product; rather they are considered a temporary work material, 
because in the implemented digital terrain model, instead of the 
Libyan Mean Sea Level altitude datum, they used the EGM96 
geod related altitude data. This is according to the (NCB) 
estimation results in 40-60 cm mean error, which they plan to 
compensate for in the final ortho photograph products. The map 
was prepared on the 7% zone of the Libyan Transverse Mercator 
Projection, which was tied to the LGD2006 datum. With this 
solution, using GPS-based measurements becomes possible 
without limits, taking into consideration that conversion 
between the WGS-84 and the LGD2006 datums can be easily 
solved. Coefficients needed to carry out the 3-parameters datum 
transformation are provided by the Surveying Department of 
Libya. In doing so, harmonization among development projects 
going on simultaneously in the city becomes possible. Further 
convenience service is reached in which GeoTiff format 
becomes available by cropping the ortho photographs along the 
edges of the Transverse Mercator projection mosaics. The 
mapping materials provided by the NCB were masked in many 
places for security reasons. Taking into consideration that the 
Metro network criss-crosses the inner city and intersects with 
some areas missing coverage in the NCB's aerial photographs, 
we had to substitute the missing parts with cuts from the 
satellite imagery, then apply elastic sheet transformation on 
these image cuts, and then they were co registered on aerial 
photographs. A further significant problem experienced is that 
the ortho photograph maps do not provide elevation data for the 
Metro network planning process. As a solution, we used 
elevation data from the above mentioned hard copy map 
scanned as a raster data. During transformation we took into 
consideration the effects of the differences of the datums of the 
photographs, and then the contours and the photograph of the 
discrete altitude points were put in vector forms. This 
practically means that plans prepared during the revision of the 
plan study of Tripoli Metro are relative to base surface 
determined by medium elevation of the Mediterranean Sea. 
Overall, it can be said that the produced final map served well 
the accuracy requirements of + Im in both latitude and altitude 
levels (horizontal and vertical directions) of the revision study 
of planning the Metro network. However, it should be noted 
that this product is not suitable for further detailed execution 
phases and tasks, for example it is not suitable for an execution 
plan. Execution plan, especially in densely built-up urban 
environments, data accuracy in the range of centimeter is 
needed. In essence such accuracy can be achieved only by 
terrestrial geodetic survey (Engedy, 2008). 
4. PLANNING THE METRO LINES 
From line designer’s point of view, it is necessary to have a 
whole detailed picture of the planning area. In Tripoli Metro 
case the choice was to use a detailed base map which was casted 
in a well-known and technically sound projection system that 
covers the whole planning area. The projection choice of a base 
map that was not prepared by classical terrestrial geodesy 
happened for three main reasons: First, the planning area was so 
large that surveying it by terrestrial means was not an option. 
Second, at the beginning of the planning process, there was no 
final decision about the exact line alignments, and consensus 
were undergoing among different Libyan authorities about the 
exact alignment locations, this fact excluded the possibility for 
the designers to consider only narrow stripes of land for the 
purpose of lines alignments. Third, the time frame allocated for 
the revision study excluded the possibility of following lengthy 
terrestrial surveying works. 
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B7, 2012 
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia 
    
The availability of satellite imagery and aerial photographs 
during different stages of the planning process has made it 
possible to flexibly change line alignments according to the 
continuous negotiations and agreements between the 
contracting Libyan (RPEMB) and other Libyan authorities. 
Figure 6, shows deep alignment line that is led away from a 
planned traffic node. Figure 7, shows subsurface alignment line 
planned not affecting buildings and leaving half the street open 
during construction. Figure 8, shows that during construction 
only half of the highway needs to be closed. Figure 9, shows 
Metro line aligned avoiding a traffic node. And figure 10, 
shows Metro line aligned avoiding a bridge foundations. 
  
Figure 6. Deep alignment line led away from a planned traffic 
node 
  
Figure 7. Subsurface alignment line not affecting buildings