3. DIGITAL STILL CAMERAS: STATUS AND APPLICATIONS
3.4 Applications
3.4.3 DEM and orthophoto generation
Digital elevation models (DEM's) are the primary data used in the analysis of catchment topography (Moore et aI., 1991) and are thus important information layers in a GIS. While stereoscopic aerial photography has been the main means of deriving DEM's in the past, the advent of modem computer-based image processing and transformation packages has meant that procedures which are analogous to photograrnrnetric techniques are now being applied to raster based digital images.
The reason that height inforrnationcan be obtained from overlapping pairs of images is simply due · to the fact that changes in ground elevation produce changes in photo scale and these in turn produce measurable relative displacernents of objects within the pairs of images (Burnside, 1979, p93). Points that are higher than the image nadir are displaced outward from the centre of the image and points that are lower in elevation are displaced inward from their true position.
King et al. (1994) selected a single pair of 0.7 m pixel digital frame camera images which overlapped by 60 percent to evaluate the potential of using the 1340 by 1037 pixel Kodak MEGAPLUS 1.4 black-and-white digital camera for elevation determination. The test site chosen was in the east end of Toronto, Canada. This area had a very small elevation variation and
consisted mainly of residential and small commercial land uses. Municipal survey control points were thus abundant.
An edge-enhancing algorithm was first applied to improve pixel identification of suitable control and test points, after which three points were chosen as control points. This small number of control points was chosen in order to simulate an operational situation where only a few control points can be measured because of cost or difficulty in identification. These three points were used in a bundle adjustmene program to perform relative and absolute orientation ofthe images.
The accuracy of elevations of other test points in the imagery were then evaluated by comparing their elevations with those obtained in a ground based survey using standard levelling and traversing techniques. Results showed that despite no rigorous sensor calibration having been performed, the method had potential for elevation determination., with elevation RMS being about 2.3% of the total variation in topography within the scene. This was similar in dimension to the pixel size. They considered the method to be applicable to obtaining large-scale elevation models.
Subsequent to this research, the same camera was evaluated for use in DEM generation in natural terrain where survey control was not abundant or evenly distributed across airborne imagery. This study is documented by King et al. (1995). The study area in this case was the Gatineau Park in Quebec, just north-west of Ottawa, Ontario. This park had rugged terrain and was mostly forested with only a few roads crossing its interior.
Digital elevation models of the area were automatically created using a photograrnmetric software package. This package used an iterative area-based matching process to determine conjugate pixels in stereo image pairs. The elevation for each matching pixel was computed using the parallax equation., following which the elevation of points between the conjugate pairs was interpolated to fill in the rest of the raster DEM. Finally, the DEM was absolutely referenced to the ground reference system using the existing control points in the imagery. In comparing the photographic and DFC imagery for elevation modelling accuracy by relating the elevations derived from the imagery to those obtained from a field survey using static differential GPS techniques,
2
Bundle or block adju~1ment is the final step in a process that is known in photogrammelry as aerotriangulation.
Aerotriangulation and bundle adjustment are complex processes. The interested reader should refer 10 Chapter 6 for a detailed explanation.
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the DFC model was found to produce mean differences closer to zero than the photogn~h;.c
model. King (1995) indicated that this may have been due to a systematic ~rrc 11'!Jr
photography such as greater influence of vegetation. Unfortunately, the precisioil." . the DFC model was lower than in the previous urban study. The standard d "'/'11; "
elevation differences was 4 times the ground pixel dimension compared to .L.; , .; • j , photography. King (1995) provided several possible explanations for thir, ;)'':.J
concluded that the potential of the sensor used was in low cost acquisition of) . DEMs for custom applications.
Maas and Kersten (1997) have performed similar research to establish the po L, j .
cameras for photograrnmetric applications. They made use oftwo test areas. 'P.
an alpine village with an area of 520 by 500 m where the use of digital photnf"
interest for cadastral applications and for orthophoto generation. The seconr ., 300 m landslide area consisting of lightly forested meadows over a steep rocy 'r,
the focus was on measuring deformations and generating a DTM.
Imagery fOf the two pilot studies was captured using a 1524 by 1012 pixel Ko I, ' camera. The imagery of the two areas was then semi-automatically aerotrialll,..
signalized control points in the form of white plastic plates and additional n' points. Results of this aerotriangulation were considered to be reasonably gO()l of2 cm and 5 to 6 cm being achieved for the planimetry coordinates and heigh .
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Based on the results of aerotriangulation, DTM's were derived from the twu . ] f.,
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Multi-image feature-based matching, which is characterised by its high reliabil..ity, \ ' . ,:' automatically derive the DTM's. Following this, orthophoto mosaics were COmpPlf'Q f ,r ti, ., I
regions. For the alpine village study, the average standard deviation of the feC",1 f , " " . ' i 1; ,..-, was 0.027,0.027 and 0.109 m in X, Y, and Z, respectively. For the landslide si~, J d'l. '1" I ' c_
standard deviations in the X, Y, and Z coordinates were 0.023,0.023 and 0.095 rn, I ,.~.·'i,
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1 In both cases these standard deviations were 0.03 percent of the flying height above. !S' 0~Jl-:The results achieved by King et al. (1995), King (1995) and Maas and Kersten (19,'/)
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promise for the use of digital still cameras for DTM generation and aerotrianguiation. Importantly, 40
both sets of researchers state that newer generation DFCs with higher resollJtions "i/,''> , :,: : p
expected to produce much better results.
There are also a few applications ofDFC's which cannot easily be categorised I .. use of DFC's in informal settlement planning, urban run-off determination.
hydro towers and lines, pipeline rights-of-way management and for verifi(':;, '(
large-scale mapping.