GNSS in Practical Determination of Regional Heights - Global Navigation Satellite Systems: Signal,Theory and Applications - page 138

campaigns and data processing strategies adopted to compute benchmark coordinates
satisfy the criteria of LSMSDPR (2005), on determination and use of local GNSS/levelling
geoids. Accordingly the geoid reference benchmarks must be the common points of C1, C2
and C3 order GNSS benchmarks and high order levelling network points. Thus the GNSS
observations of IGNA2005 project were carried out using dual frequency GNSS receivers,
with observation durations of at least 2 hours for C1 type network points (for the baselines
20 km in length), and between 45 and 60 minutes for the C2 type network points (for the
baselines 5 km in length). The recording interval was set 15 seconds or less during the
campaigns. The GNSS coordinates of network benchmarks were determined in ITRF96
datum 2005.000 epoch with ±1.5 cm and ±2.3 cm of root mean square errors in the two
dimensional coordinates and heights, respectively (Ayan et al., 2006). The levelling
measurements were done simultaneously during the GNSS campaigns and Helmert
orthometric heights of geoid reference benchmarks in Turkey National Vertical Control
Network 1999 (TUDKA99) datum (Ayhan and Demir, 1993) were derived. Total number of
the homogenously distributed reference benchmarks is 1205 with the density of 1
benchmark per 20 km 2 in the network (see Figure 7).

model point
test point

Longitude ( ° E)

Fig. 7. Geoid reference benchmarks in Istanbul (topographic data from SRTM3 (USGS, 2010)

The second case study on determining local GNSS/levelling geoids was carried out in the
Sakarya region situated in the East of Marmara sea and Izmit Gulf (between 40°30' N -
41°30' N latitudes, 28°30' E - 31°00' E longitudes). The GNSS/levelling network was
established during the Geodetic Infrastructure Project of the Marmara Earthquake Region
Land Information System (MERLIS) in 2002 (Çelik et al., 2002), and overlap with IGNA2005
network. Compared to the Istanbul area, the topography in Sakarya is quite rough and the
elevations are between 0 m and 2458 m. The GNSS and levelling observations, and data
processes were executed according to the regulation of the project. After the adjustment of
GNSS network, the accuracies of ±1.5 cm and ±3.0 cm for the horizontal coordinates and
ellipsoidal heights were derived. During the GNSS campaign of the MERLIS project, precise
levelling measurements were undertaken, simultaneously, and in the adjustment results of