Working with map service layers

A map without data layers is like an artist with a blank canvas. The data layers that you add to your map give it meaning and set the stage for analysis. These data layers come from map services published by ArcGIS Server and, occasionally, other GIS servers. The two main types of layers available from ArcGIS Server are dynamic map service layers and tiled (or cached) map service layers.

Dynamic map service layers reference map services that create a map image on the fly and then return the image to the application. This type of map service may be composed of one or more layers of information. For example, the Demographics map service displayed in the following screenshot is composed of nine different layers representing demographic information at various levels of geography:

While they can take somewhat longer to display in a client application since they must be generated "on the fly", dynamic map service layers are more versatile than tiled map service layers in that you can filter the features displayed by using layer definitions, set the visibility of various layers within the service, and define temporal information for the layer. For example, in the Demographics map service layer mentioned above, you might elect to only display Census Block Groups in your application. This is the type of versatility provided by dynamic map service layers that you don't get with tiled map service layers.

Tiled map service layers reference a predefined cache of map tiles instead of dynamically rendered images. The easiest way to understand the concept of tiled map services is to think about a grid that has been draped across the surface of a map. Each cell within the grid is the same size and will be used to cut the map into individual image files called tiles. The individual tiles are stored as image files on a server and retrieved as necessary depending on the map extent and scale. This same process is often repeated at various map scales. The end result is a cache of tilesets that have been generated for various map scales. When the map is displayed in the application it will appear to be seamless even though it is composed of many individual tiles:

These tiled, or cached, map layers are often used as base maps such as imagery, street maps, and topographic maps, or for data layers that don't change often. Tiled map services tend to display faster since the server does not have to create images on the fly each time there is a request for a map.

Operational layers are then draped on top of the tiled base maps and these are usually dynamic layers. While they can be somewhat slower in terms of performance, dynamic map service layers have the advantage of being always up to date and your application is able to change their contents and appearance before they are rendered in the browser.