Remote Sensing & GIS
Remote Sensing and Application Areas
Remote sensing can be defined as; the acquiring information and evaluation techniques about natural and artificial objects that are found at surface with measurement instruments which are connected to known distances from surface, atmosphere or space.
Nowadays, sciences related with the Earth use remote sensing. Remote sensing are used in many fields related about geo-science that are geological investigations, natural hazards, structural geology and especially natural resources investigations etc. In addition, hydrogeology, botany, agriculture and meteorology use remote sensing technology.
Acquiring Data Using Satellites
Acquiring data process is carried out by measuring the differences of electromagnetic reflectance of the objects.
The basic principle of remote sensing can be defined as the different reaction of every object for same light source in different wavelengths. According to this point, Remote Sensing enhances the data set that is present by detecting information which human eye can not detect.
Acording to spectral characteristics, two part can be defined:
Reflection Wavelength Regioni
Emission Wavelength Region
- Thermal Infrared
- Microwave
- Passive microwave, active microwave, super high and Ultra high frequency
Microwave Imaging
The wavelength of the microwave is range between 1mm – 1 m . The most important advantage is that microwave is not affected by clouds and atmospheric gasses. It is called RADAR Imaging.
RADAR (Radio Detection and Ranging), is the system that detects the existence and position of the objects via radio signals. Being active, as well as operating in the microwave region of the electromagnetic spectrum, allows RADAR to operate day or night in all weather conditions (e.g., clouds, rain, snow). RADAR data can be used to map three-dimensional surfaces in the form of digital elevation models (DEMs), to measure surface changes in time, and to detect moving objects. Longer microwaves of RADAR can also penetrate the subsurface to detect buried features.
Waves and frequency ranges used by radar (http://www.radartutorial.eu, 2014)
Wavelengths of Various Bands in the Microwave Range
| Band | Frequency (MHz) | Wavelength (cm) |
|---|---|---|
| Ka | 40,000 – 26,000 | 0.8 – 1.1 |
| K | 26,500 – 18,500 | 1.1 – 1.7 |
| X | 12,500 – 8,000 | 2.4 – 3.8 |
| C | 8,000 – 4,000 | 3.8 – 7.5 |
| L | 2,000 – 1,000 | 15.0 – 30.0 |
| P | 1,000 - 300 | 30.0 – 100.0 |
Electromagnetic waves sent by Radar transmitter antenna receives the target and a small portion of the reflected energy returns to the radar set. Radar receiver antenna gets the signal and uses it to detect the direction and distance of the target.
Basic principles of radar http://www.radartutorial.eu
RADAR systems may be divided into several major groups, including:
- Synthetic Aperture RADAR (SAR): RADAR operated from a movable platform, such as an airplane, can use its forward motion as a way to simulate a dish with a much larger diameter, referred to as a synthetic aperture. The synthetic aperture RADAR (SAR) achieves high resolution by storing and processing Doppler shift data from multiple return pulses.Spaceborne SARs have resolutions on the order of 25 meters, while aircraft and UAV SARs have resolution as fine as sub-meter. Satellite SARs typically map a swath width per pass of 100 km while aircraft and UAV have swaths on the order of 10-30 km.
- Interferometric SAR (InSAR): Interferometric Synthetic Aperture RADAR (InSAR) is an aircraft- or satellite-based remote sensing method capable of measuring minute changes on the earth's surface. InSAR utilizes two antennas to image the same terrain area at slightly different ranges. The two near-simultaneously collected images are then processed coherently (i.e., the magnitude and phase value is retained). The two "coherent" images of the same area are then phase compared, where the slight difference in phase is related to a height difference.InSAR techniques can be used to calculate flow rates of slow moving surfaces (e.g., glacier ice), to produce high spatial resolution topographic maps with accuracies of a meter or less, and to detect subtle changes in the height of terrain as in the case of subsidence before or after an earthquake.
- Pass to pass coherent SAR: "Pass to Pass Coherent Detection" SAR is a subset of the larger InSAR category, in which data are collected for the same area over two or more time periods or passes to detect subtle movements or height changes. The individual passes are "coherently" processed and, as in the case of InSAR, are phase differenced where the phase information is used as an indicator of low velocities, topography or subsidence. Changes to terrain features between the passes, such as those due to geologic activity, can be detected with this method.
