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Google Earth Blog
With the end of life of the Google Earth API approaching, Paul van Dinther of PlanetInAction.com has decided to release a tool he created for his own use that uses the Google Earth plugin for making Google Earth Tours. Paul van Dinther has created a number of games based on the Google Earth API that we have featured on this blog, possibly the most sophisticated of which is a ship simulator. When he realized the Google Earth API would eventually be shut down Paul decided to base the next version, Ships 2 Career, on Google Maps instead. It will lack the 3D that the Google Earth API provides but we believe it will still be a great game and popular with ship enthusiasts. You can follow the development on his Google+ page.
The tour maker can be found here. To use it you will need to open it in a browser that still supports the Google Earth plugin. We tested it in Firefox but we believe it should also work in Safari.
Paul has provided the following instructions for using it:
Navigate to the location where you want the tour to start. (Sorry, there is no find location).
Set up your first start view exactly as you would like it to appear. You can use all the Google Earth navigation methods to set up your view.
Click the “Add view” button to add the first view to your view list. By default the speed at that point is 10 meters per second (more about that later).
Set up your next view and click “Add view” again. You need to add the third point before the track is drawn. Make sure you keep the spacing between the points reasonably even, because otherwise the cubic spline (a smooth curve between points) becomes hard to control and starts to do funny loops.
Now you have a spline with 3 points. You can left click and drag any of the numbered view markers in the Google Earth plugin to adjust the path. The spline will flip to a low resolution spline while you are manipulating the points.
Each point is represented in the view list in the right sidebar. It shows the point index, an input box for speed in metres per second, an input box for the view altitude and a delete link. You can click on any entry in the list and the Google Earth plugin view will teleport to that view.
IMPORTANT: While on that view you can adjust your camera tilt and heading (but not roll). You do this by holding down CTRL and left-mouse-drag the view (standard Google Earth behaviour). The new camera orientation will be stored for that view point as long as the camera latitude and longitude coordinates did not change. Both heading and tilt are controlled via the spline.
The altitude is also controlled by the spline. You can change the altitude of a point by holding down the right mouse button and dragging the point. Alternatively you can change the altitude in the view list (second input box).
Timing: Each point will have a speed defined for it in meters per second. The speed varies smoothly as it is also controlled by the spline. However, it is possible to lock the duration of the tour to a precise time by entering the duration in the “Duration (sec)” input box. All the speeds are then scaled to match total duration. Clear the Duration field if you don’t want the speeds to be scaled. By default the duration is set to 60 seconds.
Once you hare happy with your track you can save it by clicking on “Save project” this causes a file with the project name to be generated and placed in your download folder. Use this file later if you want to work on this track. To load a project, refresh the page and click “Choose File” and select the file you just saved. Crude but there still is no decent local file implementation in browsers.
You can specify a Field of View (FOV) for the tour. The default Google Earth FOV is 60 degrees but you can adjust that. FOV is not animated and applies to the whole tour.
Once you have your tour and the path looks reasonable, the tour can be tested by clicking “Test kml”. The program will generate KML and show it in the text area under the view list. You can copy and paste the KML straight into Google Earth Desktop without turning it into a file. The test feature will also load the KML data into the plugin and the tour starts automatically. “Generate KML” does the same but doesn’t start a tour in the plugin.
The “Make project” button is not relevant for you. It produces data that I import into my own software that translates the tour for each of my 9 screens.
“Rebuild spline”, “update track” and “rebuild track” are debug functions. However, if you update the altitude of a point in the view list you will want to click “Rebuild track” to see the changes.
Paul also says:
This has to be the ugliest program ever written. But it does the job I need it to do. It’s not very user friendly but I produced amazing animations with it.”
In December last year Google announced the deprecation of the Google Earth API (also known as the Google Earth Plug-in). It is set to stop working one month from now on December 12th, 2015. The documentation page for the Google Earth API states that it will be shut down on that date.
The reason Google has given for terminating the Google Earth API product is that it is reliant on an ageing technology called NPAPI which is not considered secure and most browsers are dropping support for it or have already done so. In addition, it was never available on mobile platforms.
