Image Tile

Tile Interpretation

There are two types of tiles: map tiles and elevation tiles. Map tiles (introduced at ArcGIS 10.1) represent rendered map images, typically in Jpeg or Png. Elevation tiles (introduced at ArcGIS 10.3) represent elevation data and are stored as limited error raster compression (LERC) tiles.

In terms of topology, elevation tiles differ from normal map tiles in two ways:

• When the service specifies a tile size of 2ⁿ pixels (rows / columns), elevation tiles actually contain 2ⁿ+1 pixels in each dimension.
• Pixel values for each elevation tile are sampled with an offset of -0.5 * resolution relative to the matching map tile.

For a tile size of 4 by 4 pixels, these properties result in the following arrangement for a colocated map tile and elevation tile.

This arrangement allows the elevation samples (the pixels of the elevation tile) to be used directly as height values for the vertices of a regular-grid 3D mesh, and for the colocated map tile to be used as a texture for that mesh.

Deriving Tile Extent from Tile ID

Given the following notations:

Service Extent (serviceExt): {xmin, ymin, xmax, ymax} 
Tile Origin (origin): {x, y}
Tile Width and Height (tileSize): {cols, rows}
Each LOD (lod): {level, resolution, scale}
Tile ID (tileId): {level, row, column}
Tile Extent (tileExt): {xmin, ymin, xmax, ymax}
Level (lod): LODs[tileID.level]

Tile extent can be derived using the following formulas, which applies to both map and elevation tiles.

tileExt.xmin = tileId.column * tileSize.cols * lod.resolution + origin.x;
tileExt.xmax = tileExt.xmin + tileSize.cols * lod.resolution;
tileExt.ymin = tileId.row * tileSize.rows * lod.resolution + origin.y;
tileExt.ymax = tileExt.ymin + tileSize.rows * lod.resolution;

For more information about tiled elevation services, see ArcGIS for Developers documentation.