Overlaying your own caches with ArcGIS Online services in the Java Web ADF

When you create any cache that will be used with an ArcGIS Online layer in the Java Web ADF, the following items require attention:

Coordinate system of your map

The map caches on ArcGIS Online use the GCS_WGS_84 coordinate system, and your cached map must also use this coordinate system in order to achieve a successful overlay.

To set the coordinate system of your map to GCS_WGS_84:

1. Right-click the data frame name in ArcMap and click Properties.
2. Click the Coordinate System tab.
3. Expand the folders as follows: Predefined > Geographic Coordinate Systems > World
4. Select WGS 1984 and click OK.

All of the data in your map will be projected on the fly to this coordinate system. When you cache the map, it will be cached in this coordinate system.

Tile origin and tile size

The tile origin represents the upper left corner of the geographic extent covered by the cache tiles. When building any cache that will be viewed through the Java Web ADF, you should avoid placing the tile origin at any point inside the Full Extent of the map. Placing the tile origin on the upper left corner of the Full Extent is appropriate.

For example, many of the ArcGIS Online maps have worldwide coverage. Since the upper-left corner of the Full Extent on a 2D surface is -180, 90, this point was used for the tile origin when the cache was generated.

When entering the parameters for the map cache, avoid changing the tile origin that appears by default. This value is retrieved automatically from the upper-left corner of the full extent of the map document. If you want to change the tile origin, the recommended workflow is to do the following:

1. In ArcMap, open the map document containing the data frame to be cached.
2. Right-click the data frame to be cached and click Properties.
3. Click the Data Frame tab.
4. Under "Extent Used by Full Extent Command", click Other, then click the Specify Extent button.
5. Click Current Visible Extent and click OK twice to dismiss the dialogs.
6. Save the map document and close ArcMap.
7. Using ArcCatalog or Manager, stop and then start the service to be cached.

When overlaying caches in the Java Web ADF, the tile origins of the caches are not required to match. The tile sizes can also differ between caches.

Scale levels

When you add any cached service to your map, your map becomes visible only at the scale levels of that cached service. When you add two or more cached services to your map, the map becomes visible at all of scale levels in all of the cached services. However, the caches will only overlay at the scale levels they have in common.

To provide a smooth navigation experience for the users of your web applications, ensure that the scale levels in your caches match whenever possible. Failure to sequentially match the scales of the caches could result in layers not being visible at expected scales. For example, in the diagram below, Cache A was not created at scale level 3, therefore when the user zooms to Scale 3, only Cache B will be visible. Similarly, at scale level 5 Cache A will not be available, but since it is common behavior for a layer to become unavailable at a minimum or maximum scale, this may not present the same problem as the gap at scale level 3.

Many of the caches for ArcGIS Online services have worldwide coverage at a large variety of scales. You do not need to create your cache using all of these scales, but where possible the scale levels you use should match the ones used by ArcGIS Online services.

Following is a list of the scales used by ArcGIS Online services. Each cache uses all of these scales or a sequential subset of them. (The high-precision values of the scales are necessary for the caches with worldwide coverage to appear seamless at 0 degrees longitude.)

• 147748799.285417
• 73874399.6427087
• 36937199.8213544
• 18468599.9106772
• 9234299.95533859
• 4617149.97766929
• 2308574.98883465
• 1154287.49441732
• 577143.747208662
• 288571.873604331
• 144285.936802165
• 72142.9684010827
• etc. dividing by 2

When you build your cache to overlay ArcGIS Online services, you don't have to enter these values into the caching tools. Instead, in the Caching tab of the Service Properties, choose to load an existing tiling scheme. Then choose the ArcGIS Online tiling scheme from the dropdown list. This will populate all of the high-precision scale values automatically. You can then add and remove scales as needed.

Image format

The base map services from ArcGIS Online use the JPG image format, while the reference overlay caches use 24-bit PNGs. (You can use the Directory of Services page to see which caches are considered base maps and which are reference overlays.) Although the JPG format displays faster than other formats, it does not support transparency. Therefore, any base map services from ArcGIS Online will obscure layers that are placed beneath them. Similarly, if you create your own JPG cache, you will not be able to see any layers you place beneath it.

When overlaying two PNG 24-based caches, be aware that PNG 24 files do not show transparency in Internet Explorer 6. (See the related Microsoft technical article at http://support.microsoft.com/kb/294714). In this scenario, the bottom cache will be obscured.