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Comment by exit | Hacker News Reader

interesting - i guess the rgb (and luminosity?) channels are captured in succession? was this captured by a satellite?

I think that is what happened here.

Google Maps uses, among others, images from the Ikonos satellite. Here are the specs: http://www.satimagingcorp.com/satellite-sensors/ikonos.html

It has a panchromatic sensor (greyscale) and red, green and blue sensors. Ikonos has a ground speed of 6.8 km per second, so even if the sensors are only a few centimeters apart, the images would need to be recombined at an offset. The offset would be tuned for static ground objects.

So I guess the ghosting is due to both the altitude and the speed of the airplane relative to the ground.

The specs site mentions resolution of 0.82 meters panchromatic - would that be per pixel? And if so, how come the images at Google Maps have higher resolution?

Google uses a variety of data sets, including regular aerial photography and multiple remote sensing products. If houses and cars look sharp at the highest zoom, you're probably looking at aerial photography.

Exactly, and the plane's image contains small objects above that resolution threshold, so it seems that is wasn't taken by the Ikonos system.

Because the direction of the offsets in the plane is at the plane's direction it is much more likely caused by the plane's speed than its altitude or the speed of Ikonos.

Which agrees with what Luc said. Essentially he's saying that Ikonos has to factor out its own motion. Hence the only motion left that's not factored out is the plane's.

Not quite. Even if the plane had zero ground speed, and the Iknonos were offsetting the four images to account for its own motion, the effect would still appear. That's because the offsets that align features on the ground will not be the same offsets that align features on a nearer plane (no pun intended), due to parallax.

If it was a parallax issue, there would be fringes of colour on anything tall. (The plane is very low in that image, probably going to or coming from the runway just to the southwest)

Or this plane happens to be travelling in the direction of the satellite?

EDIT: here's another plane with the same (grey, red, green, blue) sequence: http://maps.google.com/maps?oe=utf-8&ie=UTF8&gl=us&#...

The sideways offset seems to be roughly half as much as the offset in the direction of movement of the plane, so at least that component would be due to the sensor platform moving...

That plane is headed straight for PHX, and isn't too far away, meaning it's probably traveling a lot slower than the other.

the resolution is to high for Ikonos. But apart from that you are right.

That's definitely not Ikonos imagery, the imagery is aerial.

It's also not the sensor distance that contributes to the halo effect for some satellites, it's the capture time. Often, each band is captured one after the other. Some satellites, like Rapideye's, have several seconds between certain bands. For instance their B/R bands are (I believe) first/last bands captured (of 5 total), so all moving clouds in their imagery get a red/blue halo.

In the UK at least, the higher resolution aerial imagery is captured by aircraft, from a couple of companies. The one I know is Geoinformation Group, which has a good historic archive as well as a continuous survey of the country.

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