Whenever we look at imagery update maps we notice certain patterns in the updates. One of the factors that causes these patterns is seasonality. We have long known that Google avoids snow cover in imagery where possible. This means that they largely avoid capturing imagery in winter in high latitudes. We thought it would be interesting to use the Google Earth API to see how strong an effect that was. The results were quite interesting and not what you might expect.
We used the Google Earth API to scan the globe and for a given location check all the dates of available historical imagery then calculate which was the most frequent month. We then colour coded the months with an approximately seasonal colour scheme and created a map. Redder colours indicate northern summer and bluer colours indicate northern winter.
To see the map in Google Earth download this KML file
As initially expected, the far north and far south are dominated by imagery captured in the summer months of the respective regions. However, this trend does not hold throughout and there is a reversal in the tropics. This is because of cloud cover. In the tropics, cloud cover roughly follows the sun, with rain mostly occurring in the summer and clear skies occurring in the winter. There are exceptions, such as along the coasts, where rainfall patterns are affected by the ocean, and deserts where there is low cloud cover year round.
Overall, the patterns are quite distinctive and it is clear that when imagery updates occur is strongly affected by cloud cover and snow cover.
A lot of aerial imagery in Google Earth does not have a specific date, but rather a date range. The start date is shown in the status bar and the end date on the ‘historical imagery’ timeline. Our map was using the end date. Some aerial imagery, especially the very old imagery, just has a year and this resulted in our algorithm seeing it as ‘December’. This is the reason for the United Kingdom and parts of Europe showing dark blue (December).
When there was more than one maximum, we picked one at random. This is not ideal, but was sufficient for showing general trends.
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.
Daniel Demonceau says
Is there a reason why the Bristish Islands are mostly photographed during the Winter season? GREAT blog!
Timothy Whitehead says
See the notes near the end of the post. The UK has aerial imagery from 1945, 1960, 1990 and 2000 and more, which has only the year listed in the date. My algorithm picked that up as December.
Maarten Andriessen says
If for whatever reason the date is not known, or in some cases if it is a collection of photos made over periods of different times during the year but distributed as one set to Google Earth, the date defaults to 12/31 or 1/1.
Surely the avoidance of extensive shadows from the sun low in the sky throughout shorter days is another key reason to avoid capturing imagery in winter in higher latitudes.
As you say Timothy, the December date for the UK is an artefact of the Google data, and on the ground signs such as bright yellow fields of rapeseed flowers in Spring and deciduous trees in full leaf indicate that the vast majority of the default imagery in the UK was captured in the lighter summer months between the Spring and Autumn equinoxes.
Timothy Whitehead says
Good point. I had thought about the problem of a near absence of sunlight for some months near the poles but had not considered the shorter days and longer shadows.
Wilmer Osario (Wguayana) says
In Venezuela there is a city called “Cumana” that every month they put a new image