Google Chrome never included support for NPAPI in its 64-bit version released in September, 2014. The 32-bit version of Chrome gradually dropped support for NPAPI (and with it the Google Earth API) initially making it harder to access in April, 2015, and finally dropping support altogether with the release of Version 45 in October.
We believe Safari still supports it.
It still works in Firefox and we still find many uses for it, such as for finding recent imagery releases. Download this KML file to see the location of imagery in Google Earth that was captured in November (less than 12 days old!).
The Google Earth API has many great uses. We showcased a number of them in this series of posts earlier this year. We at GEB believe Google should consider allowing the GE plugin to continue to work at least until a suitable replacement is in place (with the next version of Google Earth, perhaps?).
My flying skills could use some work. GEFS Online is a popular flight simulator based on the Google Earth API.
Yesterday we had a look at the progress made by Google in releasing 3D imagery for Google Earth. For this we used this KML map that we maintain that shows the areas covered by 3D imagery. This has been made possible with the assistance of a number of dedicated GEB readers who have contributed to our map. Until recently the outlines were being drawn by GEB reader Anton Rudolfsson, who also came up with the idea of the timeline section. He has had to stop drawing outlines due to other commitments. A big thank you to him for his contributions.
Since then a number of other readers have been sending in KML outlines which we incorporate into the master KML file. A big thank you to all these contributors. Also a big thank you to all the 3D hunters who find new 3D areas and let us know about them. Until now, they have been using the comments in this post to report the locations of new finds. However, there are now so many comments that it is becoming difficult to scroll to the end, so we would like to ask all contributors to start using the comments in this post from today onwards. In addition, please try and keep comments to a minimum just noting new areas covered with 3D or for messages to fellow 3D hunters. If you have any questions for us or notes about errors in the KML, please email us directly if possible, using either the Contact us page or my email address: timothy at gearthblog.com.
If you wish to submit outlines please first read through the instructions found here.
Combining the outlines we receive into the master KML file is largely a manual process. We have created scripts, such as this one, to make it easier to do this, but it is still laborious. However, given the positive feedback from GEB readers we believe it is worth the effort.
As various 3D areas are expanded and merged, getting the outlines correct can be quite complicated.
[Update: It turns out the September figures are wrong (thank you Frank) due to errors in the KML not being handled well by the Area calculator we used. Here is the updated chart.
It has been a while since we last looked at the progress of 3D imagery in terms of area covered. If we got our map and calculations reasonably correct then it seems September was a particularly good month as far as new 3D is concerned. Take the figures as approximate as we found significant discrepancies in our totals suggesting we have some mistakes in our KML map (which we are looking in to). It must be noted also that we are looking at areas not previously covered with 3D or expansions to existing areas. Google has released a significant number of updates to already existing 3D, but that is harder to track, so we have not attempted to keep a record.
Area in square kilometres of 3D released by Google. Areas calculated with the help of this site.
If you are interested in the areas by country, you can download the figures as a .csv file here. US states are listed separately as the US has more coverage than the rest of the world. California alone has more 3D than any country in the world other than the US.
It is hard to put an exact date on most of the 3D imagery, but when we have been able to identify the date the imagery is typically released six months to a year after it was captured. One reason for the long processing time is that Google puts extra effort into certain structures such as bridges, as can be seen below.
The Verrazano Narrows Bridge between Staten Island and Brooklyn, New York.
Hong Kong must have been a particularly difficult challenge to photograph, with all its tall buildings and mountains.
To see where all the 3D imagery is download our KML file
In July we noticed that Google had got the altitude wrong for the 3D imagery of the city of Crotone, Italy and it looked like it was under water. They have since fixed it. However, we have just noticed that an area in southern California has a similar problem. Maybe Google felt that some virtual water would help with the recent drought. There are interesting effects that look like levees designed to keep the water out where it meets either 3D imagery or the Salton Sea which does not show the water animation.
The Salton Sea has an interesting origin that you can learn more about in this YouTube video.
We had a look around at other parts of the world that have land below sea level and found only one other place – Death Valley, which is also in California. The water only covers the northern end of Death Valley.
It would appear that Google Earth has an internal map of what is land or sea and does not show the animation for most locations that have land below sea level, but something has gone wrong with it in the locations above.
