CONTOURS GENERATED BY A DIGITAL TERRAIN MODEL 
WITH TOPOLOGICAL TRIANGLE NETWORK FORMAT 
William Wu-Kuang Chang 
Senior System Consultant 
Design Systems, Buchart-Horn Inc., U. 
S. A. 
ISPRS Commission IV 
ABSTRACT: 
Using a Digital Terrain Model (DTM) to generate profiles, 
cross-sections, or 
contours is an impressive approach to civil engineers for increasing their 
productivity and design flexibility. 
But the question of how to create a DTM 
file by stereocompilation for generating accurate profiles, cross-sections, 
contours or other 
professionals. 
applications in 
an efficient manner is of concern to many 
The purpose of this study is to carefully examine the contours created by 
manual stereocompilation and by the interpolation of a Digital Terrain Model 
(DTM) in a Topological Triangle Network (TTN) Format, to find an optimal 
solution. 
Through comparative analysis of contours generated with different criteria in 
a DTM and contours generated by manual stereocompilation, we can approach the 
most efficient formula for creating the DTM file to increase mapping produc- 
tivity, without sacrificing the quality of the contours. 
KEY WORDS: Photogrammetry, DTM, Topology, Triangle, Format, Contours, Accuracy 
1. INTRODUCTION 
Using a Digital Terrain Model (DTM) to generate 
profiles, cross-sections, or contours is an impres- 
sive approach to civil engineers for increasing 
their productivity and design flexibility. The 
developments of computer hardware and graphic 
software places DTM applications on a new plateau, 
such as 3D Modeling, perspective viewing, build-ing 
road surface, and automatic design. DTM applica- 
tions have been performing very well on several 
commercial software packages, the question of how 
to create a DTM by stereocompilation for generating 
accurate profiles, cross-sections, or contours in 
an efficient manner is of concern to many profes- 
sionals. 
Various kinds of DTM formats have been developed 
for DTM applications, but only the Topological 
Triangle Network Format satisfies the most of 
engineers in the requirements of both computer 
processing and engineering accuracy. The reason is 
that a DTM, when created with Topological Triangle 
Network Format can represent the terrain relief 
with many different features of geographic elements 
and various densities of terrain points. 
Assuming the geographic points are placed in criti- 
cal locations, the more geographic points available 
in a DTM, the more accurate the extracted data will 
be. However, with too many geographic points in a 
DTM the collection time in stereoplotter digitiza- 
tion and the processing time in the computer would 
make it impractical. 
There is an optimal number of geometric elements 
needed to represent the relief of the terrain and 
still meet the accuracy requirements of the engi- 
neering project. The geometric elements typically 
used to create a DTM in Topological Triangle Net- 
work Format include REGULAR points, BREAK lines, 
SPOT elevations, OBSTACLE lines, CONTOUR lines, and 
EDGE line. 
REGULAR points are supplemented by random points in 
an area where there are insufficient geographic 
843 
elements to support the accuracy of the DTM sur- 
face. BREAK lines are the main features used to 
express irregular topography. SPOT elevations are 
independent points to present critical locations 
such as high and low points. OBSTACLE lines are the 
boundaries of areas that are to be omitted from the 
DTM. CONTOUR lines are lines of equal elevation. 
The EDGE line is the boundary of the valid DTM 
data. 
In order to determine the optimal number of geomet- 
ric elements, we compared the differences between 
the contours generated from various densities of 
REGULAR points and the contours compiled by the 
stereoplotter manually. 
2. DESCRIPTION 
A Digital Terrain Model (DTM) is a numerical net- 
work surface which represent the spatial location 
and the relief of the terrain. The network surface 
can be expressed with many kinds of formats. The 
data set commonly stored in a computer to represent 
a terrain surface is called a DTM data file. 
A DTM data file can be created by the following 
methods: 
1) 
2) 
Digitizing geometric elements from aerial 
photo pairs with a stereoplotter. 
Digitizing geometric elements from existing 
topographic maps on a digitizing table. 
Collecting terrain data with a total station 
theodolite and data recorder. The recorded 
data can often be directly converted into a 
specified DTM format. 
Inputting data from survey field notes using 
suitable software to convert them into an 
X,y,z format ASCII file, or a 3D graphic file. 
Converting data from an existing DTM file 
created by another system. 
3) 
4)