Overlaying your own caches with ArcGIS Online services in the .NET Web ADF
When you create any cache that will be used with ArcGIS Online services in the .NET Web Application Developer Framework (ADF), the following items require attention:
Coordinate system of your map
Ensure your map is in the same coordinate system as ArcGIS Online services, which is WGS 1984 Web Mercator (Auxiliary Sphere). If you choose to not use this coordinate system, your map will need to be projected on the fly, and you will lose the advantage of the cache.
Scale levels
You will achieve the performance benefit of both caches only where the two caches have matching scale levels. In the case of a scale-level gap (when a scale level exists in one cache but not another), the overlaying image cannot be pulled from the cache and must be generated dynamically, thereby slowing performance. Image quality will also be degraded in this scenario, because the dynamic image must be resampled from one of the existing scale levels.
In the diagram below, Cache A does not exist at Scale 3; therefore, the .NET Web ADF will generate a dynamic image to overlay the image from Cache B at this scale.
Dynamic images are not created to match scales beyond the top or bottom scale of the cache. Therefore, for Cache A, a dynamic image will not be generated at Scale 5. Instead, Cache A will not be visible at Scale 5; only the cached image from Cache B will be displayed. Similarly, at Scale 1, only Cache A will be visible.
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Many ArcGIS Online services have worldwide coverage at a large variety of scales. You do not need to create your cache using all these scales, but where possible, the scale levels you use should match the ones used by ArcGIS Online services.
When you build your cache to overlay ArcGIS Online services, choose to load an existing tiling scheme. Then choose the ArcGIS Online service tiling scheme from the drop-down list. This will populate all the high-precision scale values automatically. You can then add and remove scales as needed.
Image format
The basemaps from ArcGIS Online services use the JPEG image format, while the reference overlay caches use PNGs. Although the JPEG format displays faster than other formats, it does not support transparency. Therefore, any basemaps from ArcGIS Online services will obscure layers that are placed beneath them. Similarly, if you create your own JPEG 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 images 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 .NET Web ADF converts the top PNG 24 image to a PNG 32. The conversion is performed on the fly, potentially slowing performance.