Unless flat terrain and only simple objects are involved, final 
quality control and validation must be done visually, 
preferably on-line with adequate 3D superimposition. 
Feeding back data from the FDS database to the digitizing 
software (eg; via DXF) requires again development of a data 
interface. Such 3D topographic mapping by digitizing 
features of man made objects in stereomodels, with 
superimposition of already available vector data, can be 
done on analytical plotters (AP) and digital plotters (DP) (the 
latter also being referred to as digital photogrammetric 
workstations). Recent project work using an AP and CAD 
software for model construction was also reported in 
(Littleworth et Chandler, 1995), presenting fine examples 
and a market perspective for such products. 
For the collection of ground elevation data, one may 
consider automatic DTM generation, but this will not be 
expedient when working on large scale photographs of 
densely built up areas, since most current image matching 
algorithms cannot adequately deal with occlusions and 
multi-valued surfaces. Verifying and manually correcting a 
DTM obtained from image matching may even take longer 
than direct manual elevation measurement. Both APs and 
DPs are suited for manual and semi-automatic DTM data 
collection. 
A further problem arises when going to detailed terrain 
representations, namely, the definition of the ground 
surface. For example, are stairs part of the ground surface? In 
case of a canal or a swimming pool shall we take the water 
surface? What to do with a heap of sand next to an artificial 
lake? Obviously, object definition according to application 
context must be settled prior to photogrammetric work. 
The coding system will depend on the object definition, and 
together with the digitizing convention (eg, counter 
clockwise line strings), controls object assemblage. Actual 
coding must be tailored to the capabilities of the digitizing 
software. For object reconstruction we can distinguish two 
basic categories of objects: (a) surface, line, and point 
objects coinciding with the ground surface (eg, parcels, 
sport fields, parks, streets, rails, drainage, man holes) and (b) 
objects 'sticking out' (/e, either having significant vertical 
extension or dangling above ground, eg, buildings, walls, 
poles, fly overs, power lines). Collecting data for the first 
category should be combined with DTM generation. For the 
second category, houses specifically (with vertical walls and 
non-overhanging roofs), it is sufficient to measure the top 
when using aerial photographs and then generate the 
footprint and the walls by intersection with the DTM. 
Table 1 shows a few examples of detail to be measured and 
figure 2 illustrates the reconstruction steps of a house with 
vertical walls. 
Not all roof outlines are edges of walls that rest on the 
ground (see B1 versus B2). Coding must be sufficiently 
detailed to cope with different spatial relationships. The 
required level of detail determines what needs to be 
measured as a minimum. Digitizing and coding of surface, 
line, and point objects of the first category is straightforward. 
Vertical surface and line objects (eg, fences and poles) are 
difficult to digitize unless the photoscale is very large. Their 
height often has to be obtained by terrestrial measurements. 
Interpretation training and knowledge of the human 
operator about the subsequent structuring process are 
prerequisites for efficient manual data collection. 
  
Table 1 Examples of elementary features and coding 
  
  
  
  
  
  
  
  
  
  
  
  
Type Code D escription/Example Purpose Graphics 
=> 
Body feature B1 Roofoutline To construct the Y. 
body by plane | hel SET TTR 
sweep vertically to | inen? S 
inte rsect with DTM T | 
Body feature B2 Roofoutline Intersect with 
body 
Surface feature $i Roof facet boundaries [Replace the roof 
(ridge and drainage) outline after 
obtaining the 
body 
Surface feature $2 boundary of tennis To be part of 
court, street edge ground surface 
Line feature L1 railway To be part of 
ground surface 
Line L2 lam p-post To be completed 
feature by providing 
height 
Point feature P1 Location ofa tree, To be partof 
man hole ground surface 
  
  
  
  
  
862 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996