Satellite Image Metadata and their Applications in Remote Sensing
We tend to overlook the image metadata parameters…
In most introductory courses and lessons for Remote Sensing, the mechanics of the Remote Sensing system are explained, along with the basic mathematics and algorithms of image processing for satellite imagery. However, we tend to overlook the image metadata parameters, and sometimes, especially for Very High Resolution Imagery, they significantly affect the image processing workflows or can be even used to directly yield useful information from the images!
Metadata, as most of you know, means “data for the data”. And if satellite images are our data, then we refer to the data that describe these images. What time were they taken? What was the sensor’s geometry at that time? Where exactly, in the Earth, does the image refer to? These kinds of questions are answered by the metadata. In this tutorial, we will refer to three main types of parameters: projection info, solar angles and sensor angles. The meaning of these parameters is the same for any type of sensor (passive or active), however for the time being, we will refer only to applications of optical remote sensing.
Projection Info
In order to get an approximate positioning of the image in respect to the globe, we need to define the following:
- Geodetic Datum
- Projection
- Image orientation
- Coordinates of one pixel (Upper Left, Upper Right, Bottom Left or Bottom Right)
- Pixel size
It is perfectly fine if you are a bit rusty on Geodesy or Cartography, so I will take some time to clear up some, often confusing, principles regarding datums, ellipsoids and projections.
Imagine the rotating Earth and the oceans extending through the continents through hypothetical canals. This hypothetical shape, assuming the oceans are under the influence only of Earth’s gravity and rotation, is called geoid, and it is an irregular equipotential surface. This means that it cannot be described by basic geometric shapes (irregular) and that the force of gravity towards the Earth’s mass center is perpendicular to every point in that surface (equipotential). Now, since it is irregular, it cannot be described by analytic mathematical equations, therefore it cannot be used directly as a reference of position. However, we can use its property of being equipotential in order to approximate it accurately with a reference ellipsoid.
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