- Ground moving target indicators (GMTI): Ground Moving Target Indicator (GMTI) RADAR uses a moving target's Doppler RADAR return to distinguish it from surface clutter. This technique makes it possible to detect, locate, and track targets with the RADAR cross-section of vehicles throughout a large synoptic area when they are moving slowly on or just above the surface of land or water.
- Ground penetrating RADAR (GPR): Ground Penetrating RADAR (GPR) uses electromagnetic wave propagation and backscattering to image, locate, and identify changes in electrical and magnetic properties in the ground. Practical platforms for the GPR include on-the-ground point measurements, profiling sleds, and near-ground helicopter or aircraft surveys.GPR has the highest resolution of any geophysical method for subsurface imaging, approaching centimeters. Depth of penetration varies from meters to several kilometers, depending upon the materials' properties. Detection of a subsurface feature also depends upon contrast in the dielectric electrical and magnetic properties. Interpretation of ground penetrating RADAR data can lead to information about depth, orientation, size, and shape of buried objects. GPR has utility for a variety of transportation applications. These include: location of underground utilities (pipes, wires, fiber), soil types, and water.
Many of these sensors can be found on a wide range of platforms including earth-orbiting satellites, manned aircraft, and unmanned aerial vehicles (UAVs).
Spesification of SAR Sensors
| ENVISAT | PALSAR | TerraSAR-X | |
|---|---|---|---|
| Observation mode | IS2 | Fine beam FBS/FBD | StripMap (011) |
| Launch | December 2002 | January 2006 | June 2007 |
| Orbit height | 800 km | 692 km | 514 km |
| Periodical cycle | 35 days | 46 days | 11 days |
| Frequency | 5.3 GHz | 1.26 GHz | 9.6 GHz |
| Band | C | L | X |
| Wavelength | 5.66 cm | 23.6 cm | 3.1 cm |
| Polarization | VV | HH/HH+HV | HH |
| Off-nadir angle | 20.3 deg | 34.3 deg | 35.8 deg. |
| Coverage | 100 km×100 km | 70 km×70 km | 30 km×50 km |
| Spatial resolution | 30 m. | 10 m/20 m | 3.3 m |
| Critical baseline | 1,250 m | 16,500 m | 2,400 m |
The frequency of Radar is in between around 30 MHz and around 98 GHz. The usage of bands is optional according to projected resolution, sensitivity and distance need.
Thermal Imaging
All materials in universe emit energy above -273 0 C . An object that absorbs energy also emits energy at the same time. The energy emitted by the objects is a property of mass and temperature and can be seen by using thermal infrared sensors.
Sensors
Some Remote Sensing Sensors that are used for Geological Remote Sensing Studies
| Sensor | Launch | Country |
|---|---|---|
| LANDSAT | 1972 | USA |
| SPOT | 1986 | France |
| SPOT5 | 2002 | France |
| ERS | 1991 | Europe |
| RADARSAT-1 | 1995 | Canada |
| RADARSAT-2 | 2007 | Canada |
| IRS | 1995 | India |
| JERS | 1992-1999 | Japan |
| TERRA / ASTER | 1999 | USA/Japan |
| EO1 / Hyperion | 2000 | USA |
| BilSat | 2003 | Turkey |
| ALOS / PALSAR | 2006 | Japan |
Landsat TM
Landsat TM (Thematic Mapper)-1, is the first satellite to perform remote sensing studies, was launched by NASA in 1972. Up to 1999, Landsat-1, 2, 3, 4 and 5 were launched and in 1999 the latest Landsat was launched that is called Landsat-7 ETM+ (Enhance Thematic Mapper). It turns at an orbit about 705 km high from surface. Landsat visits and scans same area in each 16 days. One scene size of the Landsat is approximately 185 km x 185 km .
