Bismillahirrahmanirrahim.
For the last two weeks, I have already learned about LiDAR and IFSAR. Now I want to share with you about this spatial data type.
I will introduce to you about DEM, DTM, Bare Earth, Contours, Datum, Hillshade, and of course LiDAR and IFSAR, and comparison among them.
DEM
A Digital Elevation Model (DEM) is fundamental information for any three-dimensional (3-D) geo-spatial
activity. Many methodologies are currently being used to generate DEMs for different applications at various
scales, details and accuracy. Interferometric Synthetic Aperture Radar (IFSAR) technology is very effective in the creation of accurate large-area elevation datasets. Leaders in the geo-spatial community are starting to accept airborne IFSAR as a complementary cost-effective 3-D mapping technology for many applications. This is evidenced by the Shuttle Radar Topographic Mission (SRTM) and the mapping of the United Kingdom, the first completely commercial sponsored mapping of an entire country. However, IFSAR has not yet reached its full potential as a mapping tool in the marketplace. With these major efforts, it is key that the geo-spatial community understand the specifications of the IFSAR DEM product.
Geo-spatial applications such as scene visualization, modeling, animation and simulation of the real world
require three dimensions. As a result, there is a growing demand for high-quality elevation data. Recent advances in sensor development, geo-referencing technologies coupled with the continuous improvement of digital computing power now enable unparalleled functionality and flexibility in geo-spatial modeling.
A DEM is used as a means of 3-D terrain modeling, which serves as a basic source of information for deriving geo-spatial uniqueness. Currently, DEMs are being generated by many methods, such as ground survey, photogrammetry, Light Detection and Ranging (LIDAR), and IFSAR. Although different from a technological perspective, those methods are simply different ways to achieve a similar goal – 3-D elevation data generation. Technologies that can provide detailed and accurate elevation data in a timely and cost-effective way are highly desirable for many applications. DEMs generated by a particular method have their own set of characteristics. An awareness and understanding of these characteristics is essential for a successful DEM application. The characteristics mainly include data structure, resolution, quality, and limitations. A critical success factor is to determine what DEM specifications are suitable for a particular application. IFSAR has been a technique of considerable scientific interest due to its highresolution
3-D information extraction capability, quick turn-around time, and near weather-independent operation.
Interest in IFSAR has been growing since data became widely available from the microwave sensor on the ERS-1 satellite. The SRTM that flew successfully in February 2000 provided a further impetus for mapping applications using IFSAR technologies. IFSAR has a much greater economy of scale considering the capability of an airborne IFSAR sensor. Recently, Intermap’s airborne IFSAR technology has been successfully applied for several national and regional high-quality elevation mapping projects.
Bare Earth, Break Line, Contours, Datum, and DTM
Bare Earth: Digital elevation data of the terrain, free from vegetation, buildings, and other man-made structures. Elevations of the ground.
Break Line: A linear feature that describes a change in smoothness or continuity of a surface.
Contours: Lines of equal elevation on a surface. An imaginary line on the ground, all points of which are at the same elevation above or below a specified reference surface.
Datum: Any quantity or set of such quantities that may serve as a basis for calculation of other quantities. For Indonesia the horizontal datum (i.e., coordinate system in which horizontal control points are located) is the WGS 1984
DEM (Digital Elevation Model): A popular acronym used as a generic term for digital topographic and/or bathymetric data in all its various forms, but most often bare earth elevations at regularly spaced intervals in x and y directions. Regularly spaced elevation data are easily and efficiently processed in a variety of computer uses.
DTM (Digital Terrain Model): Similar to DEMs, but they may incorporate the elevation of significant topographic features on the land and mass points and break lines that are irregularly spaced to better characterize the true shape of the bare earth terrain.
LiDAR
Airborne LIDAR or laser scanning has become a widely accepted option for terrain information collection.
LIDAR is an active surface measurement technique that acquires elevation data with a high point density. A RMSE vertical accuracy of 15 cm is achievable under well-controlled conditions. Primary LIDAR applications include corridor mapping and line-of-sight analysis. LIDAR is also a viable option for acquiring local and regional terrain information. Some federal survey administrations in Germany have switched totally from standard photogrammetric methods to laser scanning for DEM generation (Jacobsen, 2002). However, LIDAR is limited by weather conditions and has a small observation ‘foot print’ along with considerable data processing requirements. These factors have positioned LIDAR as a suitable technology for some applications, but LIDAR is generally limited by cost for large target areas, for example over 20,000 km2. By nature, LIDAR generates a DSM that must be reduced to a DTM when it is needed. The success of this reduction is greatly dependent on limiting the LIDAR angular scans to be close to nadir. The additional gray value image based on the reflected intensity, available from some LIDAR systems, can be used to support the generation of a DTM.
