«RR 
  
data bases are available. The photogrammetric ana- 
lytical systems may be connected to the workstation 
of to the LAN system. 
5.2 Interface levels 
As discussed in Section 2.3, three interface levels were 
defined for GeoTeX. Level 0 consists of the set of tools 
offered by the platform's operating system —editors 
and batch commands; level 1 is composed of alphanu- 
meric ANSI-based utilities —as for instance, syntax 
oriented file editors; level 2 is (or will be) based on 
Graphic User Interface (GUI) and Graphic System 
(GS) packages. 
The reason for defining such interface levels is the 
authors' aim at creating a portable system and, at the 
same time, offering a helpful —also portable— set of 
tools to its users. To achieve that, the usage of widely 
spread GUI/GS packages has been carefully avoided. 
As it is well known, nowadays there is not a fully 
accepted GUI/GS standard. Therefore, the realistic 
goal proposed in Section 2.3, —running GeoTeX on 
DOS (the field platform), VMS (the current platform) 
and UNIX (the threatening platform) at the interface 
level 1— has been the authors’ main objective. 
A simple, character-oriented, ANSI-based, port- 
able user interface is used by level 1 utilities. Minor 
or no changes are required to migrate such utilities 
to new platforms. Hence, a standard set of tools is 
guaranteed® to the user. Once this objective has been 
fulfilled, it is possible to develop the corresponding 
level 2 utilities for specific environments. 
Note that “character-oriented” does not mean poor 
interface. The quality of a user interface depends 
on several factors. Of course, aesthetics play a very 
important role —shorter learning times, better us- 
age, understanding and acceptance of the application 
by the users, etc.—, but there are other components 
which contribute to the quality of the final result. The 
concept behind the interfacing software, sometimes 
called the interface’s foundation, is the key point here 
(see [10]). The mental data model the user has to as- 
sume, the available functions, the navigation scheme 
and the look & feel of the interface being used are the 
critical success factors to take into account. 
It is recalled that when the GeoTeX interface foun- 
dation was conceived (see again Design Concept in 
Section 2.3), a clear objective was pursued: to of- 
fer the user a professional, upgradable interface, in- 
cluding the minimum set of functionalities required to 
perform his task. These functionalities —the feel of 
the interface— have been implemented at level 1 by 
  
6Syntax oriented file editors, data screening, coordinate 
transformations, graph utilities, interface with data bases and 
plotting (PostScript). PostScript is becoming a de facto stan- 
dard for plotting in scientific environments; see as well [3]. 
662 
  
OPTIONS FILE GUI RENDERING 
\ITEM 
\TYPE = CHARACTER 
\CHOICE = ( Valuel, 
Value2, 
Value3,... 
Value N) 
\VALUE = Value3 
\ITEM 
TI INTECER 
\RANGE=(Lower,Upper 
VALUE = Semen pper) 
  
Selected 
\ITEM X Option 
\TYPE=SWITCH 
\VALUE = On 
  
  
  
Figure 2: Option files and GUI rendering. 
means of a character-oriented look, the ANSI portable 
package. Of course, it is possible to use more sophis- 
ticated tools, specific non portable GUI/GS environ- 
ments, to upgrade that look (level 2); the feel, never- 
theless, will remain unchanged (see Figure 2). 
Thus, note that GeoTeX offers to its users a 
portable, professional level 1 interface, covering a ba- 
sic range of functionalities. This interface may be 
upgraded from the “rendering” point of view using 
GUI/GS non-portable packages. Nevertheless, the 
concept behind the interfacing software is the same 
for all levels. 
5.3 ACX architecture 
A layout of the structure of ACX is depicted in Fig- 
ure 3. ACX, like GeoTeX, has been developed as a 
compromise between the limited available means and 
the paradigm of object oriented programming. Fig- 
ure 3 is almost selfexplanatory though somewhat sim- 
plified. The generation of initial approximations is, 
at the moment, done with a separate main applica- 
tion program (GAI) but the next version of ACX will 
embody GAI; this makes the program flow more com- 
plex than in Figure 3 since intermediate adjustments 
with simplified linear models have to be added. 
Note the files in the dashed boxes in the same Fig- 
ure 3. The upper left box contains the GeoTeX sys- 
tem files. The most important one contains descrip- 
tors which serve as links between abstract data types 
and data [transfer] standards The lower left box con- 
tains the ACX files whereby the most important one 
contains the network abstract data type definitions.