originally captured.
Random
The points were distributed randomly. Data
acquisition technique such as field surveying (total
station), photogrammetry (random sampling)
produces this kind of pattern. The problem with this
pattern is that the topology of the points is not easy
to defined.
Contour
This is another common way of storing DTM data.
Points with equal height were depicted in one same
line, ie. known as a contour line. Contours are
usually depicted on topographic maps. The contours
normally converted to digital form by manual or
automatic digitizing for further processings. Digital
contours data is easy to handle and manipulate, for
instance in DTM. Its easiness and simplicity to
handle, made the data popular for DTM generation.
Triangular Irregular Network (TIN)
By definition a TIN is formed by a series of irregular
triangles. Each triangle consists of three nodes, and
each node has x, y, z coordinates. These nodes
represent data points, and they are not interpolated
(i.e. every point is utilized). For example, the nodes
may come from measured random spot heights of
field surveying, or they may come from selective
photogrammetric sampling, or they may be the result
of digitizing of contours, etc. A connection of any
three ‘proper’ nodes forms a triangle, and if further
connections are made to these data points, then a
network called TIN is established. The word
‘proper’ in the preceding line, means a process of
connecting the three nodes by a systematic
triangulation algorithm, e.g. using Delaunay
triangulation (Huber, 1995). The TIN data structure
actually has been used for terrain representation for
quite sometime, back to 1970's (Peucker et al, 1978).
Now the structure is being utilized in quite a number
of GIS and mapping software (Kumler, 1994). The
structure has the following advantages:
* the size and shapes of the triangles varies
according to terrain roughness (i.e. in flat
surface, the size tend to be larger than in
rougher terrain).
* ability to handle breaklines and specific
points into the triangle network.
* ‘saddle point problem’ as in grid structure
can be avoided.
8
* some of the computation tasks are easy to
perform, e.g. slope and aspect calculation.
ee
Contours
Figure 1 Some of the most prominent DTM data
structures.
Besides the advantages offered by the TIN, it has
some limitations, e.g. need more computer storage
space to store the triangle nodes, triangle sides, and
the topology information. In this aspect, the grid is
much simpler and takes less space.
Since the structure (ie. TIN) offers several
advantages, thus we adopted it in an attempt to
identify the most developable land.
TIN-based Applications
This section reviews some of the typical DTM
applications, namely, contouring, and. slope-aspect
mapping.
Contouring. One of the common output of DTM is
contour map. The contours may be generated in
different ways, but in TIN it is appropriate to
interpolate the contours using simple method (e.g.
linear interpolation) then followed by line smoothing
(Alias, 1992). The contour interval also play a role.
Larger interval may represent course terrain, on the
other hand, the smaller interval may effect the
storage, interpolation speed, and producing touching
contour lines in steep slopes. So the chosen interval
should reflect what we are going to do with the
contours. Generally, application which needs
detailed information should go for smaller interval,
whereas larger interval for the less detail.
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
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