Web Map Caching and Tiling. Overview David M. Horwood June 2011

Transcription

1 Web Map Caching and Tiling Overview David M. Horwood June 2011

2 Web Mapping Traditional Geographic projection Select / Refresh workflow Slow, non-interactive (refresh delay) Modern AJAX (Asynchronous Java and XML) Stream pre-rendered tiles to browser Browser caches tiles, no need to resend More interactive map (dynamic pan and zoom)

3 Traditional Map

4 Modern Map

5 Web Map Caching Popularized by Google and adopted by Microsoft and ESRI (and others) Divides the world into tiles starting with 4 tiles and successively divides the world Stores tiles relative to Web Mercator Starts at level of detail (LOD) 1 and recursively subdivides each tile into 4 parts

6 Web Mercator Spherical projection Maps WGS84 LL as spherical LL Scale increases with distance from equator Scale asymptotic at poles Replicated east and west in window for seamless pan

7 Level of Detail World is divided successively into tiles Indexed by quadtree code LOD by convention defined down to level 23 (approx 2cm)

8 Tile Size 256 x 256 pixels = 192KB RGB Tiles normally stored as JPG or PNG JPG for raster imagery PNG for rendered vector Compressed size ~ 40KB JPG fairly consistent, PNG can be variable Average display 1024 x x 3 = 12 tiles = 480KB 5 x 4 = 20 tiles including overlap = 800KB

9 Tiles

10 Level of Detail and Sizes 1 Tile = 40 KB 0.1 s LOD Scale Pixel Tiles Size Source Time 1 295,829,355 : KB 0.4 s 2 147,914,678 : KB 1.6 s 3 73,957,339 : MB 6.4 s 4 36,978,669 : MB 25.6 s 5 18,489,335 : MB 1.7 m 6 9,244,667 : MB 6.8 m 7 4,622,334 : MB 27.3 m 8 2,311,167 : GB 1.8 h 9 1,155,583 : GB 7.3 h ,792 : GB 1.2 D ,896 : GB 4.9 D ,448 : GB 19.4 D 13 72,224 : TB Landsat 2.6 M 14 36,112 : TB 10.4 M 15 18,056 : E TB 3.5 Y 16 9,028 : E TB 13.8 Y 17 4,514 : E TB 55.2 Y 18 2,257 : E PB Satellite Y 19 1,128 : E PB Aerial Y : E PB Aerial 3,535.0 Y : E PB 14,139.8 Y : E PB 56,559.2 Y : E EB 226,237.0 Y

11 Cache Size Estimate 1 Tile = 40 KB 0.1 s Coverage Area (km²) Pixel Tiles Size Source Time Web Mercator 1,606,006, GB 1.2 D Earth 510,000, GB 9.2 h Land (30%) 153,000, GB Landsat 7.4 D Urban (1.5%) 2,295, TB Satellite 3.8 M

12 Cache Creation 1. Project data into target projection 2. Render data at each of the target scales 3. Store raster tiles (JPG/PNG)

13 Cache Display 1. User requests area and level of detail 2. Server sends the appropriate tiles at LOD 3. Browser displays and retains the tiles 4. As user zooms or pans, server sends new tiles as required 5. Browser handles redisplay and shifting of tiles

14 Cache Benefits No projection required for display Simple spherical equations to convert xy to LL (and for approximate distance / area) North is always north Adopted by Google, Microsoft, ESRI (and others) Existing rendered geographic context

15 Cache Problems Increase in scale from equator Develop must correct Cartesian meaurements Asymptotic at the poles Unusable > 84 N/S Must use Google approach / formulas WGS84 LL used as spherical LL Must project if non-mercator map

16 Cache Problems (in Canada) Cache increases dramatically north of 60 N 45 N => 1.4x 60 N => 2.0x 75 N => 3.9x 84 N => 9.6x Caching to same level increases cache size over 3x

17 Challenges to Improvement Current caching model solves two problems with a single solution Storage Display Any more complex system needs to insert computation either at the server or the client or both Leverage existing rendered caches

18 Middle Ground Dynamic Projection (AGS, others) GEOIDE Project Target (professional) Simple Web Map (Google, Bing, ESRI, others) Whole Earth (Google, Bing, ESRI, NASA)

19 Alternatives You cannot flatten the whole earth Either cut into pieces (i.e., not seamless) View in 3D (hides one half of the earth) Options Hybrid projection (e.g., conic + cylindrical) Unfolding projection Orthographic projection of ECEF 3D virtual environment

20 Dymaxion Map Low distortion unfolding projection Could be used in conjunction with hierarchical triangular mesh

21 Earth Centered Earth Fixed (ECEF) Coordinates actually calculated by GPS Can be used as a basis for whole earth

22 Hierarchical Triangular Mesh Can serve as both tile and index ECEF centroid is normal

23 Orthographic Projection Simple linear transformation Rotate to ECEF centre and eliminate Z Could be optimized Transform tiles on-the-fly during display Use 3D rendering API (DirectX, OpenGL) Distorts as display approaches hemisphere

24 Orthographic Projection

25 Properties of Ideal Solution Whole earth Seamless Minimize distortion Measure distance and area Efficient storage and retrieval Multi-resolution Dynamic pan North up Mobile and Desktop (Browser) Javascript, Flex, Silverlight APIs

26 Questions?