Web Mercator Projection and Raster Tile Mapsestef/talks/CCA2017_Technical_Session... ·...

Web Mercator Projection & Raster Tile Maps

Two cornerstones of Online Map Service Providers

Emmanuel Stefanakis Dept. of Geodesy & Geomatics Engineering

University of New Brunswick

[email protected]

Annual Conference – 2017 – Ottawa May 31- June 2, 2017

Online Map Service Providers

O Available for ~12 years

O Well established (difficult to remember how

life was without them)

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3

4


Challenges ?

• Technological

• Cartographical

Online Map Service Providers Framework

CLIENT

HTTP

Request (where/what/resolution)

Map Product

SERVER

End-users

Map Repository (Data+Images)

Map Service

5

Technological Challenges

O High and steadily increasing demand

O Voluminous data (vectors and images)

O Bandwidth limitations

O Concurrent access

O Security issues

O Data protection

6

Cartographical Challenges

O Visualize the map on Web browser

O flat/rectangular monitor

O Multi-resolution representation

O various zoom levels

O Multi-theme mapping

O various themes (layers)

O Multi-language mapping

O various languages

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8


Web Mercator

Projection

Raster Tile

Maps

Map Product Flat Monitor

CLIENT

HTTP


Map Product

SERVER


Map Service

9

Map Projection

X

Y

φ

λ(Χ,Υ) ƒ(φ, λ)

X

Y

X

Y

φ

λ

φ

λ(Χ,Υ) ƒ(φ, λ)

O A function f ...

10

Choose a Map Projection

11

Choose a Cylindrical

O Best fit

O Rectangular grid

12

Conical Projection ?

13

Conical Projection ?

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Cylindrical Projection ?

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Cylindrical Projection

O Simulation of a cylindrical projection

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Cylindrical Projection

O Simulation of a cylindrical projection

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Which Cylindrical?

Miller’s

Plate Carree

Mercator

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Mercator Projection

O Most popular

O Conformal

O Easy math (low complexity)

O …to serve high demand

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Mercator Projection

X

Y

X

Y

X

Y

φ

λ

2πR

2πRcos(φ) < 2πR

2πR

2πR

20

1512-1594

O Conformal

O Support navigation in Seas

Mercator Projection

2πR

2πR

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Mercator Projection

Scale factor on horizontal (along parallels at φ): 2πRcos(φ) 2πR

Mercator’s goal preserve

shapes/directions

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Mercator Projection

Scale factor on horizontal (along parallels at φ): 2πRcos(φ) 2πR

Mercator applied… Scale factor on vertical (along meridians at φ): 2πRcos(φ) 2πR

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Mercator Projection

All parallels have the same length = equator (2πR)

At latitude φ the scale factor on the parallel is equal to:

secφRcosφ2π

R2π

φ

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Mercator Projection

Mercator applied the same factor on the parallels… The actual distance between two parallels at latitude φ is equal to: He made it equal to:

dN = R secφ dφ

dN = R dφ

φ2

φ1

dN

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Mercator Projection

By integrating the last formula we get: (sphere)

2

φ45RlntandφsecφRN o

φ

0

equator

North pole

South pole

φ

N

2π R

φ

2π R

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Mercator Projection O Spherical Formulas

Easting

Northing

Sphere

P(φs , λs)

Spherical Formulas

P(Es , Ns)

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Mercator Projection O Ellipsoidal Formulas

Easting

Northing

P(φe , λe)

WGS’84

Ellipsoidal Formulas

P(Ee , Ne)

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Mercator Projection O Spherical Formulas

O Ellipsoidal Formulas

Easting

Northing

Easting

Northing

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Adopt Mercator Projection

CLIENT

HTTP


Map Product

SERVER


Map Service

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O Ellipsoid Coordinates

O Mercator Projection

Web Mercator O Make it even less computational expensive

O Ellipsoid Coordinates – Spherical Formulas

P(φe , λe)

WGS’84

Sphere

P(φs , λs)

Web Mercator

Spherical Formulas

P(E, N)

31

32


Web Mercator

Projection

Web Mercator (vs) Mercator

O Mercator basic properties are lost

O Non-Conformal

O Loxodromes Not straight lines

O Rationale (Google)

O Simpler calculations

O Map to be projected on a computer monitor

O No body can notice the differences

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Criticism

O Geodesy Subcommittee of the Oil and Gas

Producers (aka EPSG) rejected it in 2008 as

“technically flawed” & “an inappropriate geodesy

and cartography”

O National Geospatial-Intelligence Agency of the

US Department of Defense declared it as

“unacceptable for any official use, because a

general lack of understanding of its properties

has caused considerable confusion and misuse”

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A Code for Web Mercator

O An unofficial code “900913” (GOOGLE

spelled with numbers) was initially assigned

O Later, EPSG introduced an official identifier:

EPSG:3857 -- WGS84 Web Mercator

(Auxiliary Sphere)

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Web Mercator Map of the World

O Absolute square…

O Very convenient in

panning, tiling, and

indexing

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40,075,017.25

40,0

75,0

17.2

5

85.051129 South

85.051129 North

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Challenges ?

