Tuesday, February 16, 2010

Creation ofDigital Elevation Model (DEM)

For the accurate removal of the image distortions, a digital elevation model (DEM) is used to make image orthorectification. 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 ortho rectification 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 environment.

Creating anOrthophoto / Orthorectification

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.

Wednesday, February 3, 2010

Web mapping services

The GIS /Mapping services specializes in Graphical Information Systems (GIS), and has produced several interactive online maps (and printed maps) for public use.Today, a variety of Web mapping services exist, each of them providing a similar interface to extensive mapping and other GIS data.Compare various web map services and service providers http://www.sblgis.com/web-mapping-services.aspx
,http://en.wikipedia.org/wiki/Comparison_of_web_map_services .

Tuesday, February 2, 2010

What is orthophoto generation and orthophotography?

An Orthophoto or orthophotograph is an aerial photograph geometrically corrected ("also known as ortho rectified , can be done by service providers ie Orthophoto rectification service"). Unlike an uncorrected aerial photograph, an orthophotograph can be used to measure true distances, because it is an accurate representation of the earth's surface, having been adjusted for topographic relief lens distortion, and camera tilt.The concept of geo-rectification services/georectification services comes under Orthophoto rectification service.
Orthophotographs are commonly used in the creation of a(GIS) Geographic Information System and is useful in GIS mapping services. Software can display the orthophoto and allow an operator to digitize or place linework, text annotations or geographic symbols (such as hospitals, schools, and fire stations). Some software can process the orthophoto and produce the linework automatically.

An orthophoto is an aerial photograph planimetrically corrected, thus an orthophoto is generated Orthophotographs have the positive attributes of a photograph such as detail and timely coverage, and the positive attributes of a map including uniform scale and true geometry. This enables orthophotographs to be used in their primary role as a backdrop on which map features can be overlaid. Orthophotos represent the primary use of remote sensing

Orthophotos are now created by scanning aerial photographs and converting them into a raster image file format. A digital terrain model is added as a means of collecting ground points to indicate the changes in elevation. When control points are identified on the aerial photos, stereo models are developed and geometric distortions are estimated. The image is rectified and georeferened using mathematical models of photogrammetry to remove tilt, terrain, atmospheric and other digital image distortions.
Multiple photographs can be analyzed, corrected, and mosaicked / mosaicing all at once by a process called “bundle adjustment”, in which interrelated sets of equations are used to find a globally optimal set of corrections across all photographs. If a photo is black and white, each pixel is assigned a single numerical value corresponding to its light intensity. Color orthophotos are handled in an analogous way by transforming a vector of light intensities for different color bands into a single number. After the model has been scanned, the film is then developed as a negative orthophotograph, and it would usually be brought to the required scale by appropriate scaling or setting of the stereomodel and can be enlarged or reduced by the user if the scaling has not been adjusted after being scanned.

Reference : Wikipedia