Due to the technical advancements in the fields of computer, visualization, pattern recognition and feature extraction - photogrammetry has entered into the digital era. The DEM and ortho-imagery generated has now been tremendously used in the Remote Sensing and GIS community. The DEM derived from digital photogrammetric techniques need to be validated against those derived from analytical stereo plotters. This paper reflects a study comparing the DEM derived from these two techniques.
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The production of Digital Elevation model (DEM) is an important task in photogrammetry. DEM's are either stand-alone products or they are used for the purpose of generating secondary products such as orthophotos. The extraction of DEM is mostly carried out in analytical stereo plotters these days. The DEM derived from the digital photogrammetric techniques need to be validated against the DEM derived from analytical stereo plotters that are in use for long time and considered to be more reliable and accurate. So a study is necessary to compare the DEM derived from the analytical and digital techniques.
Towards that objective, this paper took up a pair of aerial photograph of the scale of 1:12000 of Hants County, Canada for comparison. The hardcopy diapositives and digital scanned images at 15 microns were available. The study area covered within the stereo pair contains diversified terrain features. The area consists of urban land (residential and commercial
complexes), dense forest and sparse coniferous forests, rivers and streams. Transportation networks such as main roads,
railways, exist in the study area. For creation of DEM Planicomp P3 (analytical) and VirtuoZo (digital photogrammetry system) have been used. Apart from creation of the DEM, an attempt to study the effectiveness of automatic image matching has also been attempted by creating surface DEM automatically by image matching technique in digital and manually in analytical system.
Methodology
The methodology followed for the exercise had 2 key steps as follows -
- Generation of DEM's- surface and ground DEM.
- Analytical method using Planicomp P3 stereo plotter.
- Digital photogrammetric software -VirtuoZo.
- Comparison of DEM created from Analytical and Digital photogrammetric techniques
Stereo Model Generation
The basic photogrammetric steps in both digital and analytical photogrammetric systems to create stereo models for DEM creation are as follows-
- Interior Orientation (IO)- Removes the distortions in the image.
- Relative Orientation (RO)- Relates the overlapping portions of the left and right image to create the stereo model.
- Absolute Orientation (AO)- Allows the stereo model to be mapped to a real world co-ordinate system.