• Technological

• Cartographical

How to deliver the map product?

CLIENT

HTTP


Map Product

SERVER


Map Service

39


40

SERVER

Map Repository(Data+Images)

Map Service

Web Mercator

One map?


41

SERVER


Map Service

Web Mercator

One map?

Map product:

Various areas


42

SERVER


Map Service

Web Mercator

One map?

Map product:

Various resolution


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SERVER


Map Service

Web Mercator

One map?

Map product:

Various theme

Various language

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One map ?

Multiple themes

Multiple Languages

SERVER


Map Service

On-the-fly rendering ?

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On-the-fly generalization ?

Use of Map Tiles

CLIENT

HTTP


Map Product

SERVER


Map Service

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Preprocessing generation of map tiles

One map not an option

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Raster Tile

Maps

Use of Map Tiles

CLIENT

HTTP


Map Product

SERVER


Map Service

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Preprocessing generation of tiles

Tiled maps

O A series of precompiled maps…

O for… various areas

O in… various resolutions

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Tiled maps – An old practice

O Old practice…

O Sheet Division and Organization of

Topographic maps

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Sheet Division & Organization

O Topographic Maps

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Sheet Division & Organization

O NRCan: National Topographic Map Index

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Web Mapping Tiled Maps

O Web mapping

O rapid growth of map data availability

O rapid growth of demand

O Use of map tiles (slippy maps)

O pre-computation and caching of map image tiles

O map servers use far fewer resources than maps rendered on demand

O delivery only limited by the bandwidth of the connection with the map server

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Tiled Maps

O Google…

O was the first major mapping provider to adopt

the tiled web maps

O Others followed…

O Bing and OpenStreetMap, Esri and Oracle

O provide functionality for map tiling and caching

of vector layers and/or raster images

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Google Maps

O Conventions…

O All map tiles

O Square-shaped and equal-sized

O 256x256 pixels

O World rendered in a single tile at the outer

most zoom level: 0

O Projection: Web Mercator

O latitude values [-85.0511, +85.0511] degrees

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Google Maps

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x

y0,0

0,0 1,0

0,1 1,1

0,0 1,0

0,1 1,1

0,2 1,2

0,3 1,3

2,0 3,0

2,1 3,1

2,2 3,2

2,3 3,3

Zoom Level: 0Number of Tiles: 1



column

row

Bing Maps

O Quadtree encoding: Quadkeys

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Google Maps – Zoom Level [0-24]

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http://www.maptiler.org/google-maps-coordinates-tile-bounds-projection/

to Zoom Level 23…












#Tiles & Pixel size / Zoom Level

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Each tile: 256x256 pixels

LEVEL TILES/LEVEL Pixel Size (m)0 1 156,543.034

1 4 78,271.517

2 16 39,135.758

3 64 19,567.879

4 256 9,783.940

5 1,024 4,891.970

6 4,096 2,445.985

7 16,384 1,222.992

8 65,536 611.496

9 262,144 305.748

10 1,048,576 152.874

11 4,194,304 76.437

12 16,777,216 38.219

13 67,108,864 19.109

14 268,435,456 9.555

15 1,073,741,824 4.777

16 4,294,967,296 2.389

17 17,179,869,184 1.194

18 68,719,476,736 0.597

19 274,877,906,944 0.299

20 1,099,511,627,776 0.149

21 4,398,046,511,104 0.075

22 17,592,186,044,416 0.037

23 70,368,744,177,664 0.019

24 281,474,976,710,656 0.009

4 ^ LEVEL 2πR/(256*2^LEVEL)

GRS80 (eq) 6,378,137.00 m

Circumference (2πR) 40,075,016.69 m

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Kotsollaris, M., 2017. A Scalable Web Tiled Map Management System. MScE Thesis. University of New Brunswick.

Map Tile Management

Interoperability across multiple Map Providers

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http://docs.opengeospatial.org/wp/16-019r4/16-019r4.html







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Web Mercator

Projection

Raster Tile

Maps

References

O Stefanakis, E., 2015. Web Mercator: the de facto standard, the

controversy, and the opportunity. GoGeomatics. Magazine of

GoGeomatics Canada. October 2015.

O Stefanakis, E., 2015. Map Tiles and Cached Map Services.

GoGeomatics. Magazine of GoGeomatics Canada. December

2015.

O MapTiler. Tiles à la Google Maps: Coordinates, Tile Bounds and

Projection. http://www.maptiler.org/google-maps-coordinates-tile-

bounds-projection/

O Tilesets: Google Maps Structure – MicroImages.

http://www.microimages.com/documentation/TechGuides/78goo

gleMapsStruc.pdf

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http://www.gogeomatics.ca/magazine/web-mercator-the-de-facto-standard-the-controversy-and-the-opportunity.htm

http://www.gogeomatics.ca/magazine/web-mercator-the-de-facto-standard-the-controversy-and-the-opportunity.htm

http://www.gogeomatics.ca/magazine/map-tiles-and-cached-map-services.htm












http://www.microimages.com/documentation/TechGuides/78googleMapsStruc.pdf



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