The word remote sensing literally means to gather information about something without getting in physical contact with it. Before 1960, the term was used for aerial imagery which was costly and covered less area on Earth. After 1960, with advent of Earth Satellites, the term broadened and was excessively used for Satellite Imagery. With the continuous efforts to increase our knowledge of Earth, came different types of Satellite Imagery.
Satellite imagery of intended resolution can be used to generate useful results in GIS applications. Satellite imagery is a powerful visual aid. It helps in extracting useful information about aspects that we are generally not accessible. And that is exactly what’s so confusing about satellites. There are so many different types and none of them is the wrong one, or the right one in that case. It all depends on what you want to do with it.
The first thing to decide is what do you want to do with the satellite imagery? Figure out your project. Think of the scale; what do you want to study? What color bands do you need. But it’s okay to be confused in the start. This article helps you with what type of imagery is available and which one you might need.
Types of Imagery
There are three basic types of satellite imagery which is commercially available; Panchromatic, Multispectral and Hyperspectral imagery.
In simple words, panchromatic imagery is black and white imagery. To detect any object, the sensor on the satellite needs a certain amount of energy. This energy comes from a light source. When a sensor “looks” at an object, it is looking at the light of the object. But all objects do not have light. A sensor shots a beam of light on the object, the beam reflects, the sensor records it. Simple enough. If the sensor is sensitive to a wide wavelength range of light, it will capture he objects it is “seeing” in more detail. Hence the panchromatic imagery is usually of the highest resolution. The highest available resolution of panchromatic band is of 0.31 meters and is of WorldView 4.
Could it get more obvious? Multi-spectral imagery generally refers to imagery with 3-10 bands. Now what is a band. A band is different images taken of the same place at the same time. But in different wavelengths. I’m sure you are all familiar with RGB color scheme. Typically, multi-spectral imagery has information stored in these three band.
RGB images capture images only in visible light. Since the window of visible light in the spectrum very small, the sensor receives less light. And resultantly …? Yes, you guessed it right; less energy. This results in a coarser resolution of the RGB imagery. Ahead of RGB, there are near infrared, short wave infrared and thermal bands as well. The usage of which depends on what information you want to extract from them.
Hyperspectral imagery contains hundreds of bands which are very narrow. In 2000, NASA launched a satellite called EO-1. The satellite recorded information from 242 bands with a resolution of 30 meters. After this successful launch, other hyperspectral satellites were sent into Space. The main difference between Hyperspectral and MultiSpectral is the number of bands.
The next question is, where can you find them? If you are a beginner, you can start with low resolution imagery from USGS. High resolution imagery is costly and can vary in prices. These variations can be because of vendors. But if you are a student, you are most likely to bag a discount of 20-30%. So you’ll have to look at that. You can also get imagery from GeoStore and Landinfo.
Make sure to explore your options before choosing a type and resolution. You don’t want to overspend on something that you won’t even need. Explore your options and then head over to find your needed imagery.