HOW GOOGLE EARTH WORKS?By Ahmad Karawash

When using Google Earth, people can take for granted the ability to zoom in, rotate, pan and tilt on an image

Google trick

The trick of Google Earth is not in compiling and storing all these images, but in getting them to your computer quickly and efficiently.

Do you know?

The Earth is approximately 40,000 km around the waist. If you stored only one pixel of color data for every square kilometer of surface, a whole-earth image would be about 40,000 pixels wide and half as tall.

=> With a 56k dial-up modem, it would take 12,400 years to download a one-meter resolution image of the Earth.

How Google earth deal with this problem?

Decrease the transfer time: One way to cut down on transfer time is by using your

computer's disk cache, If your computer can temporarily store viewed images of the Earth, it won't need to retransfer the images from the source.

how your computer processes and displays maps?

Universal Texture is used. Universal Texture uses two methods

for getting large amounts of information to your computer Mip-mapping & clip stacking

Mip mapping

Mipmaps are collections of bitmap images that work within a texture to create the illusion of depth. They work in an inverted pyramid structure, stacked on each other with each level having twice the resolution of the one under it.

These stacks ultimately create a three-dimensional representation of a two-dimensional image.

So, for example, if you provided the hardware with a nice 512×512 source image, it would compute and store 8 extra mip-levels for you (256, 128, 64, 32, 16, 8, 4, 2, and 1 pixel square)

Clip stacking

Clip stacks are portions of giant mip-maps that are clipped to a specified maximum size. The benefit of clipping these enormous files is simple - Google Earth relies on the fact that users will only need to see one piece of the mip-map at a time.

When the user tells the program where they want to fly, the program uses an algorithm to figure out which sections of the larger virtual texture are needed and uses only those sections to create the final images

what about the earth case where last level resolution is about 66 million pixels ?

Google earth clip (divide) the texture level into many smaller distinct tiles, 256 pixels square, where each gets mip-mapped individually and instead of loading and drawing the whole-earth texture all at once — which is impossible on most current hardware- The angle and height of this virtual column changes depending on our current 3D perspective.

Level of Detail (LOD)

This level consist of group of regions. Google earth use a kml language to represent data of

these regions

Region

 Region has a <LatLonAltBox> that defines a bounding box for data, the <LatLonAltBox> in a Region has North, South, East, and West boundaries. If the data contained by the Region is 3D, or is 2D at altitude, then the Region's <LatLonAltBox> also needs to include a minimum altitude, <minAltitude>, and a maximum altitude, <maxAltitude>.

Ref. Google earth technology researches

For more information:Ahmad.karawash@gmail.comPhd researches, Software DeveloperCanada, 2014.