Google Earth Blog

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historical imagery

Historical imagery and zoom

Last week we looked at animating historical imagery by using Google Earth Tours. While looking around in historical imagery we discovered an interesting effect: the historical imagery is not correctly synced to the timeline when Google Earth is zoomed out.

Last year we had a look at a number of situations in which Google Earth shows different imagery depending on the zoom level. The effect we are looking at today is similar in that the zoom level has an effect on what you see, but we think today’s effect is more of a bug or error, whereas the effects we have looked at in the past actually enhance Google Earth, as they provide access to extra imagery.

To see for yourself what we are talking about, load this KML file in Google Earth. It should switch to historical imagery, change the date to November 1st, 2002 and centre the view over Europe. Zoom out as far as you can and then slowly zoom in. As you zoom in you should notice patches of images appearing in several stages.


The same place at slightly different zoom levels without adjusting the time slider.

If you look carefully at the two screen shots above, there are more patches of imagery in the right hand image even though the only change made was to zoom in a little. Interestingly, if you zoom out until those patches disappear, then move the time slider forward a bit to 2003, the missing patches do appear.

As far as we can tell, the zoomed in imagery more accurately reflects the correct timeline, i.e., all images shown when zoomed in are dated on or before the date shown on the timeline.

We also found some cases of imagery that were not correctly synced to the time-slider at any zoom level. For example, there is a patch of aerial imagery in France dated January 1st 2002 that doesn’t show until the time-slider reaches December 2002. Being aerial imagery the date is only approximate and probably only means the imagery was captured some time during 2002, so it is possible the imagery is stored in the database with a date range and Google Earth only shows it when the upper bound is reached. Find the imagery we are talking about with this KML file .

Overall, we don’t think it is a major problem, but do keep in mind when searching for imagery of specific dates that you should zoom in as far as possible for greater accuracy and double check the actual dates of imagery rather than relying solely on the timeline.

Animating historical imagery using Google Earth Tours

Last year we showed you how to animate Google Earth historical imagery using the Google Earth plugin. However, the Google Earth plugin has been deprecated and is set to stop working on December 12th, 2015. In addition, Google Chrome and other browsers are making it harder to use the plugin.

So, we have decided to try and achieve the same thing using Google Earth Tours. In a Google Earth Tour it is possible to specify particular views that include not just the position and angle, but also whether or not to show ‘historical imagery’ and when in ‘historical imagery’ what exact date to show. It is, however, not easy to create a Tour to animate historical imagery using just the built in functionality of Google Earth, so instead we have decided to do it via JavaScript.

To use it, the first step is to decide what view you want in Google Earth. Create a Placemark in Google Earth then save it as a KML file. Do make sure it is KML not KMZ, as our script cannot read KMZ. Next, decide on your settings below and and upload the Placemark file. Then click on the “Create Tour” link to download the generated KML tour for you to view in Google Earth.

It has been tested in Google Chrome, Firefox and Internet Explorer 11.

Start: (yyyy-mm-dd)
End: (yyyy-mm-dd)
Step:
Pause: (seconds)
Placemark KML:
Create Tour

Try adjusting the settings to get the best results for a given location. Remember that the functionality works whether you want to see whole continents or zoom right in, but you will want to choose your settings depending on the availability of imagery. Although it is possible to make the pause between steps shorter than 1 second, we found that Google Earth could not keep up and simply did not refresh the screen on every step. It seemed to work alright with the 1 second setting, but if you have a slow computer or slow internet you may want to make it longer. For best results run the tour through a few times depending on the speed of your internet to cache the imagery.

We have created a few tours which you can download here to give you an idea of what is possible.


The historical imagery for Europe in the YouTube video above appears to come in waves from the top downwards. We believe this has more to do with how Google Earth refreshes the screen during the tour than the actual dates that the imagery was added.

Google Earth’s ‘background’ imagery

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.

Left: Landsat imagery as seen in ‘default mode’ when zoomed out. Right: NASA imagery (probably MODIS data) as seen when in ‘historical imagery’ mode.

Historical Imagery and the Ocean Floor

An interesting aspect of the ‘historical imagery’ feature in Google Earth is that when you turn it on Google Earth displays a different dataset for the ocean floor. Based on the copyright information, it appears the ‘historical imagery’ ocean floor is based on data from NASA, whereas the ocean floor seen in the default view is a combination of data from Data SIO, NOAA, U.S. Navy, NGA and GEBCO.

Historical Imagery view
The NASA supplied data as seen in ‘historical imagery’

Default view
The default view with data from a variety of sources

It is interesting how whole underwater mountains are completely missing in the NASA data and some locations look quite different in the two data sets. The NASA data is of lower resolution and presumably obtained by a different method.

Another interesting effect is how satellite imagery that extends into the oceans is mapped onto the sea floor topography. If necessary you can correct this in Google Earth Pro by turning off ‘Terrain’ in the layers.

As we have noted in the past the tracks across the ocean floor are not signs of alien activity, but tracks of higher resolution data obtained from ships.

We have looked at the GEBCO data before and you can view their map in Google Earth using this KML file.

We have also looked at a map from the Scripps Institution of Oceanography, University of California, San Diego, which is based on gravity measurements from satellites. You can view it in Google Earth with this KML file.

For best results when viewing the ocean floor, turn off the ‘Water Surface’ option in the ‘View’ menu.

Frequency of new satellite imagery in Google Earth

When looking around Google Earth in historical imagery, we have noticed an interesting trend with regards to the frequency of satellite imagery updates. It seems that Europe and the USA get significantly less satellite imagery than much of the rest of the world.

Although much of the less populated world has rather poor and infrequent coverage, some population centres seem to get very frequent updates. Here in Cape Town we have recently been getting several updates per month. We were recently looking at an area near Santos Dumont Airport, Rio de Janeiro, Brazil, and it has no less than 12 images so far in 2015. Even the relatively small town of Livingstone, Zambia has had two updates this year. Many large cities in the US and Europe, such as New York, San Francisco, Paris, Berlin and London still have 2014 imagery.

We have been wondering why this is. We will make a few guesses, but would welcome input from our readers too.

Aerial Imagery
The USA and Europe are covered with high quality aerial imagery and thus new satellite imagery, which is of lower resolution, is usually relegated to ‘historical imagery’. Possibly Google, or the satellite imagery providers they source the imagery from, do not see the need for satellite imagery in those regions. Aerial imagery is typically more expensive to gather on a regular basis although we expect this to change in the future with as the cost of drones and high resolution cameras continues to fall.

Weather
Google tends to avoid satellite imagery with excessive cloud cover, and most notably, snow cover. So does this essentially mean that much of the Northern hemisphere will never get good coverage over the winter months?

Some of the recent Cape Town images can be explained as a ‘special event’ where imagery has been captured and put in Google Earth because it contains something interesting:


Fires on Table Mountain

To see general trends up to October last year, this map is interesting. We hope Google updates it at some point to show more recent months.