, CSV files, KML files, feature collections, and OGC services.
The process of publishing your data to Portal for ArcGIS accomplishes the following:
- Creates a service on your portal's hosting server
- Stores service data in databases or files associated with the hosting server
- Creates a layer item in the portal that represents the data
You can, instead, create layers that reference source data (rather than have the data copied to files or databases associated with the hosting server). For example, if you register an ArcGIS Server map service as an item in your portal, a layer is created that contains a reference to your map service. The data still resides within the map service's data source and is not copied to the portal's hosting server. Similarly, you can register your source data with one of your portal's federated servers, publish a service to that federated server, and a layer will be created in the portal to represent that service. In this case, the data that populates the service also remains in your data source. If you delete the service, your data is not deleted.
Types of web layers
Layers represent geographic features such as points, lines, polygons, imagery, surface elevation, cell-based grids, or virtually any data feed that has location—for example, weather gauges, traffic conditions, security cameras, and tweets. The type of layer determines how you can interact with the layer's data. For example, you can view and query the data in a feature layer to see a feature's attributes. You may also be able to edit the data represented by the feature layer. With tile layers, you only see tiled images of the features.
In an ArcGIS portal, web layers are categorized by the type of data they contain—for example, elevation, feature, or imagery. This helps make the connection about what is displayed in the map. Icons also help show the type of data in the layer. The source of web layers is described on the item page.
Supported web layer types are as follows:
Map image layer—A collection of map cartography based on vector data. Vector data includes points, lines, and polygons. Map image layers are dynamically rendered image tiles.
Imagery layer—A collection of map cartography based on
raster data. Raster data is a grid of cells commonly used to store
imagery and other information captured by remote sensing devices. Imagery layers can be displayed dynamically or prerendered as cached image tiles.- Tile layer—A set of web-accessible tiles that reside on a server. Tile layers include prerendered
map raster tiles or 
vector tiles. 
Elevation layer—A collection of prerendered cached image tiles in the Limited Error Raster Compression format (LERC). LERC is a compression format for single-band or elevation data. Elevation layers are suitable to show terrain in scenes at global and landscape scale.- Feature layer—A feature layer is a grouping of similar geographic features—for example, buildings, parcels, cities, roads, and earthquake epicenters. Features can be points, lines, or polygons (areas). Feature layers are most appropriate for visualizing data on top of your basemaps. You can set properties for feature layers—such as style, transparency, visible range, refresh interval, and labels—that control how the layer appears in the map. Using a feature layer, you can view, edit, analyze, and execute queries against features and their attributes. The contents of some feature layers can be downloaded.
Feature layers can be categorized based on the feature source, which affects the layer's functionality.
Feature layers with the icon shown at left are likely the most common type of feature layer you'll see in your portal. They can be either hosted feature layers (with data stored in a relational data store) or feature layers from an ArcGIS Server site.
Streaming features use a live stream of data for the layer source. (Requires an ArcGIS GeoEvent Server.)
Spatiotemporal feature layers access data in spatiotemporal big data stores, which are used to store the large data volumes produced when you archive streaming feature layer data from ArcGIS GeoEvent Server and generate data from GeoAnalytics Tools.- Feature collections are another type of feature layer. These layers are created in Map Viewer when you add and save map notes, a shapefile, a CSV file, or a route layer.
Scene layer—A collection of 3D feature objects and z-values (elevation value). A scene layer can be the following types: 
point, 
3D object, 
integrated mesh, or 
point cloud.
Table—A collection of rows and columns, where each row, or record, represents a feature—such as a parcel or power pole—and each column, or field, describes a particular attribute of the feature, such as its square footage, height, or length. Tables can include location information, such as addresses, or no location information, for example, a simple list of names and salaries. Tables are not typically drawn on the map even if they include location information.
Layers are not only data however. A layer represents both data and the visualization applied to it. Visualization can include the symbols and colors used to display the data, as well as pop-up configurations, transparency, filters, and other layer properties.
Copies of layers
It is possible to create copies of hosted web layers when you want to visualize or present the data in multiple ways. When you create a copy of a layer, the layer's configuration is copied to the new layer. Typically, the data associated with the layer is not copied; the layer maintains a reference to the data. If the data is updated, any layers referencing it will reflect the updated data. This is particularly advantageous when you have data with multiple attributes. Different layers may represent the same data but are displayed using different visualizations. Each layer could be styled based on different attributes. While the data is the same between each layer, the different styling empowers you to focus on a different story with each layer. A layer can be included in multiple web maps and web scenes, meaning configurations you've saved with the layer will be honored in any web map that includes the layer.
How layers are used
Layers are the building blocks of web maps and web scenes. Every map and scene contains a basemap layer and may also contain other layers that are drawn on top of the basemap. Basemap layers are typically used for viewing purposes or context only and are usually tile layers, map image layers, or imagery layers. The layers drawn on top of the basemap can be features or imagery and are called operational layers. Operational layers are the layers you interact with. Interactions include viewing attribution information, editing features, and performing analysis. While the visualization of the layers in a map or scene is configured on the layer, properties such as extent, visibility, and layer order are configured on the map, enabling you to control how the layer collection is displayed in the map or scene.
Maps, scenes, and layers can be managed as items in Portal for ArcGIS. These items are displayed on the content page, can be shared with others, and are searchable in Portal for ArcGIS. While many layers are available as items in Portal for ArcGIS, in some cases, layers are not available as items; the layer is only available in the web map containing it. For example, you can add a CSV file directly to a map. In this case, the feature layer created by adding the CSV file is only available in the map and not as a separate item.