To see the effect, turn on ‘Water Surface’ in the ‘View’ menu. However, the levee effect remains even with the water surface turned off.
If anyone knows how long this has been in Google Earth please let us know in the comments.
In September we noted that Moldova had some unusual imagery provided by Cnes/SPOT Image when viewed from high altitudes. GEB reader S.H. let us know in the comments that the imagery was introduced on August 19th, 2015. It appears that Google has now reverted back to using Landsat imagery as is used for the rest of the world.
Until recently Moldova looked like this:
Now it looks like this:
So what happened? We can’t be sure without talking to someone in Google, but our best guess is that Google updated the ‘background imagery’ for Moldova and made a mistake by including it in the global zoomed out imagery and they have now corrected the mistake.
As we discussed here Google Earth shows Landsat imagery when you view it from high altitudes and as you zoom in, it transitions to higher resolution satellite imagery from different suppliers. Behind that high resolution imagery there is another layer we call the ‘background imagery’ that fills the gaps were no high resolution imagery is available. As we saw in our series on Google Maps maximum zoom background imagery is either Landsat imagery which is very low resolution or imagery from Cnes/Spot Image which is medium resolution. We suspect that Moldova previously had Landsat imagery as its ‘background imagery’ and this was replaced with Cnes/Spot Image imagery which is what it has now.
We also discovered that in neighbouring Romania the names for 2nd Level admin regions (which appear if you have the ‘Borders and Labels’ layer turned on) appear a bit too soon when zooming in making for a very crowded map.
On Monday we had a look at recent imagery in Google Earth. We used the Google Earth plugin to find new imagery. However, either our algorithm is not perfect, or Google has added some more imagery since then because we have found a satellite image that we had missed that shows a major flooding event in Japan.
For more about the event and some photos see this article. Note that a few of the photos are of regions of the river upstream of the area covered in the Google Earth image.
We also came across a mudslide in Myanmar. We could not find a news story about it in particular but there were apparently a lot of mudslides in the region.
And finally there are some images of wild fires in Siberia. They are black and white, but still quite spectacular.
See this article for a NASA image showing the extent of the fires and smoke.
To find the locations above in Google Earth download this KML file.
A GEB reader recently asked us how to use UTM coordinates in Google Earth and we thought it might be a good idea to explore it a bit.
If you go to Tools->Options->3D View and look in the ‘Show Lat/Long’ section there are five different coordinate systems listed and whichever system you select will be what Google Earth uses to display locations, both in the status bar as well as in the properties of placemarks. However, KML files always use decimal degrees so placemarks and other features are actually saved using decimal degrees and converted to and from that for entry and display.
To learn about the different systems and how they work, Wikipedia is a good place to start:
- Decimal Degrees
- Degrees, Minutes, Seconds (Latitude, Longitude)
- Degrees, Decimal Minutes (a variation on the above).
- Universal Transverse Mercator
- Military Grid Reference System
We found that the search function of Google Earth can deal with all the above formats. You do not need to change the options in order to search with a different notation.
When using latitude and longitude always put latitude first. This can lead to confusion when working with KML, which uses the reverse notation. It is quite flexible and you can simply leave spaces rather than degrees, minutes and seconds symbols. A comma between latitude and longitude is also optional. You can also use either positive and negative or add compass directions after the numbers. Positive is North or East, negative is South or West.
The following formats all work:
37.421966 N, 122.085010 W
Degrees, Minutes, Seconds
37 25 19.1 N 122 05 06 W
Universal Transverse Mercator
10 S 580961.91 4142077.66
10 N 580961.91 4142077.66
(the format shown in the Google Earth status bar does not work: 10 S 580961.91 m N 4142077.66 m E)
Military Grid Reference System
We chose the above location because it was offered as an example below the search box. It turns out to be the Googleplex.
A GEB reader recently remarked that the Google Earth Clouds layer (a sub layer of the Weather layer) appears to have fewer clouds than it should when compared to the ‘Radar’ layer that often shows rain and storms in apparently cloud free areas.
