Full text: XVIIIth Congress (Part B2)

  
The imagery used was a single stereopair of colour 
1:25000 scale photographs of an area of Manchester, 
UK, scanned at a resolution of 1016 dpi (22.5 microns). 
The model contained a mix of topography, including 
undulating terrain, urban features, water bodies and 
forested areas. The scanned images were utilised on 
both the digital systems with the parent diapositives being 
set up in parallel on the Zeiss P3 analytical plotter. On the 
digital systems the basic photogrammetric procedures 
were performed, from basic 'setting-up' of the imagery to 
digital terrain modelling and orthoimage generation. 
With respect to the orientation procedures, all three 
systems (the two digital and the one analytical) produced 
comparable results. It must be said that the control points 
used were coordinated terrain features, clearly visible on 
both images of the stereomodel. Experience has now 
shown that problems occur when using imagery where 
control points only occur on one of the two images i.e. 
mono pug marked control. This makes it difficult to 
perform relative orientation monocularly, particularly with 
the ERDAS system, where only mono measurement is 
possible. With the ERDAS system, control points, used in 
the triangulation, must be identifiable on both images. 
With the ImageStation it is possible to perform the 
procedure in either mono or stereo. The procedure is 
quicker and easier in mono rather than in stereo. 
Investigation from just visual inspection (through 
stereosuperimposition of the DEM on the stereomodel) 
identified that the automatic DEM generations were 
tremendously variant with respect to the viewed 
stereomodel. The consistency of the results varied 
according to the terrain being viewed/analysed, with the 
greatest consistency appearing to be achieved over rural 
areas, compared with urban environments. 
Different parameter settings varied the standard of DEM 
produced drastically. Notable problem areas included 
areas of homogenous texture, shadows, dark slopes and 
water features. Urban environments proved particularly 
difficult to model, whether attempting to obtain just ground 
heights or just roof heights. This did however improve 
with the generation of grids with minimal spacing. This 
lead to processing times being increased, but individual 
buildings were modelled reasonably well, rather than 
possibly being completely missed, which occurred with 
previous spacings. It was concluded that the photo scale 
of 1:25000 was too small to allow reasonable modelling of 
the urban features. Clearly some form of statistical 
measure is required to analyse the DEMs produced, 
rather than only visually checking them against the 
stereomodel. 
With the use of digital images, the photogrammetrist is 
forced into recognising the features which will affect the 
accuracies of the output products. The geometric and 
radiometric characteristics of the image must be 
equivalent to those of the traditional hardcopy 
photography. Output may be affected by such things as 
scanning resolution, whether the imagery is colour or 
monochrome, image compression, image resampling or 
various other filtering techniques which are now available 
with the movement into image processing. 
The image file sizes involved are large. A colour image 
(whole photograph) scanned at 10 microns occupies in 
the order of 1.7 Gbytes whereas one scanned at 20 
microns requires approximately 432 Mbytes. This issue is 
obviously significant when large projects are 
encountered. Experiences with back-up procedures have 
indicated errors with tapes and significant variability in the 
time taken to back-off or restore data, especially over 
busy networks. 
4. CURRENT RESEARCH 
4.1 Input Data 
Continued experimentation is focused on the accuracy of 
the automatic DEM. Two new stereomodels have been 
selected with larger scales than previously used and with 
a wide range of topographic characteristics. It is important 
to analyse the merits and limitations of these algorithms 
with respect to a diverse collection of physical 
phenomena. The imagery includes a pair of black and 
white photographs at 1:3000 scale containing a mixture of 
residential and industrial features which is used for 
analysis of results in an urban area. The second pair of 
images are from colour 1:10000 scale. These are used 
for parallel testing within a rural environment. The terrain 
encountered has substantial topographical variations with 
sudden changes of slope, areas of homogenous 
colour/texture and a scattering of rural buildings. 
Both sets of imagery were initially scanned at 12.5 micron 
resolution, but problems occurred with the use of mono 
control, as previously described. For consistency, both 
sets of imagery were pugged in stereo and rescanned. 
The black and white imagery being scanned at 10 micron 
resolution and the colour imagery being scanned at 20 
micron resolution. This was performed on NRSC's 
recently purchased XLVision OrthoVision 950r digital 
scanner. This produced file sizes for each image of 576 
Mb for the black and white imagery and 432 Mb for the 
colour imagery. Clearly, the storage required for only two 
images is large. If a number of models are utilised in a 
project then careful planning is required. A feature of 
future work will be the use of a run of overlapping imagery 
to introduce the testing of DEM consistency over a 
number of models. 
4.2 Parameter Variations 
With both digital systems there are a large number of 
variable parameters, for which the user must select a 
value prior to DEM generation. It is important for system 
users, whether they be trained photogrammetrists Or 
scientists grasping the benefits of this new technology, to 
appreciate the tremendous variations in output available 
due to altering these parameters. This 'black-box 
technology must be treated with caution before 
acclaiming the benefits of the output. 
With OrthoMAX and SoftPlotter there are 16 and 17 
variable parameters respectively, most of which accept a 
range of values. With the ImageStation, the automatic 
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International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B2. Vienna 1996 
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