Merge pull request #11 from jtfell/scheme-and-projection

Formalise the ability to use alternative SRS and tiling scheme

README.md

@@ -2,7 +2,7 @@
 
 Have a question? Discuss the quantized-mesh specification on the [Cesium forum](http://cesiumjs.org/forum.html).
 
-A terrain tileset in quantized-mesh-1.0 format is a simple multi-resolution quadtree pyramid of heightmaps according to the [Tile Map Service (TMS)](http://wiki.osgeo.org/wiki/Tile_Map_Service_Specification) layout and global-geodetic profile. All tiles have the extension .terrain. So, if the Tiles URL for a tileset is:
+A terrain tileset in quantized-mesh-1.0 format is a simple multi-resolution quadtree pyramid of heightmaps. All tiles have the extension .terrain. So, if the Tiles URL for a tileset is:
 
 ```
 http://assets.agi.com/stk-terrain/world/tiles
@@ -176,6 +176,17 @@ struct EdgeIndices32
 
 These index lists enumerate the vertices that are on the edges of the tile. It is helpful to know which vertices are on the edges in order to add skirts to hide cracks between adjacent levels of detail.
 
+## Tiling Scheme and Coordinate System
+
+By default, the data is tiled according to the [Tile Map Service (TMS)](http://wiki.osgeo.org/wiki/Tile_Map_Service_Specification) layout and global-geodetic system. These defaults can be varied by specifying the `projection` and `scheme`.
+
+Allowed values for the projection are `EPSG:3857` ([Web Mercator](https://en.wikipedia.org/wiki/Web_Mercator_projection) as used by Google Maps) and `EPSG:4326` (Lat/Lng coordinates in the [Global-Geodetic System](https://en.wikipedia.org/wiki/World_Geodetic_System)). It is worth noting that the `EPSG3857` projection has only 1 tile at the root (zoom level 0) while `EPSG:4326` has 2.
+
+The options for the tiling scheme are `tms` and `slippyMap`. The Y coordinates are numbered from the south northwards (eg. latitudes) in the `tms` standard whereas `slippyMap` coordinates have their origin at top left (NW).
+
+For Cesium terrain layers these options can be set in the `layer.json` manifest file. If not specified, they default
+to `EPSG:4326` and `tms`.
+
 ## Extensions
 Extension data may follow to supplement the quantized-mesh with additional information. Each extension begins with an `ExtensionHeader`, consisting of a unique identifier and the size of the extension data in bytes. An `unsigned char` is a 8-bit unsigned integer.
 
@@ -271,4 +282,4 @@ struct Metadata
 __Requesting:__ For the metadata to be included in the quantized-mesh, the client must request this extension by using the following HTTP Header:
 ```
 Accept : 'application/vnd.quantized-mesh;extensions=metadata'
-```
+```