Geoscience Reference
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Fig. 6.4 Zone division of the Gauss projection
L
1
:
5
n
0
ﾼ
ð
take integer quotient
Þ þ
1
:
3
Overlap Between Projection Zones
After a zone-dividing projection, the rectangular coordinate systems in the adjacent
zones are mutually independent. To carry out control network adjustment across
zones, to plot or use topographic maps across the zone boundaries, to show the
control points outside the mapping area (situated in the neighboring projection
zones), etc., the neighboring projection zones have to be overlapped to a certain
degree, as shown in Fig.
6.5
.
The so-called overlap between projection zones means that the control points
within a certain area have the coordinate values of two adjacent zones. Thus, on the
topographic maps in this area there are two sets of kilometer grids (the grid of the
coordinate system in its own zone and that in the adjacent zone).
For the overlap of projection zones, China has the following regulations: A
30-min longitude overlap is allowed between zones from west to east (equivalent to
the longitude width of a 1:100,000 map); and a 15-min longitude overlap is allowed
between zones from east to west (corresponding to the longitude width of a
1:50,000 map). Namely, each projection zone extends 30
0
to the east and 15
0
to
the west; thus, an overlap of 45
0
in longitude is formed near the dividing meridian,
as shown in Fig.
6.5
.
6.3.4 Natural Coordinates and False (Biased) Coordinates
In the northern hemisphere, after zone-dividing projection, all x-values of Gauss
coordinates are positive and y can have both positive and negative values. To avoid
a negative value of y, 500,000 m is assigned to it, which corresponds to a false or
biased y-coordinate of 500,000 m. Hence, the coordinate values of y are all positive.
Also, each zone is projected separately and forms a mutually independent plane
rectangular coordinate system. The same coordinate value (x, y) in each zone will
have a point corresponding to it, which is likely to cause confusion about the
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