Tiling Scheme

The tiling scheme includes the scale levels, tile dimensions, and tile origin for the cache. These properties define where tile boundaries will exist and are important to match when overlaying caches in some clients.

There are several options you can choose for the tiling scheme:

Levels of detail

Tiling schemes often contain many scales. Depending on your map, you may want to forego caching some of the extremely large or small scales in your tiling scheme. ArcGIS examines the scale dependencies in your map and attempts to suggest a minimum and maximum range of scales for caching. You can adjust this range by moving the sliders on the Caching tab.

The remaining scales are not removed from your tiling scheme, but you will not be able to create tiles for them unless you return to the Caching tab and include them in the range.

Scales

When you create a tiling scheme, the most important choice you will make is the set of scales at which tiles can be created.

There are two places you can enter your scale list:

• You can enter your scales directly into the Generate Map Server Cache Tiling Scheme tool the first time you create the tiling scheme.
• When creating a new cache, after you load a tiling scheme, you can modify the scale list in the Caching > Advanced Settings subtab of the Service Editor. This is a convenient place to enter scales if you're working with imagery, since you can type a pixel size in the list.

When you enter a scale, you can designate a ratio or the scale's denominator (for example, enter 36000 and you'll see 1:36,000 appear in the list).

If you enter a pixel size, you can type the pixel size value (in map units) to use for the generated tile. This option is sometimes useful when caching imagery. For example, you might want to add a scale to your cache where the pixel resolution is one meter.

A simple way to pick a set of scales is to determine the closest scale at which your users will need to view the map, then double the scale's denominator until you reach a scale that would contain your entire area of interest in just one or two tiles. For example, if you were designing a tiling scheme for a city, and the closest scale at which someone should view your map is 1:2,400, your scales could be 1:2,400; 1:4,800; 1:9,600; 1:19,200; 1:38,400, until you reached a scale that would cause the entire city to be visible on your computer screen at once.

Minimum cached scale

This is the minimum scale at which you want to create tiles. This does not have to be the smallest scale in your tiling scheme.

For example, you may have a tiling scheme that includes the scale 1:2,000,000, but the smallest scale at which you want anyone to create tiles is 1:500,000. You can set 1:500,000 as your minimum cached scale, and the smallest scale anyone will be able to use when running Manage Map Server Cache Tiles will be 1:500,000.

This property corresponds to the lower boundary of the Levels of Detail range you set in the Caching tab.

Maximum cached scale

This is the maximum scale at which you want to create tiles. This does not have to be the largest scale in your tiling scheme.

For example, you may have a tiling scheme that includes the scale 1:1,000, but the largest scale at which you want anyone to create tiles is 1:4,000. You can set 1:4,000 as your maximum cached scale, and the largest scale anyone will be able to use when running Manage Map Server Cache Tiles will be 1:4,000.

This property corresponds to the upper boundary of the Levels of Detail range you set in the Caching tab.

Cache directory

The server cache directory is the folder on disk where your cache tiles are stored. One server cache directory can hold multiple caches; however, you can add more than one server cache directory to your server. When you create an ArcGIS Server site, a server cache directory is created for you at a location you specify. For more information, see About server directories.

If you plan to add another GIS server machine to your site, you need to enable file sharing on the server cache directory so it can be seen by all participating machines. For more information, see Creating a server directory and Configuring a multiple-machine deployment. Be sure to give the ArcGIS Server account write access to the server cache directory so that the server can successfully place the tiles in the directory. You can optionally remove write access when you finish creating your caches.

You can open the server cache directory to examine the cache tiles and tiling scheme file conf.xml. Another file, conf.cdi, contains the default extent information of the service and is required for viewing caches as raster datasets in ArcMap. A geodatabase named status.gdb contains information that ArcGIS shows in its cache status reports.

The tiles are organized into a directory hierarchy. The top-level directory has the same name as the map service unless the service was created in a GIS server folder. If the map service is in a folder, the top-level directory is named <folder name>_<map service name>.

Under the top-level directory, you'll find a subdirectory for the particular data frame in the map being cached. Next, you'll find a single subdirectory _alllayers. Within that, you'll find subdirectories for each level of detail. The structure of these level-of-detail subdirectories varies depending on whether you're using compact storage format or exploded storage format.

• If you're using the compact cache format, you'll see bundle files immediately within the level folders. These bundle files contain the tiles.
• If you're using the exploded cache format, you'll see subdirectories for each tile row, and finally, the actual image files that represent a particular column in the row.

A server cache directory can also contain a GlobeCache folder, which contains all the globe caches stored in the server cache directory.

Area of interest to cache

This setting determines which area of your map will have tiles created if you choose ArcGIS to build the cache automatically when the service is published. The options are as follows:

• Full extent of the map—This caches the full bounding box of all the data in the map. It is the default option. Custom full extents set through the Data Frame Properties in ArcMap will be honored.
• Current extent of the map—This caches the extent of the map document as currently published; in other words, the initial extent of the map service.
• Import from a feature class—This caches any areas intersecting the boundaries of features in a feature class that you select. For example, if you wanted to constrain tile creation to just the area covering California, you could browse to a simple feature class containing the California boundary.

