Geolocate an image using the Shadow Finder Tool

GEOINT is often time-consuming, with researchers spending hours looking at photographs, studying satellite imagery, and viewing street views.

But, relatively recently, a tool has appeared that allows you to narrow your search area – Shadow Finder (using the height of an object and the length of its shadow (or angle to the sun), as well as the date and time, this code estimates the possible location of the shadow)

How can shadows help?

As many GEOINT researchers know, shadows in images can provide extremely useful clues about When And Where the image was taken.

By comparing the height of an object with the length of its shadow, you can find out how high in the sky the sun is. At any given time on Earth, there is one point where the sun is directly overhead, called the subsolar point.

At this point, the vertical stick casts no shadow. As you move away from this point, the sun will appear lower in the sky the farther you move away from it. When the sun is lower in the sky, the shadow of the vertical stick becomes longer.

The distance from the subsun point determines how high in the sky the sun is.

— At any given moment on Earth there is always a ring of places where the sun is at the same angle above the horizon.

If you know the date and time of the photo and can accurately measure the height of the object and the length of its shadow, you can determine a ring of possible locations where the photo was taken.

Tools such as SunCalcallow you to manually check the length of the shadow in certain places.

The Shadow Finder Tool eliminates the need for manual checking and allows you to search the entire surface of the Earth with the click of a button.

Practical Application: Coast

First, you need to determine the date and time of the shoot. There may be a clock in the frame or useful metadata contained in the file (use tools like Metadata2go or EXIFmeta).

It's not always possible to determine the date and time, and the metadata may not always be accurate, so keep that in mind before moving forward.

In this case, the image of the sandy beach is a live feed shot, so we know exactly when it was taken: 2024-07-10 at 10:30:46 UTC.

You can enter the time in UTC (Coordinated Universal Time) or in local time. It doesn't matter if you don't know the time zone. If you use local time, the tool will convert the time to the local time zone for each location. Because of this, you won't get a pretty ring if you use local time, but the principle is the same.

Defining the object to be measured

Once you have verified the date and time, identify a clear, visible object and shadow in the photo.

Measure the height of the object and the length of its shadow. Sometimes you can measure the object and shadow directly from the image. Keep in mind that you may have to adjust the measurements depending on the perspective of the image. Ideally, the object you are measuring should be at a right angle to the camera.

You can read more about measuring objects in an image and how to deal with perspective issues in this Bellingcat manual.

In our example, let's focus on the beach shower. The image shows larger shadows, but this object is vertical and has a clear shadow.

— Also, the shadow falls on a nearly flat surface (we see standing water from a beach shower, so any tilt is small). If the object is vertical and the surface is flat, this makes it easier to measure.

From the camera's point of view, the shadow is almost at right angles to the object, so any distortion during measurement will be negligible. Thanks to this, we can measure the image directly.

Using an image editor (eg. GIMP) or a ruler, measure the object and the shadow.

In our image, we see that the height of the object is 273 pixels, and the length of the shadow is 104 pixels. It doesn't matter what units you measure in (you can use millimeters or inches), because we are interested in the proportion between these two measurements.

Если вы измеряете и объект, и тень в одних и тех же единицах, инструмент Bellingcat для поиска теней будет работать.

Now that you have your altitude measurements and time and date data at hand, open the tool Shadow Finder

Enter the object height and shadow length, as well as the date and time, and press “▶” to run the tool.

The tool will launch (be patient, it may take a while to load the first time) and create a map.

There is a yellow ring on the map indicating the probable location of our seaside photo.

A closer look at the map shows that the ring includes several landlocked areas and only a few areas with coastlines, further narrowing the search area.

In this case, the beach is most likely located on the coast of Algeria, Italy, Croatia, Yemen, Somalia or Djibouti.

There are still a few countries on this list, but we've already ruled out a lot of places around the world. We know that looking for Portugal, South Africa or India would be a waste of time. Now we can go back to the image and use other clues to narrow down our search even further.

What can affect the measurements?

There are many ways to measure objects and shadows in images, some more reliable than others.

— Below are the ideal conditions for getting the best results using the Shadow Finder tool:

1. The time and date of the shooting must be known
2. The object and its shadow should be at approximately a right angle to the camera.
3. The shadow must be cast on a surface located at the same level with the horizon (flat floor)
4. The image should not be greatly distorted by the camera lens (as is the case with a fisheye lens).

Extended Geolocation: Re-Shot

If you have multiple images taken at different times, you can narrow down the location even further using the Shadow Finder Tool. Here's another image taken during the same broadcast the next day, a few hours later.

Notice how the shadows have shifted in the image:

If we repeat the above process and run the Shadow Finder Tool again, we will get a second map of possible locations.

This time the ring points to a different set of possible locations.

The location changed because the Earth rotated and the polar point moved. Using an image editor, we can superimpose the two images, including the new ring of possible locations and the previous ring.

By finding the places where they intersect, we can determine only two possible locations. Use the slider to see where they overlap.

The overlay shows us only two possible locations: Cameroon and Croatia. Cameroon is inland, so we can rule it out and focus on Croatia.

Zooming in, we focus on the coast and islands around Split. There is no substitute for hard work at this stage.

There are many beaches in this part of the world, but after some searching you can find the location. There is even a street view image that shows the webcam.

When geolocating any image, context is important. There are often many ways to determine location without using shadows.

— But if you know the exact time and date of the shoot, as well as the presence of a clear shadow, the shadows will help you quickly navigate where to look and quickly narrow down possible shooting locations.

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