Earlier this year ESRI announced that ArcGIS Online services will be migrating to the Mercator-based tiling scheme used by Google Maps and Bing Maps. The ArcGIS Online engineers have been busy caching tiles for months in anticipation of this switch, which is anticipated to occur by the end of this year. This post is about how the change may affect you. At the end of the post we also provide steps on how to make the change for your services.
Why a tiling scheme change?
Since the release of ArcGIS Online three years ago, the 2D services have used the WGS 1984 geographic coordinate system and a 512 x 512 pixel tile size. Google and Bing, in contrast, use a modified Mercator projection and a 256 x 256 tile size. The scale sets used by both tiling schemes are similar, but not equivalent.
Why didn’t ArcGIS Online originally choose to match Google and Bing? Interestingly enough, the first ArcGIS Online services were designed in 3D for viewing in ArcGlobe and ArcGIS Explorer. Without getting into the technical details, it’s enough to say that the coordinate system and scales used in the 2D ArcGIS Online tiling scheme have their roots in the ESRI globe technology for which ArcGIS Online was originally built.
With the broad uptake of the ArcGIS Web APIs, the 2D ArcGIS Online services have become very widely used. Some organizations struggle with choosing either the ArcGIS Online tiling scheme to match their ESRI software stack, or the Google / Bing tiling scheme to match a better-known standard. With a unified tiling scheme for the three services, the decision gets a lot easier.
A few organizations are required to support mashups with both ArcGIS Online and Google Maps or Bing Maps. After the ArcGIS Online tiling scheme change, these organizations will no longer have to maintain two caches.
What does this mean for you?
If you are getting ready to create a very large cache to overlay ArcGIS Online, it may be best to create the cache in the Google / Bing tiling scheme, anticipating the change.
If you are not able to re-create your cache at this time, the old services will remain available for a minimum of six months. They will be offered “as is” and will not receive any further data or cartography updates.
Advantages of switching to the Google / Bing tiling scheme
The biggest advantage of switching to the Google / Bing tiling scheme is standardization. This tiling scheme is well-known and widely used. Whether you love the Mercator projection or hate it, it is now customary in Web maps designed for mass consumption. In the end, simplicity of math (you can fit the world on a square at the smallest scale) determined the way most Internet users expect to see the world.
There are some subtle performance advantages to the Google / Bing tiling scheme. Because the tiles are only 256 x 256 pixels, less tile area falls outside the periphery of the map. This means you have to send less data across the wire. The smaller tiles also enhance the perception of the map loading faster, compared with waiting for the 512 x 512 “chunks” of map to appear.
Challenges associated with the Google / Bing tiling scheme
If you re-create your caches in the Google / Bing tiling scheme, you need to anticipate a few issues. First, because the tiles are 256 x 256, you’ll be creating roughly four times as many tiles as you had previously in your ArcGIS Online cache. The larger number of files in your cache will increase cache copying time. The smaller tile size also means you have to send more files to the client during a map request.
Your cache size on disk will probably also increase. Some of the 256 x 256 tiles can get very small in size if they contain few features; much smaller than the 4K default minimum cluster size on Windows. Some organizations have saved space by storing their caches on disks or partitions formatted with a smaller minimum cluster size, such as 1K. This reduces the discrepancy between “Size” and “Size on disk”.
Finally, the Mercator projection is going to stretch your map vertically, especially if you work with extreme latitudes. Besides affecting the appearance of the map, this distortion may cause you to create many more tiles than you might otherwise expect. At these latitudes, it is critical to target your cache at just the scales and geographies that you need. You may want to enable on-demand caching for the most isolated regions. The distortion introduced by the Mercator projection also means that before you measure a feature such as a user-drawn polygon, you should project the feature into a more locally-tailored coordinate system such as UTM or State Plane.
How do you create a cache to match the Mercator-based ArcGIS Online services?
Here’s the workflow for caching a map to match the Mercator-based ArcGIS Online services:
- Open your source map document in ArcMap and set your data frame coordinate system to WGS 1984 Web Mercator. You don’t have to re-project the source data, although this can make caching go faster. Some people even create a one-way replica of their geodatabase in the WGS 1984 Web Mercator coordinate system and use that replica just for caching.
- Publish a map service.
- Open the Service Properties and click the Caching tab. Choose to draw the map service Using tiles from a cache you will define below.
- Click Load tiling scheme from and select Google Maps / Microsoft Virtual Earth. Once the scales load, do not add or remove scales or change the tile size.
- Create tiles for just a small area of your map, using a feature class to constrain the caching extent if necessary.
- Test your new cache. There are already a few new ArcGIS Online services that use the Google / Bing tiling scheme, such as the World Topographic Map. Build a simple test application with these maps to evaluate how your cache overlays with ArcGIS Online in the Mercator projection. If your data doesn’t align, see the alternate steps below for creating a cache in WGS 1984 Web Mercator (Auxiliary Sphere).
- Once everything looks okay, open the caching tools again and create all the tiles for your cache.
Exceptions and Web Mercator (Auxiliary Sphere)
There are a few scenarios where you will have to follow some alternate steps to get your caches to overlay correctly with the Mercator-based ArcGIS Online services. The conditions are:
- You’re overlaying a cache with ArcGIS Online in the Web ADF OR
- Your data did not align correctly when building a test cache with the above steps. This is most often because your source data uses a datum other than WGS 1984 or NAD 83
In the above two scenarios, you need to publish your cache using the WGS 1984 Web Mercator (Auxiliary Sphere) coordinate system which is the exact coordinate system used by ArcGIS Online. The Web ADF requires an exact coordinate system match for cache overlays. Also, this coordinate system makes it easier to perform some datum transformations.
- Open your source map document in ArcMap and set your data frame coordinate system to WGS 1984 Web Mercator (Auxiliary Sphere). While you are doing this, apply any datum transformations to WGS 1984 that you require.
- Follow the steps in ESRI Knowledge Base Article 37329 to create a cache for the service.
Migrating map tiling schemes in the ArcGIS Online Help has more details about the switch and the WGS 1984 Web Mercator (Auxiliary Sphere) coordinate system used above.
Contributed by Sterling Quinn of the ArcGIS Server development team.
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