First, what a layer is not.
Layers are ubiquitous across ArcGIS and are fundamental to the ArcGIS system. The word “layer” has so many different meanings and uses that it can be challenging to describe in a useful way, but let’s try away. A simple cartographic description is that a layer is the representation of like things in a map... so a layer of schools, or customers, or roads, or study areas. But this is too simple for our needs and is easily conflated with a feature class.
“Wait up, Travis! What is a feature class?”
According to the GIS dictionary it’s a data architecture term, that is a collection of features with the same geometry, attributes, and spatial reference. Now that may sound like a layer, but feature class is the term used in geodatabase and geoprocessing contexts that describe DATA... or the GIS stuff in a data store, not the stuff in a map.
Now, what a layer is.
In my opinion, the power of Esri’s approach to GIS comes from a layer’s relationship to GIS data. The best definition I’ve heard of a layer in ArcGIS Enterprise, is “an object that references some other GIS resource and stores properties about how that resource can be used” (thanks for that one Travis). This definition can then be expanded to many forms of layers in ArcGIS like data layers in a map or scene in ArcGIS Pro; layer files (.lyr files) stored on disk; or web layer items that are shared in a portal.
Data layers are exclusively used in ArcGIS Pro for maps, scenes, or geoprocessing tools. They allow for a centrally managed feature class, or raster dataset, to be used in many resources in different ways. But in such a fashion that when the features in the feature class are edited, those edits are automagically reflected in all the resources (maps, scenes, etc.) referencing that feature class.
Layer files (.lyr), store the properties of a map’s data layers in a file that can be used by desktop users to quickly share and apply cartographic settings across projects and maps. But their usefulness is limited in ArcGIS Enterprise because they require a desktop application to be utilized. Typically layer files reference feature classes directly in a database but are not limited to only to referencing data. They could reference a GIS web service too.
What is a GIS web service? And how is it different from a layer?
As we’ve discussed in What is ArcGIS Enterprise?, the ArcGIS Enterprise system is designed for sharing GIS data and capabilities with the non-GIS professionals too, and they usually don’t use ArcGIS Pro. They use web and mobile apps that leverage GIS web services. So there needs to be another form of layer for use in web-based GIS apps.
In ArcGIS Enterprise: What is a GIS web service?, we described how ArcGIS Enterprise is purpose built to leverage GIS web services. Because of this service-oriented architecture focus, a web layer is a type of item managed in an ArcGIS portal that stores a reference to a GIS web service resource via its URL, and properties about how the service is used and shared.
Why share web layers and not GIS web services?
While data layers and layer files are helpful for GIS professionals using ArcGIS Pro. They aren’t directly usable in ArcGIS Enterprise apps. Sure, you can share a layer file as an item in an ArcGIS portal, but they can only be used in ArcGIS Pro, typically by GIS professionals.
“But Travis, I don’t want to share a layer, I want to share my GIS data.”
Travis and I find that GIS professionals typically think about GIS data as the resource they ultimately need to share with others. But the only app that directly communicates with data stores is ArcGIS Pro. Because of this limit, ArcGIS Enterprise uses GIS web services from ArcGIS Server to connect to the data stores to read or write to or from a data store.
Remember a GIS web service does more than defining what data is available via the service. The type of service also defines what someone can do with or to the data the service references. The service can have default rendering properties, or a subset of the original features classes’ fields made visible. The “problem” is that security and sharing for all items in ArcGIS Enterprise is set at the portal level, not in ArcGIS Server.
A web layer is like a book in our library.
Going back to thinking of the ArcGIS Enterprise portal like a library, web layers in an ArcGIS portal are like the books you find in a library. Think about a topic where there are many types of media that cover it, like movies, magazines or books. Some of the books might be all text, while others incorporate pictures too. It might be that many of the same books are written by the same author, but each book could be a different edition, or have a different cover, or use a different font. There might be other books on the same topic but written by other authors. These books could also have all the variety in edition, cover and the like.
ArcGIS Enterprise web layer items are very similar to this. You might say the topic is the data in your data store. The media format relates to the type of service in ArcGIS Server. So, the type of service allows for different experiences or functionality with that data. The books are the web layer items managed in the portal. Just like the differences in books described above, different web layers could be owned by different people, they could reference the same service URL but apply different styles or labels to the features available via that service. Web layers can also be shared to different audiences, to make sure the right people get access to the right information and capabilities (via a service) against that GIS data.
So, what is a web layer in ArcGIS Enterprise? You could say it’s:
• An item in the ArcGIS Enterprise portal
• A portal item that references a GIS web service URL
• An item that provides a tailored look and feel that meets a specific business need
• Made available just to the people that have that need.