  Importing a feature class is the recommended option when your cache is large or your area of interest is not rectangular. See Map caching based on feature boundaries for tips on preparing and selecting a feature class for this purpose.

The area of interest you set here applies only to caches generated automatically at publish time. It is always overridden by the area of interest you set when you manually run the Manage Map Server Cache Tiles tool. If you do not explicitly set an area of interest using Manage Map Server Cache Tiles, the full extent of the map is cached.

Tile format

This setting determines what output image format the map service will use when it creates the tiles. Your choice of image format is important, because it determines the size on disk of the tiles, the image quality, and the ability to make the tile background transparent.

When you initialize a new cache, ArcGIS examines the layers in your map and selects a default image format for you. If your map contains only vector layers, the default is PNG. If at least one raster layer is detected, the default is MIXED.

You should examine this default and determine if it makes sense based on the purpose of your map. The following is a summary of the available image formats and their appropriate uses:

• PNG—For each tile, this format determines the PNG bit depth that will minimize tile storage space without sacrificing visual quality. It is appropriate for many vector maps, especially maps with few colors, maps with large swaths of one color, or overlay networks. If you're not sure which PNG format to use, try this first.
• PNG8—You can use PNG 8 for overlay services that need to have a transparent background, such as roads and boundaries. PNG 8 creates tiles of very small size on disk with no loss of information.

  If your map uses antialiasing, you'll see higher quality of lines and labels if you use PNG or PNG 32 instead of PNG 8. If your map has a large number of colors due to gradient fills or hillshades, you may also want to use PNG or PNG 32.

• PNG24—You can use PNG 24 for overlay services, such as roads and boundaries, that have more than 256 colors (if fewer than 256 colors, use PNG 8). Do not use PNG 24 if your tiles will be viewed in Internet Explorer version 6 or earlier.
• PNG32—You can use PNG 32 for overlay services, such as roads and boundaries, that have more than 256 colors. PNG 32 is an especially good choice for overlay services that have antialiasing enabled on lines or text. PNG 32 creates larger tiles on disk than PNG 24, but the tiles are fully supported in all browsers.
• JPEG—Use this format for basemap services that have large color variation and do not need to have a transparent background. For example, raster imagery and very detailed vector basemaps tend to work well with JPEG.

  JPEG is a lossy image format. It attempts to selectively remove data without affecting the appearance of the image. This can cause very small tile sizes on disk, but if your map contains vector line work or labels, it may produce too much noise or blurry area around the lines. If this is the case, you can attempt to raise the compression value from the default of 75. A higher value, such as 90, may balance an acceptable quality of line work with the small tile size benefit of the JPEG.

  It's up to you to decide what image quality you consider acceptable. If you're willing to accept a minor amount of noise in the images, you could save large amounts of disk space by choosing JPEG. The smaller tile size also means the browser can download the tiles faster.

• MIXED—A mixed cache uses JPEG in the center of the cache with PNG 32 on the edge of the cache. Use the mixed mode when you want to cleanly overlay raster caches on other layers.

  When a mixed cache is created, PNG 32 tiles are created anywhere that transparency is detected (in other words, anywhere that the data frame background is visible). The rest of the tiles are built using JPEG. This keeps the average file size down while providing you with a clean overlay on top of other caches. If you do not use the mixed mode cache in this scenario, you'll see an opaque collar around the periphery of your image where it overlaps the other cache.

• LERC—Limited Error Raster Compression (LERC) is an efficient lossy compression method recommended for single-band or elevation data with a large pixel depth, such as float, 32-bit, 16-bit, or 12-bit data. LERC compresses 5 to 10 times better and 5 to 10 times faster than LZ77 for float data. LERC is also better with integer data. When using integer data, and the error limit specified is 0.99 or less, LERC is considered a lossless compression.

  Note:

  Limited Error Raster Compression (LERC) is only available for image services; map services are not supported.

  Elevation data, or surfaces, are mostly used within 3D views to represent the ground, such as a terrain model. You can have surfaces that do not represent the ground, such as the ozone layer, which is above the ground surface, or geological substrates, which are below the ground surface. When using a LERC compression method to serve out surfaces that are not at ground level, it is strongly recommended to publish the data with all the levels of detail. This will ensure that when you view the data, you do not zoom out beyond the coarsest resolution, which would stop rendering the content. This is less of an issue for elevation content that is contributing to the ground surface, because the surface can be used to fill in any gaps that may exist.