IFSAR
IFSAR is designed for surface data generation and has been traditionally used in a dual-pass configuration with spaceborne Synthetic Aperture Radar (SAR) systems. In contrast to other DEM generation methods, the biggest advantage of IFSAR is its weather and light independent capability. This makes IFSAR very useful to map through the smoke of a forest fire, rain clouds during a flood, or at night. IFSAR when properly configured can efficiently map large areas. When compared to spaceborne counterparts, single-pass airborne IFSAR systems have more flexible system deployment, higher spatial resolution, and a lesser degree of influence from the atmosphere and temporal target changes. These advantages provide for the creation of a DEM product with greater accuracy and greater spatial detail. During the last few years, high-resolution airborne IFSAR data has reached a wider application base and has begun significant penetration into the traditional photogrammetric market. While spaceborne IFSAR can typically provide elevation data with accuracy up to +/-5m, airborne IFSAR can generate DEM with a better than 1-m vertical accuracy (RMSE) in favorable terrain conditions. IFSAR signals interact with the terrain and thus measure distance to first surface features. Further, IFSAR is a side-looking sensor and does not view nadir and thus the potential for a ground view of the target is reduced. Because of these collection parameters IFSAR has limitations in heavily vegetated areas, areas of very high relief, and the urban core areas.
Comparison among IFSAR, LIDAR and Photogrammetry for DEM Generation
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IFSAR
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LIDAR
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Photogrammetry
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Sensor
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· Microwave sensor, most often X-Band, 3cm wavelength.
· Active, coherent system.
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· Near infrared sensor, about 1 nm wavelength.
· Active, coherent system.
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· Passive optical sensor.
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Imaging
geometry
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· Side-looking, typical incidence angles ranging from 30o to 60o.
· Angle of collection limits forest penetration
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· Nadir, typical incidence angle: +/-20o (max 35o)
· Direct polar coordinate determination.
· Better forest penetration when scan angles near nadir are used -providing a better DTM in this mode.
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· Nadir.
· Need intersection for 3-D coordinate determination.
· Almost no penetration due to the nature of intersection at the areas close to nadir.
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Typical
operational
conditions
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· Nearly weather/light independent.
· STAR-3i specific:
· Flying height: 6000 to 10000 m.
· Flying speed: 750 km/hour.
· Ground swath: 6 – 10 km, dependent upon flying height.
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· Weather dependent.
· Flying height: 300 to 2000 m.
· Flying speed: 200 km/hour
· Ground swath: up to1 km, dependent upon flying height.
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· Weather/light dependent
· Much wider range of flying height/speed.
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On-board
GPS/INS
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· Necessary for direct georeferencing.
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· Necessary for direct georeferencing.
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· Not necessary, but can be used to reduce or eliminate ground control points.
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Common
sampling
pattern
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· ‘Area-like’, cell integration
· Output regular grid directly, i.e. 5 x 5 m.
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· ‘Point-like’, irregular sampling.
· Spot diameter: 10 – 100 cm, dependent upon flying height.
· Spot separation: 1 – 5 m.
· Requires interpolation for regular grid representation
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· ‘Point- and line-like’ for manual collection. Editor controls the sampling pattern.
· ‘Area-like’ for automated image matching. Ground sample distance depends on scanning resolution.
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Nature of
DEM
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· DSM.
· Processing required to produce a DTM.
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· DSM.
· Processing required to produce a DTM.
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· DTM/DSM from manual collection.
· DSM from image correlation. Processing needed for DTM.
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Vertical
accuracy
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· 30 cm to 3 m RESE
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· 30 cm to 3 m RESE
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· Photo scale dependent, can be accurate to several centimeters.
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Cost
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· Very cost effective.
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· Relatively expensive.
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· Expensive.
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Best suitable
for
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· Timely, large area high accuracy requirement.
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· Appropriate for accurate and detailed delineation of ground features in built-up or forested areas.
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· Much wider application areas.
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