Maps in ArcGIS are composed of a series of map layers drawn in a particular order. A map layer defines how a GIS dataset is symbolized and labeled (that is, portrayed) in your map views.
A layer represents geographic data in ArcMap, such as a particular theme of data. Examples of map layers include streams and lakes, terrain, roads, political boundaries, parcels, building footprints, utility lines, and orthophoto imagery.
Each map layer is used to display and work with a specific GIS dataset. A layer references the data stored in geodatabases, coverages, shapefiles, imagery, rasters, CAD files, and so on, rather than actually storing the geographic data. Thus, a layer always reflects the most up-to-date information in your database. A layer won't draw on your map unless you also have access to the data source on which the layer is based.
When you create a new layer by adding a dataset, the layer will draw using a default set of drawing properties. So one of your first steps will be to set the layer's symbology and other display properties.
Layers are also the way that you work with GIS datasets in ArcGIS, for example:
- Layers define how features identify and report themselves.
- Layers are used to edit geodatabase datasets.
- Layers are used to define how you work with feature attributes.
Layers can be saved to a file on disk as a layer file (.lyr). A layer can also be saved with its data as a layer package (.lpk). When you add a layer file to another map, it will draw exactly as it was saved.
You can share layers and layer packages over the network, on the web, and via e-mail.
Layer properties
Layers have a number of properties that you can work with and set. Right-click a layer in the table of contents and click Properties to view the Layer Properties dialog box.
The Layer Properties dialog box is where you set symbology, labeling, drawing rules, and other options. For example, you can specify that streams are drawn using blue lines, parcels are drawn based on their land-use code, parks are drawn using a green pattern fill and are labeled with their park names, digital elevation is portrayed as a shaded relief, and so on.
In addition, other layer properties can be set, for example:
- The scales at which a layer will be visible
- Which subset of features to draw from the data source
- The location of the layer's dataset
- Attribute properties, joins, and relates for working with tabular information
Layers in the table of contents
The table of contents lets you specify which layers are displayed on the map (by turning them on and off). In addition, the order of the layer list in the table of contents determines the drawing order of layers on the map—layers higher in the table of contents are drawn on top of those that are lower. To change the drawing order, click the table of contents List By Drawing Order button 
, click and hold a layer name, then drag it up or down in the table of contents to a new position.
Working with layer attributes
You can work with attribute tables for the datasets referenced by each map layer. Right-click a layer in the table of contents and click Open Attribute Table. In the Table window, you can perform queries, make selections, locate features on the map, and so on. Click the Table window's Table Options menu 
to create graphs and reports, change the font for the table, print the table, and perform various other operations. When you select an item in a table or graph, the feature is also selected on the map (and vice versa).
When you work with a layer's attribute table, you can first set various display properties for tables. You do this by clicking the Fields tab on the Layer Properties dialog box to specify which fields will appear when you open the layer's table, what the fields will be named (using alias names), and how numeric fields will be formatted. You can also specify these options for an individual field by right-clicking a field heading in the table window and clicking Properties.
Joins and relates between layers and attribute tables
Related data is often gathered and stored in multiple layers and tables. Some examples of related data stored in different layers and tables include
- A Parcel layer and an Owner table that contain information about the parcel owners
- A States layer and a County layer that contain census data by county for each state
- A Utility Pole layer and a Transformer layer that list all the transformers mounted on each utility pole
Even though the data is stored in different layers and tables, you will often need to identify related data to perform queries and edit related data. ArcMap provides three methods to associate related data: relates, joins, and spatial joins.
- Relate—A relate defines a relationship between two attribute tables using a key common to both tables. Relates allow you to access related data when you work with a layer's attributes. A relate is similar to a simple relationship class except it can involve data from different workspaces (for example, a dBASE table can be related to a coverage) and is stored in a layer file or ArcMap document.
- Join—When you join two tables, you append the attributes from one table onto the other based on a field common to both. Joins are primarily used to label and symbolize your layer based on the associated data.
- Spatial join—When the layers on your map don't share a common attribute field, you can join them using a spatial join. This joins the attributes of two layers based on the location of the features in the layers. Spatial joins are different from attribute joins in that they are not dynamic and require the results to be saved to a new output layer.
Different types of layers
There are different kinds of layers. Some layers represent a particular type of geographic feature, while others represent a particular type of data. Each layer type has different mechanisms for displaying and symbolizing its contents and specific operations that you will perform against them. Many layers have special sets of tools for working with the layer and its contents. For example, you can use the Editor toolbar to manipulate feature layers and the Topology toolbar to work with the contents of a topology layer.
Here are a few of the common layer types:
- Feature layer—A layer that references a set of feature (vector) data that represents geographic entities as points, lines, and polygons. A feature layer's data source can be a geodatabase feature class, a shapefile, an ArcInfo Coverage, a CAD file, and so on.
- Raster layer—A layer that references a raster or image as its data source.
- Service layer—A layer used to display ArcGIS Server, ArcIMS, WMS services, and other web services.
- Geoprocessing layer—A layer that displays the output of a geoprocessing tool.
- Basemap layer—A type of group layer that provides high-performance display of basemap content.
Group layers
Group layers are used to organize a set of related layers together. They combine multiple layers that are often displayed and managed together. Group layers can help organize related layers in a map and can be used to define advanced drawing options. There are many reasons for grouping layers together to manage their display. Here are a few examples that help illustrate their use.
Thematic organization—Suppose you want to organize how parcels are displayed as a theme. You can organize a group layer that contains a subset of map layers—one for parcel polygons, another for parcel boundaries, and a third that displays parcel labels as annotation.
Group layers for each map scale—Many users build multiscale maps that portray information differently at each map scale. To do this, you can build group layers for each map scale in your map so that all the layers that portray data at a particular resolution can be managed together. You can set scale-dependent drawing for the set of layers to be drawn at each map scale. In the example map below, you can see group layers for a set of map scales. The group layer that displays that map between 1:15,000 and 1:18,000 is expanded to show its sublayers.
Common layer tasks
Here are some of the common operations that you'll use with layers in ArcGIS.
| Common task | Where to go for more information |
|---|---|
Adding data to ArcMap | |
Setting layer properties | |
Referencing datasets with layers | |
Saving a layer | |
Symbolizing data in a layer | Drawing all features with a single symbol |
Using feature attributes to create map labels | |
Specifying the drawing order for layers | |
Opening a layer's attribute table | |
Relating data | |
Setting HTML display properties for a map layer | |
Creating a map layer for delivery using KML |