This document provides guidance on how such surveys can be carried out using the PositioNZ-PP service and standard GNSS survey techniques.
LINZ can provide advice on planning post-earthquake geodetic surveys and may be able to assist with data processing. LINZ can also make the results of post-earthquake geodetic surveys available via the LINZ Data Service so that others involved in the recovery can reference a consistent set of coordinates.
General approach to re-establishing geodetic control
Re-establishing control can be done in two steps (which may take place concurrently):
- Survey a control framework, calculating coordinates using the PositioNZ-PP service. These marks should be visually stable, safe to occupy, have a well-defined vertical and horizontal reference point (eg stainless steel pin) and have good sky visibility. The survey is carried out using static GNSS techniques with at least two independent occupations of between 2 and 24 hours.
- Use the post-earthquake coordinates of the control framework marks and densify the geodetic network to the required level by surveying additional marks. This densification is typically carried out using RTK or fast-static GNSS techniques with at least two independent occupations of between 3 and 20 minutes.
Further information on each of these steps is provided below.
Step 1 - Control framework survey with PositioNZ-PP
PositioNZ-PP is a service which enables users to submit static GNSS data of at least one hour duration. A separate RINEX file is required for each occupation at each mark. Each submitted RINEX file is then processed and coordinated against the current-day position of the three closest PositioNZ stations. The deformation model is applied to remove the effects of continuous, long-term deformation to give a post-earthquake NZGD2000 coordinate that reflects movements due to the earthquake.
Full details of how to use PositioNZ-PP and where to submit data are located here.
Step 2 - Control densification survey with GNSS or levelling
Control densification is typically carried out using RTK or fast-static GNSS methods. This provides sufficient accuracy for the majority of requirements in the immediate aftermath of an earthquake.
Sometimes, however, there is a requirement for height accuracy that is greater than can be provided by GNSS. In these situations, GNSS and PositioNZ-PP are still used to provide a control framework. For example, if Order 1V heights are required, then two control framework marks at least 4km apart can each be surveyed with two 16-hour independent occupations. This will provide heights accurate to 10mm. Control densification is then carried out using precise levelling techniques.
A local authority requires height control in an earthquake-affected town (area approximately 4km by 4km) to support repairs to the stormwater and wastewater networks. Due to the earthquake, there is no reliable geodetic control in the vicinity. A mark is required every 500m and each mark must have a height accurate to 30mm. Existing geodetic marks are to be selected where possible so that the difference between pre and post-earthquake heights can be calculated.
The surveyor has access to a set of RTK GNSS equipment (base and rover). Based on the equipment specifications and the surveyor’s experience, it is capable of measuring heights relative to the base station over a distance of a few km with an accuracy of 20mm (95% confidence level), if two 3-minute independent occupations are made at each mark. A total of 20 marks will be required to meet the density requirement. These will be surveyed from two base stations set up on opposite sides of the town. These two base stations will be the control framework marks and the remaining 18 occupied by the rover will be the densification marks.
The total height accuracy required for the job (30mm) is a combination of the accuracy of the rover measurements from the base station (20mm) and the accuracy of the base station height (which will be calculated using PositioNZ-PP). To add (or subtract) accuracy values, the root-sum-squared approach should be used (based on standard error propagation principles). That is, square the accuracy values, add or subtract them as required, then take the square root of the result. Doing this, the required height accuracy of the base station(s) is 22mm.
From Table 1, a height accuracy of 20mm at the base stations can be achieved if two 4-hour occupations of each base station mark are made.
The survey is carried out as follows:
- Morning of Day 1: Set up the base station over a proposed framework mark on one side of the town and log data for 4 hours while occupying nine of the densification marks and the second framework mark with the rover
- Afternoon of Day 1: Re-set the base station over the same mark and log data for another 4 hours while occupying the other nine marks and the second framework mark with the rover
- Morning of Day 2: Set up the base station over the second framework mark on the other side of town and log data for 4 hours while occupying nine of the densification marks and the first framework mark with the rover
- Afternoon of Day 2: Re-set the base station over the same mark and log data for another 4 hours while occupying the other nine marks and the first framework mark with the rover
- Process the logged data at the base station through PositioNZ-PP to calculate the post-earthquake coordinates for the framework marks
- Recompute the rover data in terms of the post-earthquake base station NZGD2000 coordinates sourced from PositioNZ-PP
- Use the NZGeoid2016 model to calculate NZVD2016 heights at each mark
- Ensure epoch, datum, datum version and projection metadata is recorded with the coordinates