GNSS OBSERVATION ACCURACY – IS IT WHAT YOU EXPECT? Toni Hill, Senior Survey Advisor, Tony Nikkel, Cadastral Surveyor, Genevieve Abrey, Senior Survey Advisor, Garth Falloon, Cadastral Survey Advisor – Toitū Te Whenua Land Information New Zealand
G
NSS technology has changed significantly in the
meet the accuracy requirements. The locations and
past 20 years and is now the preferred equipment
environments that some of the marks were found in clearly
of choice among many surveyors. The advances in
indicate a questionable use of GNSS measurement meth-
GNSS surveying equipment and satellite signals have been
ods to achieve the required accuracy. In other locations,
accompanied by claims that the equipment now has the
the inappropriate use of GNSS technology may not be as
ability to work in ‘high multipath and signal shaded envi-
obvious and the low accuracy achieved is surprising.
ronments’ allowing surveyors to work more efficiently and
Despite manufacturer and surveyor claims, careful
in more challenging environments than ever before. There
consideration still needs to be given to the factors that
is also an expectation that with the increased number of
affecting GNSS accuracy, including:
constellations and satellites, ‘survey-grade accuracy with
Number of available satellites
trusted reliability’ can be achieved almost anywhere, every time. It’s easy to get carried away with what can be achieved but does the reality meet expectations? The question should still be asked: how reliable is the GNSS data you
Position or geometry of the satellites Weather or atmospheric conditions Surroundings or environmental factors (buildings, trees, terrain, power lines)
are collecting and what checks and balances need to be
The length of base to rover vectors
completed to have confidence that the outputs meet the
Measurement methodology used (including the num-
required accuracy tolerances of the rules? Toitū Te Whenua LINZ is receiving an increasing number of reports from surveyors working over GNSS surveys, finding discrepancies between the measured vectors and positions of ground marks and other documented evidence. This is further supported in LINZ field audits. The photos included in this article represent survey marks measured to during field audits that failed to
ber of epochs recorded and duration of measurement time to record and reduce a measured vector). Most surveyors are aware of the dilution of precision (DOP) or coordinate quality (CQ) values the GNSS calculates which gives an indication of the strength of the satellite configuration and the estimated precision of the data collected. Settings are generally established to only accept
SURVEYING+SPATIAL
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Issue 106 October 2021 17
