Solution

• Set JPEG as the tile format.
• Set MIXED as the tile format.
• Do nothing.

More information

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.

The default image format is PNG, but you may often need to change this based on the type of map you are creating. Below is a summary of the available image formats and their appropriate uses.

• PNG—Use PNG for overlay and basemap services where you are either unsure of the number of colors in the map or would like the caching process to optimize storage for simple areas in the map while preserving fidelity for other areas. For each tile, this format picks 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—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.

  Do not use PNG 8 if your map contains more than 256 colors. Imagery, hillshades, gradient fills, transparency, and antialiasing can easily push your map over 256 colors. Even symbols such as highway shields may have subtle antialiasing around the edges that unexpectedly adds colors to your map.

• 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—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 MSD-based 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 are willing to accept a minor amount of noise in the images, you may 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 smaller 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 will see a nontransparent collar around the periphery of your image where it overlaps the other cache.

Learn more about map cache properties, planning a map cache, and creating a cached map service.