Imagine you want to build a hydropower plant, somewhere in the middle of nowhere on a large river. You have some coordinates but that’s all. What to do?
Ok, you can open Google Earth and get a first overview of the location and landscape. But for planning you need more…
5 facts about satellite images you need to know:
1. Orbit and repeat cycle
Satellites fly around the planet earth on an orbit with a defined altitude and they pass the same location every few days. This means you get, for example, a picture of your garden every 3 days.
2. Optical and radar sensors
On the satellite, a sensor is installed which detects solar radiation reflected from the ground or any object on earth. The reflection is recorded in different wavelengths. Depending on the wavelength of the radiation, there are two types of sensors: an optical sensor for shorter wavelengths of visible light and infrared or a radar sensor for the longer microwaves.
Radar sensors actively send microwaves to the earth surface and measure the reflection of the signal. One of the results is an elevation model of the earth surface showing the topography like valleys, mountains and plain areas. This Digital Elevation Model helps to find the best place for the dam of my hydropower project and enables us to calculate the extension and volume of a future water reservoir.
Optical sensors record the wavelengths of visible light and infrared in different channels, the so-called spectral bands. For visualization of the satellite image, you can combine three spectral bands. If you combine the three bands of visible light (blue, green and red), you get real-color images like in Google Earth. But if you choose the near-infrared channel for example, you get a picture with red areas which represent vegetation like forests, garden or fields. These pictures can give you information about density, type, vitality and other characteristics of vegetation.
How detailed the satellite image is, depends on the spatial resolution of the sensor. Some satellites can cover huge areas on a picture, but you can’t see details like cars or trees. Other satellites like the old spy satellites for example have a very good spatial resolution with pixel size of less than 50cm and you can count the number of houses of a village or see small paths.
4. Mapping features
You can analyze the satellite image and map with Geographic Information Systems (GIS) what is interesting for you. In our case of the hydropower project, I need to know in the beginning if there already are access roads to the project area. Maybe there are also some villages, agricultural areas or archaeological sites which might be important for social and environmental impact studies.
5. Multitemporal analysis
One of the most interesting possibilities for analysis is the comparison of satellite images from the same location but taken on different dates. This method allows you to monitor not only changes in landscape resulting from natural events like floods, tsunamis or volcanic activities, but also the long-term impacts of climate change or the development and expansion of human structures. With a temporal difference of a few years for example, you may discover a new artificial lake in the highlands of an African country, created by a new hydropower plant.
All these characteristics of satellite images play a significant role in the initial phase of a planned hydropower project. Especially the Digital Elevation Model is essential to define the best location for the project and the further design of the hydropower plant.
But don’t think satellite imagery is for free! While you can download old Landsat scenes with a low resolution from the image archive at no charge, you have to pay up to a few thousand euros for current images with high resolution and of course without clouds hiding the landscape.