SOFTCOPY PHOTOGRAMMETRY AND ITS USES IN GIS 
Shears J C, Sales Manager & Allan J W, Managing Director - ERDAS International 
Commission IV, Working Group 4 
KEY WORDS: 
ABSTRACT: 
Softcopy Photogrammetry, Orthoimage, Spatial Modelling, DEM, 3D Visualisation 
Two years ago, the term "softcopy photogrammetry" was almost unheard of in GIS circles. Today, the availability of low cost 
softcopy photogrammetry systems has opened up a vast range of data provision and updating options to GIS users. The 
two primary datasets that are created by softcopy photogrammetry are terrain data, in the form of a Digital Terrain Model 
(DTM) and an orthorectified image (Orthoimage), which is a georeferenced image, free from any sensor or relief distortion. 
This paper discusses why Softcopy Photogrammetry is needed and covers, in brief, the digital processes involved in the 
production of such data, together with a comparison of these process with traditional manual methods. It closes with a 
description of the type of GIS projects currently integrating softcopy photogrammetry. 
THE IMPORTANCE OF TERRAIN DATA 
There are a large number of military and commercial GIS 
applications that rely entirely on the ready availability of 
digital terrain databases. Their success or failure is 
dependent on the timely production and ultimate accuracy 
of the terrain models that are fed into them. The military 
applications range from simulation, mission planning and 
mission rehearsal to terrain referenced navigation and 
weapons guidance systems. Commercial applications 
include land use monitoring and assessment, such as the 
EC MARS program and base mapping for oil and gas 
exploration activities. At a time when budgets are under 
scrutiny, there is now a greater need for digital terrain data 
to be generated more cost effectively, whilst still being 
made available in a timely manner but also without 
sacrificing or compromising the accuracy of the data. 
With this in mind, new softcopy photogrammetric 
techniques have evolved which go some way to providing a 
solution for this. To understand how this has been 
achieved, it is necessary to look at conventional techniques 
for building terrain databases. There are three common 
methods, the most popular of which is using traditional 
analytical photogrammetry. An alternative is digitising 
contours or spot heights from hardcopy maps and creating 
a surface from them. The derivation of surfaces from 
digitised height features, will involve a degree of 
interpolation, hence the surface will be inherently more 
generalised than a photogrammetric compilation. Given 
that maps and charts are themselves derived from aerial 
photogrammetric surveys, then any errors in the 
photogrammetric compilation will also be propagated once 
they are digitised, hence true photogrammetric compilation 
will always provide a more accurate terrain database than 
map digitising. 
A third source of terrain data is to use existing products. 
Terrain databases do already exist in a digital form and are 
available as standard products like the US Defence 
Mapping Agency's Digital Terrain Elevation Database 
(DTED). Similar products are also available from other 
mapping organisations, such as the USGS and the UK 
70 
Ordnance Survey and a number of other National Mapping 
Agencies throughout the world. DTED has been the most 
widely available dataset for military applications and is 
widely accepted, but its predominance masks some 
inherent problems associated with it and also with other 
digital elevation model (DEM) products. 
With DTED, the resolution of the height data is fixed and is 
generally available at either 100m spacing (Level 1) or at a 
nominal 50m spacing (Level 2). However, due to the higher 
resolution of Level 2, it is only available to authorised users 
on a restricted basis. Whilst 100m may be suitable for 
broad area applications, higher resolution DEM data is 
needed to provide greater levels of detail to fulfill the 
potential of terrain based applications. The Level 1 product 
represents a very generalised view of the terrain even at 
100m resolution and some of the finer terrain detail is lost. 
This is sometimes done deliberately to protect compilation 
sources and provide collateral against national sources or 
because there was simply insufficiently accurate source 
material available at the time of compilation. Either way, the 
relatively poor resolution constrains the potential 
capabilities of the applications. 
This highlights the second major drawback as the user has 
no control over the accuracy and quality of the DEM. DTED 
is compiled from what ever sources were available for a 
given area using either photogrammetric extraction 
techniques or digitised mapping. RMS figures are provided 
for both horizontal and vertical accuracies but in some 
instances the application may demand higher levels of 
accuracy and detail. Mission planning projects for example, 
require higher quality at terminal locations whereas a lower 
quality may suffice for en route positions. The user needs 
to be able to both specify the resolution required and vary it 
as required for the application as well as the ability to edit 
the DEM to increase the accuracy if needed. The same 
criteria apply to commercial applications, where accuracy 
has an impact upon commercial decisions, rather than 
human lives. 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996 
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