The first thing to know about the Clouds layer is that it is not an optical photo of the clouds as they are seen from space. If we look at the images from a weather satellite such as Himawari 8, that we have looked at in the past, we can immediately see one of the problems that would arise from purely optical images. Approximately half the Earth is in darkness at any given time.
Left: Google Earth Cloud layer. Right: Image from Himawari weather satellite.
Weather satellites have a range of sensors that can detect clouds even in darkness. They make a map of the clouds that is then converted into an image with white where the clouds are that is then used in Google Earth. However, they appear to focus only on thick clouds, which results in an image showing a lot less clouds than can be seen in an typical photo from a weather satellite.
The Weather layer has an Information layer that includes the option to download animations of the Clouds and Radar layers. It also tells us that the data for the Clouds layer comes from the Naval Research Laboratory (NRL) Marine Meteorology Division. We had a look around their website but were unable to determine exactly which satellites are used for the Google Earth Cloud layer or anything about the process used. We think the data may come from MODIS (or Moderate Resolution Imaging Spectroradiometer) an instrument found on the Terra and Aqua NASA satellites. Learn more about them here. We also found this highly technical article about the algorithms used to collect cloud data and it is clearly a complicated process.
If any of our readers knows more about where the Google Earth Cloud layer comes from and how it is processed please let us know in the comments.
Google has added quite a lot of imagery to Google Earth recently, especially in Australia and East Asia. We have been using the Google Earth plugin to find new imagery. However, we have been having problems because of a bug we have mentioned in the past, which has Google Earth not showing all historical imagery at all zoom levels. It turns out that this affects the timeline as well and if you are zoomed out too far you do not see many of the dates on the timeline. We found that, as of this writing, almost all imagery dated July or August, 2015, does not show when you are zoomed out. For example, if you look at the whole of Europe, switch to ‘historical imagery’ and step backwards through the timeline, it will go almost daily through October and September and then skip straight to June even though there is imagery in Europe from July and August.
Nevertheless, we did manage to map out a lot of new imagery. We have probably missed some, as our system is not perfect due to the problems mentioned above. There is some overlap with the ‘Satellite imagery updates’ layer found in Edition 2 of the Voyager layer. Also, we have only mapped imagery with recent date stamps (July to date) and as you can see from our colour coded version of the ‘Satellite imagery updates’ layer, Google typically releases a lot of old imagery as well as recent imagery. We have no way of finding the older imagery.
California has seen a lot of large fires this year due to the ongoing drought. We already looked at some of the resulting scars using Landsat imagery. Now Google has added some imagery covering another area where a fire known as the Butte Fire took place.
Burnt area from the Butte Fire is clearly visible.
In our coverage of Google Crisis Response we mentioned a map featuring satellite imagery of flooding in South Carolina. Google has since added some of that imagery to Google Earth.
Flooding in Columbia, South Carolina.
On October 2nd there was a landslide in Santa Catarina Pinula, Guatemala. We learnt about it via DigitalGlobe’s ‘First Look’ map.
This year Black Rock City, the site of the Burning Man festival, has only received one image, captured near the start of the festival. In previous years the festival received much better coverage, including the current 3D imagery, which was captured in 2013.
Also visible in the imagery, are:
- Sarnia Artwalk, an art festival in Sarnia, Ontario, Canada.
- Fires in Indonesia similar to ones we already looked at with Landsat imagery.
- Albuquerque Balloon Fiesta, a Balloon festival in Albuquerque, New Mexico. The image was captured the day after the event ended, so there don’t appear to be any balloons in the image.
- A new image of the Tianjin crater, showing the area has been cleaned up quite a lot since the explosion.
To see the maps of recent imagery and the locations above download this KML file.
The biggest news for October was the release of Edition 2 of the Voyager layers. This included the introduction of a new layer called ‘Animal View’ and updates to the other layers. We colour coded the ‘Satellite imagery updates’ layer and talked about what the other layers show and how to access Edition 1.
There has been quite a lot of new imagery added to Google Earth during the month. Some of it can be found using Edition 2 of the Voyager layer, but there is a lot of imagery not included in that layer. We had a look at the Tianjin crater and a number of other locations that can be seen in the new imagery. More imagery has been released since then which we have not yet looked through.
