sentinel

The sinking Millennium Tower, San Francisco

December 21, 2016

We often feature imagery from the European Copernicus programme’s Sentinel-2 satellite. The programme has other satellites, including Sentinel-1A and Sentinel-1B, that gather radar data rather than imagery. They seem particularly good at measuring altitude changes over time. For example, we saw a map of surface deformation after a major earthquake in Chile.

A recent story on the European Space Agency (esa) website is about how the satellites have been able to determine that the Millennium Tower, a sky scraper in San Francisco, is sinking by a few centimetres per year relative to other buildings in the vicinity.

Image credit: ESA. The colour scale ranges from 40 mm a year away from radar (red) to 40 mm a year towards radar (blue). Green represents stable targets.

Given the sensitivity of this data it makes us wonder whether or not Google could use it to improve the accuracy of their altitude data. We have noted in the past some fairly major errors in their data, such as the sunken island of Gorgona and significant inaccuracies around Rio de Janeiro.

Animating the new Landsat/Sentinel global mosaics with a dynamic tour

December 2, 2016

Earlier this week Google added to Google Earth global mosaics based on Landsat and Sentinel 2 data from 1985 to 2016. Google Earth Engine provides animations for the data using a web based interface. We promised to release a way to animate the data in Google Earth.

Simply download this KML file and open it in Google Earth. Then switch to ‘historical imagery’ and find a location of interest then play the ‘Animation’ tour found in the KML. If you wish to move to a different location, close the tour, move to the new location, wait a moment, then open it again. Depending on your internet speed, either let the tour run through a couple of times to load all the imagery, or you can manually go through each year first to make sure it is loaded, then run the tour to see it animated.

Speed in milliseconds per image:

The growth of Kuala Lumpur, Malaysia, over 30 years (1985-2016) using the new Google Earth global mosaics.

The imagery covers a period of just over 30 years and there is almost no place on the planet that hasn’t changed significantly in that time, so get exploring!

Remember these are long term changes, not month-to-month changes.

We chose a timing of one second per image. We would have liked to go faster, but Google Earth cannot handle faster tours and starts to skip images if you increase the speed.

More about the new Landsat / Sentinel 2 data in Google Earth

November 30, 2016

Yesterday Google surprised us by adding global mosaics created from Landsat and Sentinel 2 data to the Google Earth’s ‘historical imagery’. The data comes to Google Earth via Google Earth Engine, which has long had this time-lapse feature that has just been updated to include imagery up to 2016. The timelapse page also tells us more about the imagery:

Using Earth Engine, we combined over 5 million satellite images acquired over the past three decades by 5 different satellites. The majority of the images come from Landsat, a joint USGS/NASA Earth observation program that has observed the Earth since the 1970s. For 2015 and 2016, we combined Landsat 8 imagery with imagery from Sentinel-2A, part of the European Commission and European Space Agency’s Copernicus Earth observation program.”

Google Earth Engine has also published this post about the update.

On Friday we will release a KML file that will allow you to view similar time-lapses from directly within Google Earth.

Today we are making a few observations about the imagery.

The first observation comes from GEB reader ‘haflaa’ who points out that although the mosaics are global for every year between 1985 and 2016, there are some locations, such as the Maldives, where the imagery is identical for a number of years. In the case of the Maldives, the imagery is identical from 1985 to 1999.

We also note that the Google Earth Engine animation starts in 1984, whereas the Google Earth data starts with December 1985 (which we presume represents the data for the whole of 1985).

The path of the satellite is often noticeable in the imagery. For more on the orbit of the Landsat satellites see this post and this post.

In many locations you will notice narrow stripes in the imagery. This is because of a faulty part on the Landsat 7 satellite. Learn more about it in this post.

The mosaics are created by trying to select imagery from throughout a given year then selecting cloud and snow free pixels where possible. However, there are a few locations on earth that are almost always cloudy, a problem we discussed in this post.

This location in Cameroon features both Landsat 7 stripes and clouds that just could not be eliminated.

Google Earth now has global Landsat and Sentinel 2 data from 1985 to 2016

November 29, 2016

Thank you for GEB reader Jonah for being the first to let us know. Google has added yearly global mosaics using Landsat and Sentinel 2 data going back to 1985. Google has long had these mosaics on Google Earth Engine which also provides a ‘Timelapse’ tool. As of this writing, the Google Earth Engine timelapse only goes up to 2012, but that will likely be updated too.

To see the mosaics, simply zoom out a bit and switch to ‘historical imagery’.

We plan to create tools to animate the new data, but that may take a few days.

Here are a few ‘before and afters’ showing changes between 1985 and 2016.

The Amazon, 1985 vs 2016

Dubai, 1985 vs 2016

The region around Shenzhen, China, 1985 vs 2016

Note that the new data is created by blending all Landsat/Sentinel 2 data for a whole year to remove clouds and snow cover. The result is that changes that happen on timescales less than a year, such as seasonal changes, will not be visible.

Google has not (as of this writing) updated the historical imagery layer with all the high resolution imagery that has been added since June 2016.

There is one disadvantage to the new data – it is much harder to find historical imagery, as you need to zoom in very close to the earth to see the high resolution historical imagery.

The Kaikoura Earthquake Landslides

November 21, 2016

On November 14, 2016, the South Island of New Zealand experienced a 7.8 magnitude earthquake named the Kaikoura Earthquake after the town of Kaikoura near the quake’s epicentre. The affected region is mountainous with steep slopes and the earthquake resulted in a large number of landslides, including creating some landslide dams (a topic we have covered in the past).

The Landslide Blog has done a number of posts on the Kaikoura landslides (1, 2, 3 and 4). It also mentions this article, which shows a map of the locations of the landslides so far identified using Sentinel 2 imagery.

We thought it would be interesting to examine the sentinel 2 Imagery in Google Earth. The image in question has quite a lot of cloud cover, but in the gaps between the clouds we can see the scars of a large number of landslides. It must be noted that landslides appear to be common in the region, with many landslide scars being visible in older imagery, too. Here are a couple of ‘before and afters’ showing just how many landslides there were in some places.