THE USE OF LINEAR FEATURES AS REFERENCE DATUM IN 
DIGITAL MAP REVISION 
Jussi Heikkinen 
Anita Laiho 
Helsinki University of Technology 
Institute of Photogrammetry and 
Remote Sensing 
Finland 
Abstract 
In this paper we shall present the idea of connecting the concept of linear features to a digital map revision 
process. The purpose is to simplify map revision with digital or digitized images, by using linear features 
derived from GIS as reference datum instead of pointwise control, and by digital mapping. Some simulation 
results will be given to show the capability of linear features in close-range and aerial photogrammetric 
tasks. 
Key words: Map Revision, Linear Features, Geoinformation System 
0. INTRODUCTION 
Traditionally, the main part of the time spent in 
mapping processes has been required for prelimi- 
nary tasks i.e. field surveys. For minor mapping 
processes, e.g. map revision purposes, very careful 
and precise design have been required in order to 
achieve sufficient accuracy. Usually, standardized 
methods have been used concerning the number of 
control points, their distribution and the size of 
overlap. This kind of procedure obviously guaran- 
ties the quality of mapping, but the relation be- 
tween the amount of time and money spent on the 
process compared with the size of the mapping area 
can be poor specially in the case of map revision 
projects. For this reason a new, more flexible 
method could be applied and, still, fulfil the requi- 
rements of accuracy. 
. An alternative approach to accomplish a project is 
to use old, existing objects of the area in question 
as the control datum. In very many cases we have 
some a priori knowledge of the area. Very often 
surveys have been done in earlier times and their 
results have been stored and can be found. In an 
ideal case the information is already in a numerical 
form, e.g. in a GIS database. The objects in ques- 
tion could be roads, their edges, or blocks of build- 
ings. It is required that the data is stored with 3- 
dimensional coordinates. At the moment, this kind 
of data is, unfortunately, seldom available. Also, 
some information about accuracy is desirable. Now- 
adays 3-D LIS/GIS implementations are rapidly ex- 
panding and already some companies and institu- 
tions are providing such databases. 
In order to replace control points with objects, we 
must use methods of feature based photogrammet- 
ry. The basic idea is to determine some linear fea- 
tures in a parametric form from the original image 
observations on several photographs. Vice versa, 
resection can be performed based on a 3-dimension- 
al model of some features to determine the position 
and orientation of the cameras. Intuitively it is 
true, it is hard to achieve the same accuracy on 
control features as by measuring geodetic points. 
On the other hand, a linear feature in parametric 
form consists of data in a very compact form. We 
claim that the increased number of control data 
substitutes the requirement of their accuracy. Lin- 
ear features are more stable and their parametric 
form stabilizes the solution of exterior orientation, 
because the image observations of one feature are 
more dependent on each other. 
1. USE OF LINEAR FEATURES 
1.1 Parametric formulation of linear features 
As mentioned, parametric form of linear features is 
very compact and consist of an enormous amount of 
information about the object curve. In this sense, 
the position of features and its orientation on photo- 
graphs have effect on the results. Power of these 
methods is shown later with examples. 
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