Orthophotography removes the distortions in the aerial photographs which occurs due to relief displacement, atmospheric refraction and lens displacement. When the photo is produced in digital format it is called Digital Orthophotography .
Orthophotography was first done in 1960. At that time it was expensive and required lot of time. Technological improvments in 1970 made it affordable and its use started increasing. Digital Orthophotography became prominent only two or three years ago due to affordable price of high technology computers.
Transfering an aerial imagery to orthophoto involves three steps:
• Correction of scale
• Removing errors due to relief displacement
• Values of X,Y coordinates are assigned to the photograph
Creation of an image by Digital Orthophotography follows the following steps:
• X,Y,Z values of grond control points must be known.
• Depending on the values of the point coordinates are assigned, the process being called aerotriangulation.
• After aerotriangulation, Digital Elevation Model is produced.
• The Digital Elevation Model standardizes the scale across the imagery.
• Scanning is done after the Digital Elevation Model is complete and the photograph is transformed into a digital image.
• Errors due to relief displacement is removed through computer based processes which relates Digital Elevation Model with rastered aerial imagery.
• Radiometric corrections remove the tonal difference to improve the quality of the image.
Quality Control of Digital Orthophotography :
• Spatial Accuracy which is the location of the position of pixels in reference to its actual position on the earth surface should be acquired for attaining a high quality of Digital Orthophotography
• Removes tonal difference across the imageries to produce a standardized Digital Orthophotography .
Digital Orthophotography provides with accurate:
• Measuring distance
• Calculation of area
• Identifying shapes pf the features
• Calculating directions
• Coordinate determination at a given place
Pixel Resolution in Digital Orthophotography :
Digital Orthophotography is an image processed by computer consisting of pixels. Ground disatance is represented by each pixels in the Digital Orthophotography . Higher pixel resolution produces very clear imageas the map is zoomed in. Pixel resolution varies in different Digital Orthophotography .
Proper photogrammetic quality products available from the Digital Orthophotography are:
• Digital Elevation Model and Digital Terrain Models in all kinds of format.
• At any kind of scale Digital Orthophotography is delivered as paper plots or as TIFF images on CDROM that can be made available as Geography Information System Packages.
• Elevation contours as separate TIFF images and DXF files.
• For fulfilling any kind of request slope and aspect maps can be drawn.
• A three dimensional view of the areas
Digital Elevation Model in Digital Orthophotography :
Digital Elevation Model or DEM are used to control large volume of data and mathematically represents the earth. Sophisticated high technological tools are used for creation of Digital Elevation Model .
Use of Digital Orthophotography :
• Geography Information Systems (GIS )
• Land Information Systems
• Automated Mapping and Facilaties Management
• Assessing and Analyzing Environmental Impact
• Studying project site
• Monitoring area of project
• Planning for Irrigation
• Hydrology and Hydrography
• Revision of Digital Line Graphs
• Revising Toposheets
• Resource Management
• Montoring Change in Growth Process
• Civil Engineering
• Pollution Controlling
• Soil Studies
• Geological Studies
So, Digital Orthophotography can truly produce an updated image of earth surface.
Remote Sensing and GIS