Google Earth Tips

Understanding Google’s Imagery Updates map

April 8, 2015

Last week Google published a map titled ‘Monthly Google Earth Imagery Updates’, which contains a layer called ‘Satellite and Aerial Imagery Updates March – 2015’. So what does this map tell us?

The map displays red outlines of areas where Google has obtained satellite or aerial imagery and added that imagery to their various databases during the month of March. The first thing to note is that the imagery itself was not necessarily captured during the month of March. Some of the imagery was captured in March, such as the image of Wadi Sayyidna, Sudan, captured on March 4th, 2015. However, imagery is typically a month or two old, and sometimes much older, such as an image of Taipei, Taiwan, captured on January 25th, 2014.

Google has several different imagery databases and it is usually the case that the imagery is added to these databases at different times. These databases are:

The default view in Google Earth.
The ‘historical imagery’ in Google Earth.
The Google Maps imagery as viewed in ‘Earth’ view.
Classic Google Maps in ‘Satellite’ view.

A popular method for finding or confirming new imagery is to compare the above sources and look for differences. Typically, the Google Maps imagery is eventually synchronised to be identical to Google Earth’s default imagery. However, imagery that goes straight into Google Earth’s ‘historical imagery’ and not the default layer may never be visible in Google Maps.

Left: Google Earth default view. Right: Google Earth ‘historical imagery’.

Left: Google Maps Classic. Right:Google Maps ‘Earth’ view.

As can be seen above, the latest image of Livingstone, Zambia, captured by Digital Globe on January 27th, 2015, as of this writing, has been added to Google Earth, but not yet to its ‘historical imagery’. It has also been added to Google Maps Classic, but not yet to the new Google Maps.

It is often the case that when Google releases an imagery update map, that some of the imagery is destined for the ‘historical imagery’ database only and has not actually been added to it yet. Thus we can see outlines on the map, but cannot view the actual imagery it is referring to, and must wait a week or so for it to get put into ‘historical imagery’.

Two red outlines over the northern half of Taiwan.

As can be seen in the above screen shot, Google has outlined two areas over the northern half of Taiwan. As of this writing, the left hand outline does not match any image found in ‘historical imagery’ nor does it obviously match any imagery found in the default layer. It is most likely an image that will in the near future be visible only in ‘historical imagery’. The outline on the right, matches an image from January 25th, 2014 which can be seen in ‘historical imagery’ but not in the default layer.

Note: To see imagery dates in Google Earth, first deselect the ‘Monthly Google Earth Imagery Updates’ map layer if you have it open.

Earth Hour

March 27, 2015

Earth Hour takes place this Saturday, 28th of March at 8:30 pm local time. Earth hour is an annual event designed to raise awareness of global climate change. Participants turn off their lights for a period of 1 hour. It would be nice to see what this would look like from space, but due to obstruction from cloud cover and lightening, it might be difficult to see the effect. Nevertheless, this would be a good opportunity to look at what the earth looks like at night and see just how much energy we waste on lighting.

Google Earth by default shows endless day. There is an option on the tool bar to show the real-time location of sunlight and the night time shadow, but it doesn’t give an accurate picture of what the earth looks like at night. There is a layer called ‘Earth City lights’ found in Gallery->NASA that was first added in 2007. This is a cleaned-up image removing the clouds and lightening that would otherwise obstruct the view. Rather than using the ‘Earth City Lights’ layer, we recommend trying a newer, higher resolution version that can be found here on the Google Maps Gallery and viewed in Google Earth using this KML file

The South China Sea around the Island of Hainan, China, has almost as many lights as the land. These are presumably mostly fishing boats.

We found it interesting how many lights there are in the seas and oceans. We have made a KML noting some locations of interest that you can download here.

Planes in flight and the rainbow effect

March 11, 2015

Ever since Google Earth was first released, users have been spotting aircraft in flight in the imagery.

