Have you ever wondered what time a particular image was captured? Google Earth does not show times, only dates. We noticed in the Google Earth Enterprise documentation that ‘Acquisition date’ only accepts a date and not a time. We assume that Google’s equivalent software for Google Earth has the same limitation.
So, we wondered how easy it would be to tell the time based on shadows in the imagery. Google Earth has the option to show the sun, and if you are lucky, you can adjust the time until the sun lines up with a shadow. However, Google Earth does not allow you to look upwards except in the Flight Simulator, so if the sun is too high in the sky you cannot get it in the view.
[ Update: Frank pointed out to me that you can look up using the ‘eye’ tool in the centre of the compass. I am so used to my 3D connexion mouse that I forget there is any other way to navigate.]
Also, accurately lining it up is tricky.
Note that the time on the ‘time’ toolbar appears to be shown in the computer’s ‘local time’. So the above image was captured at about 1:15 am CAT (UTC +2) which is 11:15 pm UTC or 4:15 pm in California (where the image was captured). (We didn’t check that for daylight savings time complications).
We thought it would be fun to write a tool to calculate time based on shadow angle. As a bonus, the shadow length can tell you the height of the object. We found some open source code here that was created for the ‘Suncalc.net’ website. The code isn’t ideal as it converts time to the sun’s angle above the horizon and azimuth whereas we need the reverse, so we run it for each minute of the day and find the closest match. To use it, first switch to ‘historical imagery’ and find an appropriate image and some shadows. Draw a path in Google Earth from the base of an object to the end of its shadow. Use only two points in the path as only the first two points are used by the code. Next, right-click on the path and select ‘Snapshot View’. This stores the date from the historical imagery time bar in the path. Make sure you selected the appropriate date on the time bar first. Now save the path, or a collection of such paths, as a KML file. Upload the file below, and it should show a table listing the approximate time the image was taken in UTC and the estimated height of the object. Unfortunately, we do not know of an easy way to convert time to the correct time zone based on latitude and longitude. Using the tool below for the location in California shown above, we got 11:06 pm UTC, and a height of 14 m for the street light.
Ideally, do a number of shadows in the same image for greater accuracy. We found variations of over 10 minutes due to the difficulty of accurately marking the shadow angle.
St George Wharf, London, UK.
Image captured on 2015-10-08 at approximately 11:01 am UTC. The tallest building measured 182 m based on its shadow. Using Google Earth Pro’s 3D measuring tool and 3D imagery we got 179 m. According to Wikipedia it is 181 m.
As usual, let us know in the comments if you find any bugs or have ideas for improvements.