Basic rock types identification (magmatic, metamorphic, sedimentary), mapping volcanic activities, dome - caldera structures, determining wide regional structures, determining linear and circular structures, determining the hydrothermal alteration zones, for geothermal studies etc geological purposes it is used.
| Band | Spectral Range (µm) | Resolution (m) | ||
|---|---|---|---|---|
| 1 | 0.450 - 0.515 | Blue | Visible | 30 |
| 2 | 0.525 - 0.605 | Green | Visible | 30 |
| 3 | 0.630 - 0.690 | Red | Visible | 30 |
| 4 | 0.750 - 0.900 | Near Infrared | NIR | 30 |
| 5 | 1.55 - 1.75 | Shortwave Infrared | IR | 30 |
| 6 | 10.4 - 12.5 | Thermal Infrared | TIR | 60 |
| 7 | 1.09 - 2.35 | Shortwave Infrared | IR | 30 |
| Pan | 0.520 - 0.900 | Visible | 15 | |
Landsat TM Bant Properties and Application Areas
| Band | Usage |
|---|---|
| 1 | Difference between vegetation and soil boundary, forest area and shore line mapping |
| 2 | Green vegetation |
| 3 | Healty vegetation, water body boundary determination |
| 4 | Bitkilerin miktarını saptamada, litolojilerin tanımlanmasında, toprak/litoloji ve kara/su arasındaki kontraslığı gösterir |
| 5 | Arid areas, water contents, detection of difference between snow and ice |
| 6 | Temperature, vegetation, thermal pollution and determining geothermal places |
| 7 | Determination of the boundary between lithology and soil body, discrimination of water in soil and vegetation |
Landsat TM bands and indices
| index | operation |
|---|---|
| Bitki indeksi | B4-B3 |
| Normalize Fark Bitki İndeksi (NDVI) | (B4-B3) / (B4+B3) |
| Demir Oksit | B3/B1 |
| Kil Mineralleri | B5/B7 |
| Demirli Mineraller | B5/B4 |
| Mineral Kompozisyonu | B5/B7, B5/B4, B3/B1 |
| Hidrotermal Kompozisyon | B5/B7, B3/B1, B4/B3 |
ASTER
r sensor ( Japan ) is located on the TERRA Satellite Platform which is belonging to NASA (1999). It has an orbit located 705 km above the Earth. The ASTER Sensors are sunsyncronous. It passes around 10:30 local time and acquire images. TERRA also contains five sensors other than ASTER. ASTER has moderately spatial resotions and it has 14 bands that have different spectral ranges. Moreover, one scene covers 60 km x 60 km
| Band | Spectral Range (µm) | Spatial Resolution (m) |
|---|---|---|
| 1 | 0.52-0.60 | 15 |
| 2 | 0.63-0.69 | |
| 3 | 0.78 - 0.86 | |
| 3N | 0.78 - 0.86 | |
| 4 | 1.60-1.70 | 30 |
| 5 | 2.145-2.185 | |
| 6 | 2.185-2.225 | |
| 7 | 2.235-2.285 | |
| 8 | 2.295-2.365 | |
| 9 | 2.360-2.430 | |
| 10 | 8.125-8.475 | 90 |
| 11 | 8.475-8.825 | |
| 12 | 8.925-9.275 | |
| 13 | 10.25-10.95 | |
| 14 | 10.95-11.65 |
ASTER images especially are used for determining rock type, mapping volcanic activities, discrimination of linear and circular structures, determining hydrothermal alteration regions and preparing basic minerological zoning maps, determining geothermal zones, stereoscopic images etc. Since it has 14 spectral range to obtain spectral characteristics of the object we can produce more detailed mineral and alteration maps. After this operation detailed information is gathered about potential mineral areas.
On the other hand, by using 3D stereoscopic images gathered from ASTER, fotogeological studies can be applied.
Geographical Information Systems
GIS is a collection of computer hardware, software and users to make up a system, which collects, stores, manipulates and displays spatial information about the earth.
Data Models
In GIS, there are 2 types of mathematical constructs for representing geographic objects or surfaces as data.• Vector Data Model, Raster Data Model
Vector data model: A coordinate-based data model that represents geographic features as points, lines, and polygons. Each point feature is represented as a single coordinate pair, while line and polygon features are represented as ordered lists of vertices.
- Point Feature: A geometric element defined by a pair of x,y coordinates. Point feature is Used for geographic elements which have small boundaries.
- Line Feature: A map feature that has length but not area at a given scale, such as a fault or a river on a world map or a street on a map.
- Polygon Feature: A map feature that bounds an area at a given scale, such as formations, lakes, a city on a map)
Raster data model: A spatial data model that defines space as an array of equally sized cells arranged in rows and columns, and composed of single or multiple bands. Each cell contains an attribute value and location coordinates. Groups of cells that share the same value represent the same type of geographic feature