Special issue to commemorate the 70th Anniversary of the Battle of ...

Decoding the Enigma of the Fall of the Shing Mun Redoubt Using Line of Sight Analysis
or MicroSurveyCAD 2009.
In generating the Lines of Sight/Fire covers, the
commercial GIS software, “ESRI ArcGIS,” with its
extension package, viz., 3D spatial analysis, was
used to construct a three-dimensional view of the
interpolated surface. There was, indeed, other GIS
software (e.g. Global Mapper; MicroDEM; ERDAS
Imagine, and IDRISI) on the market that was also
capable of generating 3D views and contained a similar
analyzing function, but we selected ArcGIS to conduct
the analysis for the following reasons:
• It was probably the most common GIS software
used in Hong Kong, if not the world. Many GIS
and land-related professionals have used the
software since their training/studying periods.
• In light of its popularity in the industry, the
software’s problems of data conversion, data
format, and data compatibility could be minimized.
• It contains many user-friendly, built-in functions
that can be used to generalize results.
• Other GIS software contained different limitations
in building up the surface model.
After all the surveyed details were generated and
plotted in AutoCAD format, the data was then, layer by
layer, incorporated and geo-referenced into the ArcMap
with the B1000 as base maps.
By abstracting the digital contour information, as
contained in the base maps, a Triangular Irregular
Networks (TINs) model was then built to create the 3D
terrain model of the Shing Mun Redoubt. TINs, which
are digital geographic data in the form of a vector base,
were established by triangulating a set of vertices.
These vertices were connected with a series of edges to
form a network of triangles.
Upon completing the TIN model, the elevation values
for any location on the TIN surface could then be
interpolated using the values of the triangles’ nodes.
In ArcMap, some built-in tools could be used to
analyse the visibility of the parts of the resulting model
surfaces. In this project, two major tools, viz, the Line
of Sight and Viewshed, were used:
• The effective observation range of the MG was
600m at all angles.
• There was an average plant height of one metre at
the Shing Mun Redoubt in December 1941.
• There was no material change in the natural
topography between December 1941 and the
present except that which was already reflected in
the survey plan.
• The heights of the observers at the PBs and OP
were 1.2m above the floor level.
• The heights of the observers at the concrete firing
trenches were 0.7m above the floor level.
• The heights of the observers at the dam were 1.2m
above the floor level.
• The targets at the PBs and OP were one metre
above the floor level.
• The targets at the concrete firing trenches were 0.5m
above the floor level.
• The targets at the dam were at the ground level.
• The average refraction factor was applicable.
• The Earth’s curvature was negligible.
The key findings are shown in Figures 2 to 5 and
Table 2.
Figure 2 shows the horizontal surveyed layout of the
entire Shing Mun Redoubt as it existed in Winter 2008
with the names of the tunnels (which were actually
inscribed on the walls of the tunnels and the open
trenches, to which numbers T1 to T9 were assigned by
the authors for ease of reference. The question as to
whether there was one or are two systems of tunnels
has been answered: there were two separate systems.
This shows that the tunnels were deliberately not built
in straight lines, but in a zig-zag manner with varying
widths. This was certainly a design feature to prevent
the enemy’s ‘enfilade fire’ from easily shooting inside
the tunnel. During the survey, we identified from aerial
photos and measured a concrete bunker or block house,
now in ruins, very close to Jubilee Dam. Marked on
our maps as “Position X”, this site had never been
reported, but would have altered the course of the
fight for the Redoubt had it been manned as a forward
post with machine guns. This post covered the entire
Jubilee Dam and the slopes of Needle Hill descending
to the dam.
• The Line of Sight tool identifies whether or not one
location is visible from another and whether or not
the intervening locations along a line between the
two locations are visible (ESRI, 2006).
• The Viewshed tool can be used to find the places
that can be seen from one or more observation
points or lines. It calculates for each cell of a
raster surface and codes them to indicate if they are
visible (ESRI, 2006).
For analysing firing arcs and lines of sights, we made
some basic technical assumptions:
• The elevation and depression observation angles of
the Vickers Machine Gun (MG) were 20 degrees
and 30 degrees, respectively.
SBE
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