  • If you select LERC, specify the compression value using the Compression input dialog box. The compression represents the maximum tolerated error value that is applicable per pixel (not an average for the image). This value is specified in the units of the mosaic dataset. For example, if the error is 10 centimeters and the mosaic dataset is in meters, enter 0.1.
  • LERC-based cache services will not re-sample to draw if the source data is not supported at a given scale or if overviews are not present. LERC-based cache services will also contain a properties file that lists whether or not the service contains elevation data. To provide full coverage while drawing 3D scenes, LERC-based caches use a cache tile size half-a-pixel more than the standard 256 x 256.
  • The LERC format uses a non-transform compression algorithm, therefore the compression value or maximum error allowable value will vary from 0 to 1,000.

Before committing to an image format for a large cache, build a small cache of a representative area of your map, and examine the tile quality and performance in a test application. If you'll be working with multiple caches, build a small test cache for each, and add them to a test application to make sure they overlay as expected. This will allow you to make adjustments before you create the entire cache.

To make a small test cache, use the editing tools in ArcMap to create a new feature class consisting of a small rectangle around the area you want to test. Then use the option at the bottom of the Manage Map Server Cache Tiles tool dialog box that allows you to create tiles based on the boundary of a feature class. Browse to the feature class containing your test area, and create the tiles.

Compression

Compression refers to the amount of JPEG compression that will take place when using JPEG or mixed image format caches. Higher values signify higher JPEG quality and therefore less compression. For imagery, values of 55 to 75 are usually sufficient without causing any visible loss of quality. For vectors and other sharply defined features or regions, a higher quality of 90 is recommended as a starting point.

Create tiles on demand

When this box is checked, tiles for previously uncached areas will be added to the cache when they're first requested by a client. This is a good way to ensure that people always see a map when viewing your cache, even if you didn't have time to initially create all the tiles at every scale level.

Learn more about when to use on-demand caching

Tile origin in map units

The tiling scheme origin is the upper left corner of the tiling scheme grid. The origin does not necessarily represent the point at which tiles begin to be created; that happens when the full extent of the map or area of interest feature class is reached. Using a common tiling scheme origin for your caches ensures that they can overlay each other in web apps.

In most cases, you should keep the default tiling scheme origin of the software. The default is the upper left point of the coordinate reference defined by the map document. If no coordinate reference is defined in the map document, the upper left of the two times the maximum of the union of extents of all the layers in the map is used.

If you change the tiling scheme origin to something other than the default, be aware that tiles can be created only in the map area to the lower right of the tiling scheme origin. If you want to cache only a certain area of your map, creating tiles based on a feature class boundary is a better option than changing the tiling scheme origin.

Dots per inch (DPI)

Dots per inch (DPI) refers to the resolution of the cache tiles that the server will generate. The default value of 96 is almost always sufficient unless you are working primarily on a network where the majority of your client machines have a different DPI. Be aware that adjusting the DPI affects the scale of the tiles.

Tile height and tile width

The default tile width and height is 256 pixels. It's recommended that you use 256 or 512. If you're building a cache that will overlay another cache, be sure to use the same tile width and height for both caches.

Choosing a smaller tile width and height may improve performance of the application requesting tiles from the cache, as less data will need to be transferred. However, with exploded caches, a smaller tile size results in a larger cache size and longer creation time.

Storage format

You can choose whether to group your tiles for more efficient storage and mobility or store each tile as a single file. The following are the storage options:

• Compact—In compact format, each file in the cache contains a bundle of tiles. The bundles are 128 tiles wide by 128 tiles high, containing a maximum of 16,384 tiles. Compact caches copy much faster than exploded caches, and they take less space on disk.
• Exploded—In exploded format, each tile is stored as a file on disk. This makes it easier to see the tiles in your cache; however, it results in larger caches that take longer to create and can be cumbersome to copy.

The enormous number of files and folders in the cache can sometimes cause inefficiencies in copying utilities, including Windows Copy/Paste. For this reason, compact caches greatly reduce cache copying time. If you have an automatic cache update workflow that creates tiles on a staging server and copies them to a production server, it's recommended that you use compact caches. Additionally, if you have limited disk space available, you should use compact caches.

Allow clients to cache tiles locally

When this box is checked, Esri client applications, such as ArcMap, are allowed to keep a local cache of any tiles they retrieve from the server. This can help the client view the tiles more quickly when returning to the area. Clients must manually clear their cache to get any newer tiles from the server; therefore, consider unchecking this box if you'll be updating your cache frequently.

Allow clients to export cache tiles

The ArcGIS client APIs allow apps to download map tiles from the server for offline use. When you check the checkbox Allow clients to export cache tiles, you authorize these downloads to occur.

Large downloads of tiles can negatively affect your server performance and overwhelm the client device. Use the Limit export cache to setting to define the maximum number of tiles a client can request at once.

Exported tiles are placed in the server output directory. When you've allowed clients to export tiles, you may want to increase the maximum allowed age for files in the server output directory. The default of 10 minutes may not be long enough for the client to retrieve the tiles before they are cleaned up. See Editing a server directory in Manager for steps on how to make this edit.