Clear and complete referencing of coordinates is important because ongoing movement can mean that they change several times after an earthquake.
It is particularly important after an earthquake that coordinates are provided in terms of the appropriate national datum. These are:
- New Zealand Geodetic Datum 2000 (NZGD2000) for horizontal coordinates and ellipsoidal heights. This will ensure that any deformation models can be applied when calculating coordinates.
- New Zealand Vertical Datum 2016 (NZVD2016) for normal-orthometric heights. This will ensure that heights can be efficiently established using GNSS and are in a consistent datum over the extent of the earthquake-affected area.
Referencing post-earthquake coordinates
After a large earthquake, it is normal to have ongoing post-seismic movement and/or movement due to significant aftershocks. Survey marks may physically move between two surveys and therefore have different coordinates.
Coordinate epoch (date)
It is important to record the date for post-earthquake coordinates (known as the epoch). Coordinate epochs may be recorded as decimal years or dates in year-month-day format. For example 6 December 2016 could be recorded as 2016.93 or 20161206. Some software, including PositioNZ-PP, uses the term “observation epoch”. Coordinate epoch and observation epoch have the same meaning when referring to a set of coordinates.
Coordinate epoch when using PositioNZ-PP
When new post-earthquake coordinates are sourced by processing data using PositioNZ-PP, the coordinate epoch is the date than the data was collected. This is because PositioNZ-PP reference station coordinates are updated daily using a specific model of the movement at each station.
Coordinate epoch when using local post-earthquake geodetic control
When new coordinates are sourced by connecting to local post-earthquake geodetic control marks, the coordinate epoch for the new coordinates will be the same as for that local control. This is normally different to the date that the new data is collected.
For example, a new survey carried out on 20 December 2016 connects to some nearby geodetic marks that have post-earthquake coordinates published on the LINZ Data Service with a coordinate epoch of 20161129. The coordinate epoch of the new post-earthquake coordinates is also 20161129.
This assumes that all marks have moved consistently over the area of the survey since the local post-earthquake control was surveyed. If this assumption is not valid (eg where there has been a significant aftershock after the coordinate epoch for the local geodetic control), then additional data should be collected as part of the survey and processed using PositioNZ-PP, rather than using the local post-earthquake geodetic control.
NZGD2000 coordinate epoch vs NZGD2000 reference epoch
Note that the reference epoch (also known as the nominal epoch) for NZGD2000 refers to the epoch used for calculations using the secular (long-term continuous) deformation model. This is always epoch 2000.0 or 20000101. It should not be confused with the coordinate epoch of a set of coordinates.
Other information to record
The datum, datum version, projection information and the source of coordinates must also be recorded. Including the datum version for NZGD2000 ensures that coordinates can be updated if a new version of the deformation model is published. The deformation model may be updated several times over the course of the recovery.
Coordinate referencing example
The following table shows the type of information that should be included to reference post-earthquake coordinates.
REF KA KAIKOURA PENINSULA
Ellipsoidal hgt: 120.978
Circuit North: 805717.168
Circuit East: 414663.672
Ellipsoidal hgt: 2.243
PositioNZ-PP (processed 2/12/2016)
WITH at epoch 20161123 sourced from LINZ Data Service data.linz.govt.nz/layer/3527