The Ghosts of LA live on the freeway
March 31, 2006 8:35 PM   Subscribe

Inspired by this excellent FPP, I was looking at the LA River on Google Maps, when I noticed something odd.

If you look at this map, there are images of "ghost" cars on the highway at the top. It doesn't change as I scroll around, so it's not like it's a seam effect. It doesn't go away when I switch from hybrid to satellite. And I haven't seen it on other highways I've looked at (not that I've done an exhaustive search). Is this caused by Google, or is it an artifact of the satellite imagery caused by the motion of the cars? And if so, why is it a ghost and not a blur?

What causes this? The deeper the explanation, the better ("yah it's the motion" doesn't do much for me).
posted by Eideteker to Technology (12 answers total)
I saved a copy locally, with a few annotations. I circled just a few of the cars, in case you weren't sure what I was talking about. I also think the one pair of ghost trucks maps to the pair of real trucks, though the distance between them looks different (could be moving at different speeds).
posted by Eideteker at 8:50 PM on March 31, 2006

Well, you're right that it's not the motion of the satellite. Ghost cars also appear on the highway that runs perpendicular to the one in question but they're still on the highway, not shifted off to one side. The dislocation is temporal, not spatial. I'm guessing the ghosts are the result of stitching multiple pics together in the original image creation (i.e. not by Google). The ghosts look a little like what happens in Photoshop if you subtract one imprecise copy of a layer from another. (the inexactness, which accounts the ghost image, would be caused by car or satellite motion.

Note to others: zoom in one more level to make it clear what Eideteker's talking about.

Some info (though not an exact answer to your question) on satellite image stitching here (scroll down to "image surface area").

On preview: yeah, the annotation helps, thanks.
posted by zanni at 8:55 PM on March 31, 2006

There are similar images of aircraft on google earth hacks here and here. The forums on that site don't really add anything to what zanni said. It seems that while airplane ghosts are common LA is the only area with car ghosts. Maybe the satellite that did LA had a timing problem.
posted by thrako at 9:09 PM on March 31, 2006

I don't think that the labeled pairs are made up of "ghosts." Enlarge the photos a bit and look closer. Flip back and forth between regular size and enlarged size.

The two trucks on the left are different. One has a white cab and one has a red cab.

The two cars circled in the lower middle of the picture don't really look like they have the same shape either. The lower vehicle is probably a car and the upper vehicle might be a smallish truck. And the lower vehicle is angled too, unlike the top vehicle.

Given these discrepancies I would guess that the other pair of trucks are just your basic silver/gray tractor trailer and cab. You've seen one, you've seen them all. I don't think the upper car set has the same shape either.

Very interesting question, though. Thanks for the puzzle.
posted by bim at 9:24 PM on March 31, 2006

zanni: I had noted that the cars stay on the highway. Scroll to the right on Google's site, and you'll see the road curves but the cars don't appear over the trees; they stick to the road. If they're the product of a subtractive process, why are there no ghosts of stationary objects? Also, if things are temporally shifted, it may be important to note that the ghosts are after the actual vehicles.

thrako: Note, however, that these are only the outlines of the cars, not a chromatically shifted image. There is, in fact, no surface at all, as far as I can tell; just an outline. Most peculiar.

bim: are we looking at the same thing? I'm saying the red cab and the white cab tractor trailers BOTH are duplicated to the right in the image. There are two pairs of semis, one real, one ghosted.
posted by Eideteker at 9:42 PM on March 31, 2006

One explanation might be that it is because of the overlap in aerial photography. Look here for a more detailed explanation.

I think they might be 'adding' the pictures to eachother, so on the overlap areas you would be looking at the car in two places.
posted by sebas at 10:04 PM on March 31, 2006

Unsubstantiable wild speculation: It appears to me as if the ghosts are caused merging a number of exposures, one of which is a partial exposure (or whatever the equivalent is on a satellite). Possibly the effect of seeing the outline most distinctly is the way the satellite deals with the input, searching for edges first before finding detail.
posted by MetaMonkey at 10:09 PM on March 31, 2006

The outline ghosts are the result of a sophisticated blending scheme that does more than just cross fade between overlapping images.
posted by StickyCarpet at 10:23 PM on March 31, 2006

Awesome. I think that looks about right. So it's not done by the satellite, but as a part of the stitching? I'd love to hear more about this.
posted by Eideteker at 11:04 PM on March 31, 2006

Well, remember, those aren't satellite photos you're looking at. They were taking from a plane or, more likely, a helicopter. They have too much resolution for satellite photos.
posted by kindall at 7:11 AM on April 1, 2006

I think StickyCarpet is on the right track with regards to detail vs blurred information, but I don't think this is an effect of stitching adjacent images or.

To me, both the LA map and both of the airplane pictures look like the combination of two exposures: One exposure captures the edges and detail of the image with no color, while the second captures a lower-resolution image with color values. This might be done because of sensor limitations, where both color and the desired level of detail cannot be captured by a single sensor at once. So then the two exposures are combined, adding the detail/edges from the one image to the other that has the color information.

Think of two full-resolution color images taken one after the other. The first is sent through a high-pass filter (example), and the second retains its color information but is put through a low-pass filter. Combining them would produce images like the LA map and both aircraft images, with a detailed "ghost" either ahead of or behind a blurry, but correctly colored image.

I'm not saying that these filters have been applied to a single image like this, but the effect from combining what I assume are two separate exposures with different characteristics is the same. One sensor captures luminance detail while the other captures chrominance values in a lower resolution.

This works well for stationary objects, because their detail and color data will line up correctly even when captured at slightly different times. But moving objects will exhibit the ghosts we see here.

Look at the airplane examples. See how the trailing plane is much more detailed, yet has the color of the terrain underneath, while the leading plane has correct color but is blurry? Same with the vehicles on the LA freeway, but the detail image was taken after the color image, so the ghosts are ahead of the colored vehicles. Also notice that any stationary cars in the LA freeway image (for example in the parking lot to the SW of the interchange) are more detailed than colored images of the moving cars on the freeway.
posted by whatnotever at 9:16 AM on April 1, 2006

Another very interesting example, just outside the mouth of the LA River.
The ghost is leading the barge/tug in its motion.
posted by UnclePlayground at 12:22 PM on April 3, 2006

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