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Fig. 3.5 First-order
triangulation chain with
astronomical points
closure, the mean square error of angle observation should not be greater than
0. 7
00
.
The initial side length at the crossing of the chain sections should be determined
with a relative accuracy of no less than 1/350,000. The astronomical longitude,
latitude, and azimuth are measured at the two endpoints of the initial side and the
former two are also measured at a point in the center of the chain. The determined
mean square error of the astronomical longitude, latitude, and azimuth should be
less than
0.5
00
, respectively. All points with measured astro-
nomical longitude and latitude will provide data for computation of the deflection
of vertical. As astronomical surveying is involved in the plans for network estab-
lishment, the national horizontal control network is also called the astro-geodetic
network.
0.3
00
,
0.3
00
, and
Second-Order Triangulation Network
Set within the area circled by the first-order triangulation chain loop, the second-
order network is the overall basis for densification of the third- and fourth-order
networks, as shown in Fig.
3.6
. The average side length of the second-order network
is 13 km and the density of such a network basically satisfies the needs of the
1:50,000 scale mapping. The second-order network, together with the first-order
chain, belongs to the national high-order network. Hence, accuracy should be the
primary concern whereas density is secondary. The mean square error of angle
observation computed through the triangle closure should be less than
1
00
.An
initial side and azimuth are to be determined at the center of the network. For larger
chain loops, the initial azimuth should be measured as well. Angles of triangles in
the network should be no less than 30
. The second-order network on either side of
the first-order triangulation chain should be connected with the first-order chain to
form a continuous triangulation network.
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