My Google Map Blog

A couple of years ago a GEB reader asked us about a row of dots he had found in the Amazon. At first we thought it might be a small aircraft and a similar effect to the well-known ‘rainbow effect’.

However, we had a look around the region and found a number of other nearly identical rows of dots. They all occur in imagery labelled ‘CNES/Spot Image’. This is low resolution imagery that has been used in a number of places in Google Earth to fill in the gaps where there is no high resolution satellite imagery available. Sometimes it is just two or three dots, sometimes a long row. All the rows line up in the same direction which strongly suggests that they have something to do with the way the satellite captures the imagery.

There have been seven SPOT satellites so far and since Google Earth does not put a date on the SPOT imagery, we don’t know which satellite in particular the imagery comes from. Many satellites use either a single sensor or a row of sensors, which in combination with a moving mirror can be used to photograph a large area. We have discussed this before with the Landsat 7 satellite. The best explanation we have at present is that a single sensor malfunctions temporarily and this leaves a trail of dots in the image as a result. We considered the possibility that that sensor is being blinded by something highly reflective on the ground such as a tin roof. However, although there are some rows of dots in farmed areas, we could find no obvious correlation between human habitation and the dots.

To see all the dots we have found so far in Google Earth, download this KML file. Do not read too much into the distribution of our placemarks as that has more to do with the extent of SPOT imagery and our search method than anything else.

If any of our readers has noticed a similar effect in other parts of the world, or has a better explanation for their origin, please let us know in the comments.

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As we have discussed before Google Earth shows different imagery depending on the zoom level and whether or not you are in ‘historical imagery’ mode. We have also talked about how the ocean bathymetry data changes in ‘historical imagery’.

When you are looking at the whole globe in ‘default’ mode, the imagery used for the land was cleverly created by taking Landsat imagery captured over a period of time and automatically removing cloud and snow cover by combining multiple images of each spot. As you zoom in, this transitions to higher resolution satellite or aerial imagery. For much of the world, the individual satellite images or patches of aerial imagery can be identified by means of the ‘historical imagery feature’. However, there are some ‘background’ sets of imagery that are not at first obvious.

If you view the whole globe in ‘historical imagery’ and move the time slider all the way to the left, the image you will see, looks very similar to the Landsat imagery seen in the zoomed out ‘default’ mode. However, this image is a different one and the attribution shows NASA as the provider. It is most likely MODIS data, which comes from Nasa’s Terra and Aqua spacecraft. Now, still in ‘historical imagery’, if you move the timeslider all the way to the right, all the satellite and aerial imagery found in ‘historical imagery’ gets superimposed on top of the background image. However, because the background image is such low resolution it is fairly easy to see where it shows through and there is no other satellite or aerial imagery available to date. If you find one such location, zoom right in, and then switch back to ‘default’ mode instead of the NASA imagery, for most parts of the world, Google Earth shows imagery attributed to CNES/Spot image. This is imagery that has been obtained via an agreement with Spot Image that we first reported on in 2007 when the imagery was first added in parts of Europe. Since then, most of the rest of the world has obtained similar imagery.

None of these ‘background image’ sets have any dates associated with them, nor, as far as we are aware, are they noted on Google’s Imagery update maps when they receive updates.

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