Thank you to GEB reader Ali Shareef for letting us know in the comments that the crane that collapsed onto the Grand Mosque in Mecca killing 111 people and injuring 394 is visible in the imagery.
We had a look at Google Crisis Response and how to find your way around the different maps. Thank you to Christiaan Adams of Google Crisis Response for providing in the comments a link to satellite imagery relating to flooding in the Carolinas. Some of that imagery has since made it into Google Earth’s ‘historical imagery’.
Three new countries, Turkey, Ukraine and Macedonia got Street View.
Google continued releasing new 3D imagery throughout the month. A significant number of the additions were updates to areas that already had 3D imagery. Since Google does not show dates for 3D imagery nor provide an ‘historical imagery’ option, it can be difficult to identify these updates. We tend to know about them because they do usually show a colour change between the new and old imagery and the borders of the area covered may change. This month we showcased some 3D areas that are not primarily centres of population.
A big thank you to all the GEB readers that have been contributing to our 3D imagery KML file.
We had a look at how Google Earth calculates measurements and how the alignment of imagery may affect this. We also had a look at some locations using historical imagery animation to give you an idea of how far off imagery alignment can be and how it is affected by terrain.
- A tool for merging Polygons or Paths
- A tool for numbering the points along a polygon and displaying the distances between points
With Halloween this Saturday it is a good time to dress up Google Earth with the pumpkin image overlay that Frank created back in 2006. Download the KML file here, which also includes enhancements we added last year. For best results turn off ‘Atmosphere’ in the ‘View’ menu.
To get into the spirit of Halloween you can also visit a couple of spooky places in Street View:
First, there is the Zombie Manor House.
The blurred faces are because Google respects the privacy of Zombies too.
This is a real place near Manchester, UK, where you could meet zombies and learn how to fight them. It appears to no longer be operating, but there is a Zombie Shopping Mall that is still taking bookings. However, the shopping mall does not appear to have Street View.
You can also view the Zombie Manor House with sound here. Although we could not get the sound to work in Chrome it did work in Firefox and Internet Explorer.
The second is Diagon Alley of Harry Potter fame.
To see these locations in Google Earth download this KML file.
We have included a couple of other Harry Potter related placemarks, such as Platform 9 3/4 and a Harry Potter themed shop. There is a Harry Potter theme park in Orlando, Florida, but it doesn’t have good Street View coverage, so can only be viewed through user submitted photos that can only be seen in the Google Maps version of Street View
The ‘Earth View landscapes’ layer which shows the locations of the landscapes featured in the ‘Earth View’ Google Chrome extension, has not been updated and does not appear in Edition 2 of the Voyager layers.
The ‘Street View highlights’ layer has been updated with all new locations.
The ‘3D cities’ layer only shows recent additions of 3D imagery, rather than the complete set. It also does not include the very latest releases. It covers additions from July 1st, 2015 to about September 22nd, 2015. Edition 1 went up to the end of June.
We have already looked at the ‘Satellite imagery updates’ layer and noted that it covers recent releases but missed some releases since Edition 1 and there have been more releases since Edition 2.
Although Edition 1 of the Voyager layer is no longer displayed in Google Earth’s Layers it can be accessed by clicking the ‘Download’ link found in the Voyager layer. This copies the Voyager layer into your ‘Temporary Places’ and includes both Edition 1 and Edition 2 of the layers. You can then either save the layers as KML or move them to your ‘Places’. Note that if you wish to keep a permanent record in case Google ever removes them from the default Layers, you should save all the sub layers individually, as the main layer contains network links to KMLs and saving it will not save all the data.
The advantage of downloading the KML version of the Voyager layers is that you can then manipulate them, as we did for the ‘Satellite imagery updates’ layer or as you can see below, you can easily change the colour of the icons in one of the editions and then view the two editions together for comparison.
‘Street View highlights’ layer. Edition 1: dark purple. Edition 2: light purple.
Google has been rolling out 3D imagery for over three years. The focus has been almost entirely on centres of population. However, there are a few places where Google has mapped interesting geographical features rather than centres of population. Today we are featuring the main ones we are aware of.
