« Amsterdam in 3D | Main | New historical Street View sites in Italy and France »
March 30, 2011
How Google Earth displays dates on their imagery
As you probably know, when you're looking at an area on Google Earth, the date the imagery was captured appears in the lower-left corner, as shown here:
However, what does that date actually mean? As some of you have pointed out, the date doesn't always correspond with the imagery (snow on the ground in July, etc).
For standard satellite images, it's simply the date the imagery was taken, which makes sense. Easy enough. The discrepancies arise when Google loads imagery for a large area from a commercial aerial provider. In those cases, they're given a range of dates for the imagery. The date you see on the screen is the "oldest known date" for that imagery, while the tic mark in the Historical Imagery sliders is the "newest known date". In many cases, those date ranges can be up to a few months apart.
To confuse it further, some providers don't even have exact dates for a batch of imagery; they might simply say "April-June, 2010". In those cases, Google considers that to be "April 1 - June 30, 2010", and then displays the date as explained in the previous paragraph.
While the system obviously isn't perfect, it's certainly improving. Google Earth didn't start showing the date in the corner until version 5 came out (so you had less of an idea of when the imagery was captured), and the Historical Imagery tool was certainly a great addition to Google Earth.
As the pace and quantity of imagery updates continue to increase, I expect we'll see some refinements to this system over the coming years to help it become even more accurate and useful.
Posted by mickmel at March 30, 2011 8:13 AM
Comments
Posted by: Chris at March 30, 2011 1:16 PM
One of the problems that I find with that date is that they use 'American' date format, so a date of 1/7/2010 to the rest of us means 1st Jul, but it is actually 7th Jan, which can be very confusing.
Posted by: Erik at March 30, 2011 2:40 PM
This is good to know. We use the maps to date sites and their history. This will help us to understand what is happening a little better.
Posted by: Jim Cole at September 3, 2011 6:02 PM
Hi. I have a google immage with a date in the lower left corner but I don't have the coordinates. If I know the country that it was taken in, how can I use the image date to narrow down my search for the location?
Posted by: Erin at January 12, 2012 10:59 AM
Post a comment:
NOTE: Please use English. Comments are moderated.
Many thanks for this very helpful explanation.
I guess the obvious inconsistencies raised doubts about the dates as a whole, but now we know the date should usually be correct to the year or better.