When an aircraft is captured in flight in Google Earth it is not uncommon for it to have a second ghostly image next to it, or in some cases a rainbow effect. This is caused by the way satellite cameras are designed. Satellites have multiple cameras for capturing imagery in different wavelengths of light. A common setup is to have a high resolution monochrome camera and then a separate camera that takes photos with various colour filters in quick succession. The multiple images are then combined to form what you see in Google Earth. However, if there is a fast moving object in the scene such as an aircraft, it will have moved between exposures and the ghosting or rainbow effects can be seen, depending on what type of camera the satellite is using. In addition to the aircraft’s movement, the satellite itself is moving and due to parallax the aircraft will appear to have moved in relation to the ground. This often results in the multiple images being offset from the direction the aircraft is travelling in.

The satellite took images in quick succession with blue, green then red colour filters, and finally a higher resolution image in monochrome.

In this image, the satellite took the high resolution monochrome image first, and then red, green and blue colour filters. We don’t know how the double image of the aircraft tail happened.

This image shows an aircraft and its shadow both exhibiting ghosting.

Satellites are also capable of taking images in spectral ranges far outside the visible range such as in the infrared. These images have a variety of uses including mineral exploration, environmental monitoring, agriculture and military. Digital Globe’s World View 3 that was launched last year and whose imagery we recently looked at shows on its datasheet that it is capable of capturing a variety of bands in the infra-red range.

Google Earth Hacks has a couple of collections of aircraft in flight: civilian aircraft and military aircraft. Do our readers know of any more comprehensive collections?

To find the aircraft featured in this post, download this KML file.

Google Maps ‘Earth’ view FOV

March 9, 2015

We recently noticed that if you look at a given place in both Google Earth and Google Maps’ ‘Earth’ view, they look a bit different, with distant objects looking closer in Google Maps than in Google Earth. The main reason for this is that Google Earth by default has a different Field of View from Google Maps.

Last year we showed you how to change the field of view in Google Earth using a tour KML. After some experimentation we reckon that Google Earth by default uses a horizontal field of view (HFOV) of about 60° and Google Maps uses a HFOV of about 35° for its ‘Earth’ view.

So to match up a given location you need to download this KML file, which adjusts Google Earth’s HFOV to 35 degrees. In addition, you may want to try turning on ‘Photorealistic Atmosphere’ in Google Earth’s settings to get the colours to match up a bit better.

Praia Grande, Brazil, with Google Earth’s default settings. Note how the marked building and hill in the distance look further away than in the Google Maps screen shot below.

Praia Grande, Brazil, in Google Maps’ ‘Earth’ view.

Praia Grande, Brazil, in Google Earth with an HFOV of 35° and ‘Photorealistic Atmosphere’.

Google Earth Pro: regionation

February 26, 2015

We have recently been looking at some of the features exclusive to the now free Google Earth Pro. Today we are looking at the ‘regionate’ feature.

The regionate tool is a way to make large datasets easier to work with by limiting the number of items displayed in Google Earth at any given time. How it does this is explained in this YouTube video from the Google Earth help page:

We decided to try it out using a KML file that we found on the Geo-Wiki site that we looked at yesterday.

The file we tested with, the Geo-Wiki validations dataset from the downloads page, contains just over 58,000 features. When we opened it in Google Earth it made the application sluggish and it was quite difficult to navigate around to view the data. We used the regionate tool, and it split the KML into 6,589 separate files all linked together from a master KML file. All the data was still there, but you don’t see all the features until you zoom in, making Google Earth much more responsive. However, the regionation means that you loose the overall picture and it is no longer easy to see which areas have large clusters of points. In addition, although it would be possible to distribute the regionated files as a zip file, there appears to be no easy way to distribute the regionated data as a single file that can be loaded directly in Google Earth. So although this feature would clearly be very useful and practically necessary for very large data sets, it may not be the most suitable solution in all cases.

Before regionation, Google Earth was sluggish and the map is cluttered.

After regionation, a lot lest clutter, and Google Earth works smoothly, but large scale patterns are lost.