Answers to the self-test

Answers to the self-test:

Properties of remote sensing which may have a clear added value in addition to traditional field work are:

The most obvious source of electromagnetic energy and radiation is the sun. The sun provides the initial energy source for much of the remote sensing of the Earth surface. The remote sensing device that we humans use to detect radiation from the sun is our eyes. They can be considered remote sensors - and very good ones - as they detect the visible light from the sun, which allows us to see.

There are photographic films in black and white as well as colour emulsions, which are sensitive to the reflective portion of the infrared band up to 1000 nm and these are used for scientific and artistic purposes too. But no photographic films exist to directly record emissive infrared (heat). If they did, then they would have to be cooled (and kept very cold during use), which would be very impractical. Otherwise the temperature of the camera would be recorded. However there are a number of electronic devices which detect and record thermal infrared images.

Detecting and recording the ultraviolet and blue wavelengths of radiation is difficult because of scattering and absorption in the atmosphere. Ozone gas in the upper atmosphere absorbs most of the ultraviolet radiation of wavelengths shorter than about 0.25 mm. This is actually a positive thing for us and most other living things, because of the harmful nature of ultraviolet radiation below these wavelengths. Rayleigh scattering, which affects the shorter wavelengths more severely than longer wavelengths, causes the remaining UV radiation and the shorter visible wavelengths (i.e. blue) to be scattered much more than longer wavelengths, so that very little of this energy is able to reach and interact with the Earth's surface. In fact, blue light is scattered about 4 times as much as red light, while UV light is scattered 16 times as much as red light! As a result the sky looks blue for the human eye.

Around noon on a sunny, dry day with no clouds and no pollution would be very good for remote sensing in the visible wavelengths. At noon the sun would be at its most directly overhead point, which would reduce the distance the radiation has to travel and therefore the effects of scattering, to a minimum. Cloud-free conditions would ensure that there will be uniform illumination and that there will be no shadows from clouds. Dry, pollutant-free conditions would minimize the scattering and absorption that would take place due to water droplets and other particles in the atmosphere.

The light originates from the sun (of course), hits the earth, bounces up to the (dark side of the) moon and then comes back to the earth and into your eye. A long way around - isn't it?

Indeed. The passive microwave radiometer, for instance, does not carry an illumination source, relying instead on detecting naturally emitted microwave energy. Such an instrument can be used for detecting, identifying and measuring marine oil slicks, for instance.

In order to combine data from different sources, it should be possible to link these data. Therefore, geographic data should match geometrically, that is be geometrically registered.

Geo-referenced means that they contain a coordinate system with x and y coordinates. This is not necessary. One may use, for instance, an image (with image coordinates) as a reference and register all other data to this one reference image.

Because both types of trees will appear as similar shades of green to the naked eye, imagery (or photography) using the visible portion of the spectrum may not be useful. Trying to distinguish the different types from aerial photographs based on tree crown shape or size might also be difficult, particularly when the tree types are intermixed. Looking at the reflectance curves for the two types, it is clear that they would be difficult to distinguish using any of the visible wavelengths. However, in the near-infrared, although both types reflect a significant portion of the incident radiation, they are clearly separable. Thus, a remote sensing system, such as black and white infrared film, which detects the infrared reflectance around 0.8 mm wavelength would be ideal for this purpose.