Hoover Dam, Colorado River, Arizona/Nevada. We featured this when looking at Google Earth Pro’s Movie Maker feature
Sassolungo Mountain Group, Italy
Arches National Park, Utah (screenshot is not actually in the park but within the same 3D area)
A section of The Grand Canyon has 3D imagery. The rest of the Grand Canyon already has fairly high resolution terrain that is not part of the 3D imagery.
Zion National Park, Utah
El Capitan, Yosemite National Park and surrounding area. El Capitan also has vertical Street View.
Yellowstone National Park, Wyoming
Crater Lake, Oregon
A couple of weeks ago Google updated the Voyager layer, adding a new sub-layer called ‘Animal View’. The layer includes some nice arcs between placemarks. However, there is no automatic way to create these arcs with Google Earth or in KML. We do not know how Google created them, but when we have seen it done in the past (such as in this KML) it was done programmatically.
We thought a utility to create these arcs might be useful.
In order to draw an arc we first needed to work out the location of points along a great circle between two points. We did this with assistance from this website. We have used a simple sine wave and by default make the maximum altitude of the centre of the arc one fourth of the distance between the two points. There is also the option to set the maximum altitude of the arc. We have used altitudes relative to the ground, which has the advantage of making sure the end points start from the ground, but as can be seen in the screen shot above, if there is a significant dip in the ground altitude it is mirrored in the arc as well (notice dips where the arc crosses the river). This effect is not noticeable for larger arcs.
You can include multiple features, such as Placemarks, Polygons or Paths. All Placemarks will be put together and arcs drawn between pairs of consecutive points. Polygons and Paths will have an arc between each consecutive point.
If you don’t like the styling, you can adjust that later in Google Earth or in the KML.
No of segments to divide into:
Altitude method: AutomaticFixed
Maximum altitude: (metres relative to ground)
Extrude:YesNo (shades the area below the arc)
We recently came across this image from NASA’s Earth Observatory, which shows smoke from fires on the island of Sumatra, Indonesia. Given the scale of the phenomena we thought it should be visible in Landsat imagery.
To get the imagery, we used the technique outlined in this post. Landsat 8 takes 16 days to cover the entire earth and adjacent strips of imagery are captured on different days. We looked for images close to the target date that showed some fire activity.
When you import an image into Google Earth Pro using drag-and-drop, it states that ‘the imported image is larger than the maximum size supported by the hardware’. There are options to create a super overlay, scale or crop the image. However, we found that if you scale the image, you can in fact edit the image overlay and set it back to the original unscaled image without problems.
We wanted to display multiple adjacent images, so we needed to get rid of the black background. We first tried editing the images and making the black background transparent. This did work, but it required saving the images as PNG files, because the JPEG format does not support transparency. But PNG files are significantly larger (about 10 times bigger for these files) and thus not suitable if you wish to share them with others. If you are just using Landsat imagery locally, then transparency might be the easiest solution.
So, what we decided to do was to use a graphics editing program (GIMP) to rotate the images and then crop out the black area, keeping them in the JPEG format. We could then use the previously created image overlays as a guide to create new overlays with our rotated and cropped images. To do this easily, in the properties of the image overlay, go to the ‘Location’ tab and click ‘Convert to LatLonQuad’. This makes it easy to line up all the corners with the existing image overlay.
It must be noted that some slight loss of image quality occurs and that image alignment is not perfect, but for our purposes is is good enough.
To see the results in Google Earth download this KMZ file. When looking at the fires and smoke trails, be sure to look at the scale in Google Earth.
Street View has been added to Turkey, Ukraine and Macedonia. The last major additions prior to this one were the Philippines and shortly before that some Kenyan parks. We saw some ‘historical Street View’ in Ukraine dated 2011, suggesting that there were political hurdles to cross before releasing Street View there.
Street View changes from September 16th, 2015 to October 20th, 2015. Changes are marked in red. Large version.
Quadcopter drone being flown in Istanbul, Turkey.
Rumelihisarı fortress, Istanbul, Turkey.
St Nicholas Cathedral, Kyiv, Ukraine.
Church of Saint Clement of Ohrid, Skopje, Macedonia.
To find the above locations in Google Earth, download this KML file.
