After creating a global map of historical imagery density last week, we have been experimenting with higher resolution versions for a small area.
High resolution historical imagery density Latitude 0°-15° Longitude 0°-20°.
As you can see above, there is a curious pattern of vertical and horizontal lines. At first we thought this was an artifact of the way the Google Earth plugin reports historical imagery, as is the case over the oceans, but on further investigation we found it has to do with the way satellite imagery is collected. We were aware of this, but don’t think we have talked about it before.
Satellite imagery appears as either squares of imagery (or rather parallelograms, depending on the angle they were captured) or strips. The strips almost always run in the North-South direction, but occasionally are in the East-West direction and very rarely at some other angle. All of the above are typically 17-20 km on the shortest side. The interesting feature we are looking at today is the fact that the majority of the strips are lined up with lines of latitude and longitude with strips starting and ending on exact degrees.
The best way to see this is to turn on the Grid (found in the ‘View’ menu) and then zoom until the grid shown matches single degrees.
The effect is very noticeable in sparsely populated regions, such as the Sahara desert above, where most of the imagery is in strips.
The pattern can be seen at all latitudes. In Canada, above, even horizontal strips can be seen to line up with whole degrees of longitude even though the distance on the ground is about half that for a degree of latitude in this location.
The pattern seems to be followed by both DigitalGlobe imagery and CNES / Astrium imagery, although the later is rarer and so harder to check. We do not know whether this has something to do with the way the satellites capture the imagery, or whether it has to do with the way satellite imaging companies catalogue and sell imagery. If any of our readers knows more about it, please let us know in the comments.
About Timothy Whitehead
Timothy has been using Google Earth since 2004 when it was still called Keyhole before it was renamed Google Earth in 2005 and has been a huge fan ever since. He is a programmer working for Red Wing Aerobatx and lives in Cape Town, South Africa.
Where new imagery update from last month?
The shape of the imagery is because of the orbit. These satellites (IKONOS, QuickBird2 etc) are in polar orbits, which means they are flying north/south patterns. Some satellites have methods to allow them to capture east/west imagery but it is more difficult and requires a gimble holding the camera or a mirror the camera is focused on.
The reason polar orbits for these satellites are used is that it means that any part of the planet can be imaged at some point in the day. Sometimes the angle is rather extreme for a good picture, but excluding cloud cover and pollution problems, you can get a picture of it. Quite a few satellites for scientific studies are also in polar orbit for the same reason so that they can collect data on the entire planet every day.
Thanks. But I talk about release New Imagery Update from Google with release map. Where is release map kml?
I wasn’t replying to you. I was replying to the original post.
These patterns are due to the majority of the historical imagery coming from aerial platforms – not satellites. If you examine those blocks of imagery and the dates they were taken, most of them happen before we had satellites with the spatial resolution visible in those images. Generally these were collected for some particular reason, like the NAPP imagery in the USA. Search the NRC (Natural Resources Canada) Earth Observation catalog at https://neodf.nrcan.gc.ca/neodf_cat3/index.php and you’ll see that many of the aerial photo collections listed there match up to the patterns on the map of historical imagery.
If it were from satellites, it would not be nice north and south shapes like that. Munden’s comment has some truth in it, but it is important to note that no Earth observing satellites have a truly Polar orbit. They have Near Polar orbits between 97 – 99º (Quickbird-2 has an orbital inclination of 97.2º, Ikonos-2 is at 98.1º) for the purpose of passing over the same latitude at the same local time. For example, Ikonos-2 passes over the equator at 10:30am local time on every orbit. The result is that any image swath would be a parallelogram angling across the grid at about 8º or so. If it is satellite imagery and it is in 1×1 degree blocks (or variations on that) it is due to processing and Google Earth purchasing X number of square miles which have been stiched together from many different scenes.
One more point to make is that all of the truly high resolution Earth imaging satellites have swath widths MUCH smaller than most of the patterns you are seeing here. Worldview 3, for example has a swath width of just 13.1 km. For comparison, Landsat images are about 180km swath width & would take 16 days to capture the entire globe (no off-nadir viewing). Keeping the off-nadir look angle below 20º on Worldview 3 means revisit times for any particular point are over 4 days (every day if you are willing to go for extreme look angles).
All the satellite specs are checked using the ESA Satellite Mission Database (https://directory.eoportal.org/web/eoportal/satellite-missions)
Timothy Whitehead says
The strips of imagery I am referring to are definitely satellite imagery and the majority come from DigitialGlobe. I am aware that the the North-South pattern is because of their orbits. My real question is why the strips line up with exact degrees of latitude and longitude. It that how they are gathered and is it purely for cataloguing reasons?
I should also mention that I have seen up to three strips gathered on the same day but in three different passes (so different times of day). Based on the clouds, I can confidently say that any given strip is gathered at one time although whether it is a single imager or stitched images I am less certain.