Not all tile services are created equal
Most of the web mapping community uses a standard tiling scheme. As far as I know, there isn’t an official name for the tiling scheme, though I’ve heard of it referred to as Google Maps style tiles.
What makes different tile services interoperable is the ability to request a tile at a particular place, using a simple URL, and get back a square image which is 256 pixels wide and high. Although the URL format differs slightly between providers, there are typically three key parts: level (aka, zoom), row (y), and column (x). Given a particular combination of values, each map service should return a unique image of the exact same place.
Seeing is believing
Here is an example tile shown using ESRI’s World Imagery map service, MapQuest’s satellite imagery, and a tile service from MassGIS.
Let me point out a few differences which you may have already noticed:
- The first two tiles align perfectly, but the last one does not. The last tile is the closest tile I could find to the other ones, but uses different values in the URL.
- The MapQuest tile swaps the row and column values in the URL, and puts a file extension at the end (.jpg). Most map controls make it easy to deal with these kind of differences in URL format.
While the first two tile services can be easily dropped into a web map, or used with a service that has configurable layers (e.g. Fulcrum), the last can only be used where the map control knows about ESRI map services. It uses a non-standard origin, which is why the “row” value is different, and the image is shifted from the others.
Are non-standard tile services bad? No, they’re not, but they are less generally useful. ESRI’s map control, the ArcGIS JavaScript API, knows how to talk to ArcGIS map services, and can handle these kinds of differences with ease. That’s one of the benefits of building on the ESRI stack (inner-interoperability, so to speak).
Producing non-standard tiles could provide benefits like optimized tile storage or deliver (e.g. MBTiles & Vector Tiles), or spatially faithful imagery (no, Greenland is not larger than the continental USA). Today’s non-standard method might be tomorrows standard.
A word of advice
If you’re providing a tile service for general use, please make it generally useful. I am using the term “standard” in a relative manner. No standards body declared Google Maps style tiles to be “standard”. However, the community has adopted that method, and the most widely available tools have followed. It’s been this way for a while now.
If you’re building a map and looking for a tile service to use, there are many different options available. Every one of them has a Terms of Service document, so be sure to read it, especially if you’re using it for commercial purposes or relying on it for heavy usage (e.g. the New York Times website).