With the recent release of the ‘Voyager Edition 2’ layers, including a map of recently added imagery, we can, as we have done for previous updates, have a look through the imagery to see if there is anything interesting. We forgot to mention in our previous post on the layer that the dates are all given as one day earlier than the dates displayed in Google Earth. We do not know which dates are correct.
A good place to start when checking to see whether imagery was captured for specific events is DigitalGlobe’s FirstLook map. Many of the locations on the map do feature relevant imagery in the latest Google Earth updates, but not all of the locations have anything interesting to see, or we were unable to find it. It must also be noted that the imagery is useful to first responders in disasters even when not much relating to the disaster is visible in the imagery. In fact, some of the imagery added is years old, but has been obtained to assist first responders as good maps or satellite imagery might not previously have been available.
Note that some of the locations featured are from imagery more recent than the Voyager Edition 2 map.
Flooding in Bulani, Pakistan.
Flooding in Rojhan, Pakistan.
The Hajj pilgrimage, Mecca, captured a few days before the official start of the Hajj.
Hellfest Open Air 2015 in Clisson, France.
Oktoberfest in Munich, Germany.
To find the above locations and more in Google Earth, download this KML file.
We chose three locations that we know have significant elevation variation as well as a lot of satellite imagery. They are Devil’s Peak, Cape Town, South Africa, Rio de Janeiro, Brazil, and Calama, Chile.
If you look carefully, you can see that some locations move more than others.
The buildings at 1 and the bridge at 2 seem to move much less than the surrounding imagery. This suggests that the terrain model for these locations is closer to the true altitude than for surrounding areas.
You can download the Google Earth tours we used to create the video here.
The post Seeing imperfect orthorectifcation in Google Earth imagery appeared first on Google Earth Blog.
As we saw last week Google Earth’s measuring tools are actually quite accurate when measuring distances based on latitude and longitude, and can even take altitude into account. However, many people will be measuring distances between objects visible in satellite imagery and will not have the actual latitude and longitude of the objects in question. So, it is important to take into account various issues relating to what we see in satellite imagery, such as alignment issues.
Aligning satellite imagery correctly is complicated. We have created many image overlays (such as this one showing the surface deformation after the Chile earthquake) and we have never managed to get exact alignment between the overlay and the underlying satellite imagery in Google Earth. Even when we are trying to align an image that is already in Google Earth, such as the DigitalGlobe image of Shanghai, China, from this post, exact alignment is not possible. This is because satellite imagery must go through a process known as orthorectification, which uses a model of the terrain and knowledge of the position of the satellite at the time the photo was taken and the angle at which it was taken to adjust the image.
As we can see with the tallest building in the world, the Burj Khalifa, if the satellite takes an image from anywhere other than directly overhead, the top of the building can be quite a long way from the bottom of the building in the resulting image.
The Burj Khalifa is, according to Wikipedia, 829.8m tall. In this image, the top experiences an apparent shift of over half that distance at 482.4m
If you go through historical imagery for the Burj Khalifa, or any city with sky scrapers, you will see that the degree of displacement in the above image due to the satellite not being directly overhead is not that unusual. For sky scrapers or other buildings this is typically left as is and as a user, it is simply a matter of making sure you make your measurements from ground level objects. For mountains and other geographic features, the imagery is orthorectified by moving the top of the mountains the appropriate amount to put it in the right place. For very steep mountains this results in the effect we can see below:
Some steep mountains in the Himalayas show a streaking effect where the slopes of the mountains away from the camera had to be stretched out to get everything in the right place.
The process of orthorectification depends on having an accurate model of the terrain. The accuracy of Google Earth’s terrain model varies from place to place. We also don’t know whether or not the same terrain model is used for orthorectifying the images. What is clear though is that the process is not perfect and some displacement does take place, especially in mountainous regions. Cycling through historical imagery usually shows some movement between images.
Whether caused by this or other alignment issues, a quick check of a location in Cape Town showed differences of up to 30m between historical images. We have not yet done the same test in other locations to see whether there are more extreme variations elsewhere.
We marked the location of a feature on various historical images to see how much the location varies.
So, when making measurements in Google Earth based on objects seen in satellite imagery, keep in mind that the imagery alignment may be off by a significant amount.