Map layers |
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Release 9.3
Last modified July 28, 2009 |
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When you add a dataset to ArcMap, a layer is created. Each map layer is used to display and work with a specific GIS dataset. A layer represents geographic data in ArcMap such as a particular theme of data. Example map layers include streams and lakes, terrain, roads, political boundaries, parcels, building footprints, utility lines, and orthophoto imagery.
A layer references the data stored in geodatabases, coverages, shapefiles, rasters, 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.
Layers have a number of properties you can work with and set. You can right-click a layer in the table of contents and click Properties to view the Layer Properties dialog box, where you can set symbology, labeling, drawing rules, and other options. For example, you can specify that streams are drawn with all blue lines, parcels are drawn based on their land-use code, parks are drawn using a green pattern fill and are labeled with the park name, digital elevation is portrayed as a shaded relief, and so on. In addition, other properties include defining the scales at which they can draw, which features to draw from the data source, where that data is located in your database, attribute properties, joins, and relates for working with the tabular information.
Layers can be saved to a file on disk (.lyr) so they can be shared and reused without sharing the entire map. When you save a layer to disk, you save everything about the layer, such as the symbolization and labeling. When you add a layer file to another map, it will draw exactly as it was saved. Others can drop those layers onto their maps without having to know how to access the database or classify the data; this can be helpful when sharing data stored in a multiuser geodatabase with nontechnical staff members. You can share layers over the network as well as e-mail layers, along with the data, to people or enclose the layer within the data's metadata.
When you add a dataset to a data frame in your map, by default ArcMap draws all the features using the same symbol and color. If you open the properties of a layer and click the Symbology tab, you can change the way that information is drawn. Depending on the type of layer that is created, you will find different ways to symbolize the data contained within the layer. Most often, information contained within the dataset itself is used to determine how the data is symbolized.
Sometimes all the features in a dataset are drawn using a common symbol or color. For example, all streams are drawn using blue lines and labeled in the same font and size.
However, chances are that you will often want some layer displays to be more sophisticated and will want to assign symbols based on attribute values. For example, the circle symbols representing cities in the map below are classified into sizes based on population.
Categorical data such as this is drawn using unique symbols for each category. You can work with continuous numeric data fields by using classification tools to create a series of class ranges. Then, you can assign symbols to draw each class or use a color ramp to assign color values to each class.
ArcMap contains numerous alternative methods for depicting and rendering map symbols and displays. Learning about various alternatives for layer rendering and portrayal is important for effective use of ArcMap.
Here are examples of three frequently used methods for symbolizing layers.
Layers are used to create map labels using attribute values. You can specify a field that can be used as the source for label text and specify a series of drawing rules for the labels. In this case, administrative boundaries will be labeled using their name taken from the NAME field.
Learn more about text in ArcGIS
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 layers 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 order, click the table of contents Display tab, click and hold a layer name, and drag it up or down in the table of contents to a new position.
Layers with complete coverage of the area, such as filled polygons or a raster, are usually drawn underneath other map layers.
Learn more about layer drawing order
Learn more about using the table of contents
You can work with attribute tables for the datasets referenced by each map layer. Right-click a layer in the table of contents to open its attribute table. In the table window, you can perform queries, make selections, zoom and pan to features on the map, and so on. Click the table window's 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).
Before you begin to 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.
Related data is often gathered and stored in multiple layers and tables. Some examples of related data stored in different layers and tables include
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, the Topology toolbar to work with the contents of a topology layer, and the Annotation toolbar to create and edit annotation (text) layers.
Here are a few of the common layer types:
Common task | Where to go for more information |
Adding data to ArcMap
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Adding layers to a map |
Setting layer properties |
Setting layer properties
Displaying layers at certain scales Displaying a subset of features in a layer |
Referencing data with layers |
Repairing broken data links
Referencing data in the map |
Saving a layer (.lyr file) | Saving a layer to disk |
Symbolizing data in a layer |
Drawing all features with a single symbol
Drawing features to show categories Drawing features to show quantities |
Using feature attributes to create map labels | Displaying labels |
Specifying the order layers draw in a map |
Changing a layer's drawing order
Using the table of contents |
Opening a layer's attribute table | Adding and viewing tables in ArcMap |
Grouping layers | Managing group layers |
Relating Data | About joining and relating tables |
Setting HTML display properties for a map layer |
Setting HTML pop-up properties for feature layers
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Creating a map layer for delivery using KML | Working with KML in ArcGIS |