Reference Station user strategies in a tectonic environment

Author:Penny Boviatsou

Earthquakes are a response to the motion between tectonic plates. As two plates push together at a steady rate, the rocks along the plate boundary become more and more stressed until eventually something has to give — an earthquake occurs along a fault somewhere in the plate boundary zone.

New Zealand lies at the edge of both the Australian and Pacific tectonic plates. The Australian and Pacific plates push against each other along a curving boundary and generally don't move smoothly past each other. They move in a series of small rapid motions, each of which is accompanied by earthquakes.

SmartFix基于世界上最大的参考站网络HXGN SmartNet，是新西兰最广泛的GNSS参考站网络，在全国范围内拥有70多个电台。SmartFix提供百分钟水平，实时运动学校正，后处理解决方案和子级GIS校正。HXGN SmartNet是一款集成的24/7 GNSS网络RTK和校正服务，构建在世界上最大的参考网络上，可实现具有GNSS能力的设备，以快速确定精确的位置。

“Providing a real-time kinematic (RTK) correction service over the internet and RINEX files for post processing, our customers can enjoy the benefits of SmartFix,” said Bruce Robinson, director at Global Survey Limited. “If their GPS or GNSS receiver can connect to the internet, SmartFix can deliver the data they need.”

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Earthquakes shaking up New Zealand

New Zealand has two types of earthquakes. Deep earthquakes under North Island form a well-defined band (seismic zone) running northeast from Marlborough through White Island. Shallow earthquakes tend to occur to the southeast of this seismic zone, while the deeper ones occur towards the northwest.

The New Zealand GeoNet, a partnership between the Earthquake Commission (EQC), GNS Science, and Land Information New Zealand, monitors all geological hazards in the country, and locates between 50 and 80 earthquakes each day – about 20,000 a year. Earthquakes can occur anywhere at any time in New Zealand.

These earthquakes are continually altering the shape of New Zealand and while a lot of the movement is small and unfelt, it is continual and the result is perceivable, measurable and locally variable. The measurable and variable part is what the reference station network SmartFix needs to deal with.

There are many examples of rapid movements that combine lateral and vertical movements. Such movements are clearly not uniformed; this variability creates challenges for those who need to establish reliable survey control or those who run reference station networks.

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Investigating earthquake activity

Even though New Zealand is continuously moving and deforming under the influence of the Australian and Pacific tectonics plates, the country’s datum is designed to provide constant unchanging coordinates for features.

Datums define how coordinates, longitudes and latitudes or heights, relate to physical locations. Projections are different ways of representing a position in a datum, for example as northings and eastings used on topographic maps. Together, they define New Zealand's coordinate systems.

To manage this deformation, the datum itself is moving and deforming along with the New Zealand land mass – it is a “plate-fixed” datum.

The New Zealand datum is known as NZGD2000, and this datum is based on International Terrestrial Reference Frame (ITRF) 1996 as at the position on the feature on January 1, 2000. Because the datum is deforming, NZGD2000 coordinates no longer reflect the true positions of points relative to one another.

The local area distortion, however, is small, and for most applications, the deformation can be and is ignored. This allow for distances, bearings and areas to be calculated directly from NZGD2000 coordinates. This deformation, however, needs to be continually addressed and managed by the reference station administrators.

The error in ignoring the variation is no more than one millimetre in a kilometre for each year since 2000. A deformation model is periodically applied to account for deformation due to earthquakes, regarding calculating NZGD2000 coordinates, and as the measurement of the deformation model becomes more accurate.

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Challenges when running a reference station in New Zealand

The NZGD2000 datum was set on January 1, 2000 – the last time everything fit together nicely. Since 2000, New Zealand has had 17.5 years of differential movement, and while the coordinates of the mark have not changed, the relative position has. A point surveyed today, from three different reference stations, will likely have three different NZGD2000 coordinates dependant on the reference station used.

“Education of clients is a major part of SmartFix administrator job here in New Zealand, as we are upskilling users to understand the geodetic implications of station selection,” said Robinson.

The second challenge is obtaining and choosing the two coordinates for the reference stations that the Leica GNSS Spider reference station software requires. The two coordinates being the current epoch coordinates to ensure the reliability of the network and the NZGD2000 coordinate. The GNSS Spider solution is an integrated software suite for centrally controlling and operating GNSS reference stations and networks and helping to manage complex environments such as New Zealand.

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A reliable and efficient solution

使用一个网络像HxGN SmartNet (SmartFix在东北w Zealand) saves field crews time as it precludes the setup of separate base stations, and it avoids the risk of having control fail due to a damaged or stolen base and removes the issues of radio interference. Surveyors have long used GNSS networks to save time and money as well as remove potential sources of error.

Instead of carrying around:

• GPS receivers
• batteries and cables
• two radios
• a tripod and a pole
• and setting up their own local base for each project,

network users simply carry a GPS or GNSS smart antenna (rover) with an internal modem or an external mobile phone and use that equipment to quickly access a network of permanent reference stations. The combined data from those permanent stations is used to generate RTK corrections and provide accurate positioning at much greater distances than traditional radio solutions.

“At Survey Global we are committed to the growth of the SmartFix (SmartNet) network,” said Robinson. “It enables our customers to get reliable and repeatable NZGD2000 coordinates, despite the tectonic challenges, and for the user it makes them more efficient, allowing them to complete projects on time and on budget.”