components to correlate on. Image content is the single 
most contributing factor to correlation success. Low 
frequency areas will generally be correlated slower, 
although the templates will automatically increase in size 
until sufficient texture exists in the template to allow 
correlation. The correlator will visit and attempt to height 
every point in the DEM and if a correlation cannot be 
accurately computed, then a height is interpolated. The 
interpolation is performed by a weighted technique based 
on radial distances of points in a neighbourhood. 
Evidence has shown that collecting hierarchically from a 
very coarse resolution through successively finer levels, 
reduces the possibility of generating a false fix. By reducing 
the scale of the imagery, changes in elevation are less 
pronounced in image space, thus the effect of height 
variations are minimised and searches over broad ranges 
of elevation are quicker. Also correlations at reduced scales 
tend to reduce the confusion between similar appearing 
objects by locking into gross areas which include the 
objects. The heights derived from each level are used as 
an estimate for the next higher resolutiori collection level. 
EDITING TOOLS AND ACCURACY 
Central to generating an accurate DTM is the ability to edit 
the computer generated model. This is required either 
where man made features (with sharp edges) need to be 
highlighted, cliff edges need to be added as 'breaklines' or 
simply where the computer has failed to find suitable 
matching points from which to generate a height. 
Correlated points will be designated a 'quality figure' based 
on a user-defined set of signal-to-noise ranges, as either 
'good', 'fair or 'poor. Failed attempts at correlation are 
labeled as ‘interpolated’ and are all rankings are made 
available to the operator to give assistance for the editing 
stage. 
Whilst the automated collection will generate heights based 
on statistical correlations, it is important for the sake of 
ensuring accuracy that the height values can be validated 
by the operator. The software displays all the points at their 
correlated positions in a stereo view, colour coded to allow 
a rapid visual inspection of the whole model. 
The editing tools provide an interactive method of modifying 
the height of points deemed to be in error by the operator. 
In this way, the resulting DEM has both a statistical 
statement of accuracy and one that has also been verified 
by an operator using their skill and judgment. As the 
accuracy of the correlation is dependent on the accuracy 
with which the stereo geometry was computed, the 
software provides a full summary of all mathematical 
calculations including standard deviations for all final 
computations of camera positions and attitude. 
DERIVED PRODUCTS 
As well as being used in a range of military and commercial 
spatial analysis applications, the terrain database can be 
used to generate additional products, such as orthoimages. 
These are images that have been corrected for 
72 
displacements due to relief variation and sensor 
imperfection. In any imaging system, each imaged point will 
have a particular perspective geometry and in order to view 
each pixel in an orthogonal projection (i.e. from a nadir 
view, as if each pixel were being viewed from directly 
above) the effects of terrain have to be removed. The DTM 
is used to model the relief variation present in the image 
and each pixel in the raw image is resampled into an 
orthogonal projection which the user can define. 
The orthoimage is vital, especially if perspective views are 
to be rendered for mission planning or visualization as this 
will ensure that all features are in their true position with 
respect to the underlying elevation model and curious 
effects such as rivers flowing uphill can be avoided! It is 
also vital for use an highly accurate, up to date base map 
for commercial applications, including database updating. 
USES WITHIN GIS APPLICATIONS 
Uses with GIS applications can be divided into two primary 
types; those requiring height information (the DTM) or a 
derivative (slope, aspect) and those requiring high precision 
base maps (the orthoimage), either for backdrops or as a 
source of vector data. 
Many spatial modelling applications, such as site location 
and route planning require height derived "layers" as part of 
the process. For example, where new housing 
development projects are being planned, using soil type 
combined with slope can show areas where land slippage 
may occur. In route planning in military applications, slope 
again is important as certain vehicles may only be able to 
negotiate low angle slopes. One area where aspect (i.e. 
south facing, north facing etc.) is important is in vineyard 
location - it is important that vines are planted at the 
optimum location to produce the best quality grapes! 
DTMs can also be used in visualisation, specifically in 
environmental and military applications. The siting of new 
facilities can first of all be generated using the spatial 
analysis described above. The proposed site could then be 
viewed in 3D, with the facility "added' to the DTM, either by 
adding a polygon of the appropriate value or "height" in 
mono view or by accurately adding the height of the facility 
in stereo. This enables the user to check on its visibility 
from surrounding areas. Viewshed analysis can also be 
used in the opposite sense to show if your own location can 
be seen from other areas for the purposes of concealment. 
Orthoimages, as described above, provide the most 
accurate (and up to date!) base maps of all. Many natural 
resource management applications now simply use a 
symbolised base map instead of a complex vector based 
map. The old adage "a picture is worth a thousand words" 
could easily be changed to " a pixel is worth a thousand 
vectors" in this instance! However, the largest demand for 
orthoimages lies in the data provision aspect of GIS. 
An orthoimage can be used for generating vector map and 
other measurement information directly from the computer 
screen. In the past this has to be done by the 
photogrammetrist on an analytical stereo plotter using 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996 
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