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Estimating the size of a map cache

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To help you make a wise decision about whether to build your cache automatically when publishing or manually, ArcGIS provides estimates of the anticipated cache size:

  • The Estimated Cache Size estimate, displayed on the Caching tab of the Service Editor dialog box, is a very rough approximation based on average tile sizes for the image format you chose and the number of tiles expected in your cache. The estimate assumes a vector map for non-JPEG image formats and raster imagery for JPEG and MIXED formats. For JPEG and MIXED formats, the estimate assumes a compression quality of 75.
  • The Manage Map Server Cache Tiles tool reports cache size estimates next to each scale check box on the tool dialog box. These are rudimentary estimates, derived the same way as the Estimated Cache Size above.
  • The Calculate Cache Size utility, accessible through a button on the Caching tab of the Service Editor dialog box, provides the most accurate estimate of your cache size. This utility builds random sample tiles for your service and uses them to extrapolate a size estimate. You can increase or decrease the amount of sample cache built, thereby affecting the accuracy of the estimate.

If you're sure of the image format and scales you're going to use, it's best to configure those before you look at any estimates. However, you can use the estimates to help understand the variations in cache size that will result from choosing different image formats.

Using the Calculate Cache Size utility

To calculate a size estimate for your cache, follow the steps below. Note that the Calculate Cache Size utility is only available before you publish a service; you cannot run it on an existing service.

  1. Begin the process of publishing a service in ArcMap, and proceed until you reach the Service Editor dialog box.
  2. Click the Caching tab.
  3. Ensure you have chosen to draw the map using tiles from a cache and click Calculate Cache Size.
  4. Set the Estimate Quality property to determine how accurate your estimate will be and how long it will take to be calculated. Better target estimates require more sample tiles and take longer to calculate.
  5. Click Start. The estimates appear scale by scale, beginning at the largest cached scale allowed by the server administrator. This is the scale that will take the most space on disk.
  6. Optionally, click Skip Level to skip estimating a scale level.
  7. Click Stop when you want to stop calculating estimates.

After you close the utility, you'll notice that the Estimated Cache Size now displays the estimate you derived from the utility.

Deriving estimates when your area of interest is a feature class

It’s important to note that the estimates only reflect rectangular map extents. If you specified a feature class as the area of interest for your cache, the estimate is based on the rectangular bounding box of each polygon in the feature class, not the exact feature class boundary. If you're going to cache an irregularly-shaped geographic feature, you may get a more accurate estimate by clipping a grid to the boundary of your feature class, effectively breaking up your feature into large grid cells. You can use the Create Fishnet tool to make a grid.