Thursday, January 29, 2009

Ortho/Orthorectification process

In the areas of GIS data acquisition, visualization and general mapping,
digital satellite imagery and aerial photographs have a significant place.
Photographs obviously provide a solid visual effect. Imperceptible spatial concepts are more clearly understood by viewing the photographs. These are not photographs taken by ordinary cameras. These are very professional high-end cameras with higher zoom and clarity.

Another important role of these photos is to provide a foundation for collecting the spatial information needed. Data in the form of the satellite images or aerial photographs must be taken without any distortions, if you need to gather information useful for a mapping or GIS system in the case of roads, marine forms or vegetation.

This process of correcting the distortions of a satellite image of aerial photograph is called Orthophoto rectification .This process allows you to make direct and precise measurements of areas, distances, angles, positions etc.

Why an aerial photograph needs correction?

The main challenge of an aerial photograph compared to a non-aerial photograph is that an aerial photograph needs perspective correction. An aerial photo is usually captured at an angle to the object being photographed. Here, the perspective of the photograph will be incorrect with near objects compared to distant objects. By perspective correction, the objects in the image and the real world will have equal size.

Process of Orthophoto rectification service

As topographical variations in earth’s surface and the tilt of the satellite or the aerial sensors can affect the display of the features on the satellite or aerial image with regard to their distance. The image distortion will be more as the topographical diversity of the landscape is more.

Image data acquired by airborne and satellite image sensors are affected by systematic sensor and platform-induced geometry errors, which introduce terrain distortions when the image sensor is not pointing directly at the Nadir location of the sensor.

There can be hundreds of meters of terrain displacement. Image data over an area with at least a kilometer of vertical relief, with the sensor having an elevation angle of 60° (30° from Nadir), the image output will have nearly 600 meters of terrain displacement.

Errors in setting the reference elevation can cause further terrain displacement. Other than this, low elevation angles of images, imperfect terrain models, and inconsistency of sensor azimuth and elevation angles within an image alters the accuracy potential if image orthorectification is attempted. To overcome this defect, high elevation angles of the sensor is used with new high resolution satellite image of irregular terrain.

Creation of digital elevation model (DEM)

For the accurate removal of the image distortions, a digital elevation model (DEM) is used to make image orthorectification or Ortho mosaicing. The requisite DEM can be produced by semi-automatic DEM extraction software from stereo satellite scenes obtained by the QuickBird, IKONOS, SPOT-5, or ASTER satellite sensors, and stereo aerial photography.

In situations where higher mapping accuracy standards are required, the DEM is extracted from the already existing topographic maps with reference to a standard scale or collected using stereo satellite image data.

To obtain this accuracy standard, adequate GPS-derived ground control points (GCPs) are necessary. Other remote sensing techniques can also be used such as radar interferometry or LIDAR.

When a particular vector data needs to be extracted from satellite or aerial image data by raster-to-vector translation, the process of orthorectification of the remotely sensed image data can rectify all digital images of geological, environmental, topographic or any other source map which will be used in the GIS Mapping service environment.
For more informations visit SBL Geomatics

Ornithology and GIS

During spring, flocks of migratory wading birds arrive from their natural habitat, which would be usually intolerably cool during winter, to a critical non-breeding habitat on the tropical places.

Protection of these migratory birds is a concern that needs real attention. Some of these birds from northern hemisphere fly more than 20,000 km a year in search of a suitable dwelling place for survival during the winter season.

The use of GIS and remote sensing technology can be used as an integral part to trace the migrating location of these birds from field mapping to reporting of the location.

One tip to find the birds of migration is to identify their food habit. This would give an idea of their prospective migrating location with regard to the availability of the specific food.

For instance, if we take the birds that usually migrate from Siberia to the tropical North coast of Australia. These migratory birds feed on small animals that live in mud such as crabs, snails and worms. These birds naturally migrate to the area of low muddy lands of Australia to feed and refill their energy for their journey back to their natural habitat.

Using compatible and innovative GPS units and enough field staff, samples can be collected from various points of the expected area of migration by producing progress maps and occasional species maps. By these procedures, even the presence of any new species in the area also can be identified.

To cite another example, some migratory birds have time and again halted in Malaysia during their roosting season that usually lasts from November to March because of its Matang Mangrove Forest.

The arrival of these migratory birds was observed by The Department of Wildlife and National Parks and they have decided to create a GIS database in order to study the biodiversity and sustainability of migratory birds.

Finally, they made a GIS database for the migratory birds and conducted an overall analysis on the captured data. The methodology run from need assessment to data collection, database development and system integration. This finally resulted in an analysis on the trends of bird migration, the properties of ecosystem, environment sensitivity analysis and spatial statistic analysis on the distribution of the migratory birds.

As already mentioned, the resultant migratory bird’s database contain statistical results on the trend of bird migration which in turn helped to identify the endangered species of migratory birds. When the endangered species are classified, measures and procedures for the maintenance of the mangrove areas are taken.

The database of the migratory birds with reference to the diversity and sustainability of the birds can been developed using ArcView 3.2, MapObject 2.0, Microsoft Visual Basic 6.0, AutoCadMap 2.0 and S-Plus 2000.
Geospatial services
Article By: RARIMA N S