Elements of Geo-information technology
1. Elements of Geo-information technology
Geo-information technology consists of Remote Sensing technology, Geographic Information System and Global Positioning System. Details of these technologies are as follows:
2. Remote Sensing
Remote sensing refers to data acquisition through the use of remote measuring instruments and information extraction from measurement data for analysis and processing. The procedure starts from delivering energy from energy sources in order to acquire data, to information extraction from measurement data, and the use of data to support any decision making process. The details of each element are as follows:
- Data acquisition consists of energy sources, interaction of energy with other objects on the global surface, data measurement system, and data recording.
- Information extraction for analysis and processing consists of various steps, including pre-processing calibration development and printing, as well as interpretation which requires the interpreters’ background knowledge and understanding, together with field inspections to make the maps and information management.
The history of Remote Sensing development began in the 20th Century, with the use of aerial photographs taken under visible light waves of electromagnetic radiation. The word “photograph” and “image” have different meanings in the aspect of remote sensing. A photograph is a picture recorded on the film while an image is obtained from the record on film or digital record via a scanner system. Therefore, the data is obtained through the use of electromagnetic radiation that has longer wavelength than the visible light waves, such as the Thermal infrared or the Microwave. Since 1960, a number of earth observation satellites have been launched into orbit, such as Landsat, Spot, IRS, and JERS-1, which have spatial resolution of less than 100 meters, and are applied for environmental monitoring. The NOAA Satellite that has lower resolution but cover a larger area is used for meteorological purpose. Other satellites used for meteorological purpose are such as Meteosat or GOES, which are in geostationary orbit following the direction of the Earth’s rotation. They orbit around 36,000 kilometers above the Earth’s surface, with two remote sensing systems—Passive remote sensing system and Active remote sensing system.
1.1 Passive remote sensing system
This is a measurement of electromagnetic energy reflected or emitted from the surface. The energy source in this passive measurement system is solar energy that can give measurable energy in visible and infrared wavelengths. The electromagnetic spectrum that can pass through the atmosphere is called “Atmospheric window” which happens only in the wavelength that is longer than Ultraviolet. If it is shorter, the wave will be absorbed by the atmosphere. The atmospheric window has different wavelengths. The wavelengths that can pass through the atmosphere include visible light waves, Near-infrared radiation, proportion of Thermal infrared (3-5 micrometers and 8-14 micrometers), and Microwave. The energy interaction in atmosphere which causes wave change while it travels to the earth is the scattering process which happens when small particles in the atmosphere have uncertain direction. There are three types of scattering—Rayleigh scattering happens when the particle diameter is smaller than the incident wavelength, causing smog in the atmosphere; Mie scattering happens when the particle diameter is about the same size as the wavelength, such as water, water vapor, and dust particles; Non-selective scattering happens when the particle diameter is larger than the wavelength. The example is water droplets that reflect the visible light waves almost as equally as the infrared, making clouds appear white. Refraction, meanwhile, happens when the light travels through the change of atmospheric density, which causes deviation of image position. Therefore, reflection of a recorded object depends on the atmospheric condition and the wavelengths during measurement.
The examples of passive remote sensing satellites are such as Landsat MSS (Multispectral scanner) which comprises data in four wavelengths, from green to near-infrared, with spatial resolution of 80 meters, while Landsat 5 TM (Thematic Mapper) consists of data in six wavelengths with spatial resolution of 30 meters, and 120 meters at one Thermal wavelength. The Spot 1, 2 and 3 satellites of France consist of data in two systems. The HRV system has three-wavelength data with spatial resolution of 20 meters, and the Panchromatic system has spatial resolution of 10 meters. Besides, the radiation emission from an object that has temperature above 0 degree Kelvin is also passive remote sensing.
1.2 Active remote sensing system
This is a system in which human creates energy and sends the energy to hit the target objects in the microwave spectrum, such as Radar (Radio Detection and Ranging) system which has the wavelength of 1 millimeter to 1 meter. The earth observation satellites that take photos with Radar system include Canada’s Radarsat which takes photos at 5.6 cm wavelength (C-Band). Various types of photos are taken, making different image resolutions and width. The ERS satellite, which was sent to the space by the European Space Agency (ESA) can take similar types of photos as that of Radarsat. Both satellites give out electromagnetic waves in different directions. The scattering direction of Electromagnetic fields is called polarization.
The Radarsat sends and receives electromagnetic waves horizontally, while the ERS sends and receives electromagnetic waves vertically. Presently, the earth observation satellites are designed to carry both active and passive remote sensors, such as the JERS-1 and Alos of Japan, and Envisat and ERS of the European Community.
1.3 Image analysis and interpretation
The satellite data records images in a digital system in order to represent objects on Earth. The data is stored as arrays of pixel, each pixel having gray level resolution, and being positioned by row and column. The pixel value or digital number is the value recorded from energy reflected from objects on Earth to the measuring device.
Procedures in digital data processing and analyzing will help enhance maximum benefits due to the purpose of data use. The steps of image processing are as follows:
Raw data obtained from satellite photography will go through the so-called “radiometric correction” process to correct the pixel values deviated during data recording, which is probably caused by the atmospheric noise, such as fog and water vapor, while “geometric correction” is used to correct geometric deformation which happens from data recording and the Earth’s rotation. The data will then be adjusted to geo-referenced positions, which require Ground Control Points (GCP) for image adjustment and correction.
Image enhancement is aimed to adjust the gray–level values of image pixels to the new clearer pixel values for easier image interpretation. The adjustment is made in the image histogram.
- Image processing
This is a procedure or method to classify pixel values into layers of data classification for pixels grouping under the fixed condition. The first type of image classification is “supervised classification” that will categorize reflection of the wavelengths into various sample groups. Then a training area is designated to represent a variety of characteristics for statistic calculation, such as the average value of each type of data. Such statistical values are used to represent data classification, and this type of image classification requires the use of ground-based data. The other type of image classification is known as “Unsupervised classification”, which will classify the data from reflected wavelength values of different objects. The process is called clustering.
- Visual image interpretation
Visual image interpretation requires experience, knowledge and understanding about characteristics of the study areas and activities taking place in those areas at different periods of time. Elements of image interpretation include tone and color, size, shape, texture, as well as height and shadow.
2.Geographic Information System
Geographic information system (GIS) is the system that compiles, stores and analyzes data systematically. It can help for data search and improvement, as well as using the analyzed data for decision making process. The data compiled and stored in the system can be used for management and analysis of spatial data, which is linked to attribute data that explains detail of a phenomenon and characteristics of the spatial data to induce correctness and precision of data use.
The purpose of GIS use is to support decision making in various issues such as the planning for use of natural resources and management of man-made environment. The GIS data can answer the questions about where the places and things we search for are located and how they are involved with the surrounding objects. Besides, it indicates what choices are available, what their characteristics are, and also makes comparison to find the best option. Data from the geographic information system is a spatial data which can tell the position of our interested Geo-referenced data. The GIS consists of varying data including physical and social characteristics both in terms of quantity and quality of the studied objects. It also indicates the position and time of the objects we are studying
3. Global Navigation Satellite System (GNSS)
The GNSS will receive satellite signal to find geo-referenced position at any spot on Earth throughout 24 hours, without weather limitations. Therefore, it is presently recognized as a good navigation system. The GNSS functions by receiving signals from at least three satellites, which can calculate a position in two dimensions, that is horizontal position. And if the GNSS can receive signals from four satellites up, it can tell a position in three dimensions, which are positioning and the height.