Reduced level for unaffected stratum boundaries – Rule 20.10

Note: this guideline is issued by the Surveyor-General under section 7(1)(ga) of the Cadastral Survey Act 2002 about the Rules for Cadastral Survey 2010 and is not legally binding.

Rule 20.10 requires any pre-earthquake reduced level (RL) defining a stratum boundary that is not an affected boundary to be corrected to be in terms of earthquake movement and recorded on the Diagram of Survey [r 20.10(a) which refers to r 9.6.10].

The survey report must also record the basis for determining any change in RL [r 20.10(b)].

The term ‘affected boundary’ is defined in Rule 20.1 as a boundary where earthquake movement has changed it in excess of the relevant accuracy tolerances and it has not already been redefined in terms of that earthquake movement and recorded in an approved CSD.  This means a stratum boundary, where there has been uniform uplift or subsidence of the ground surface over the full horizontal extent of the stratum boundary, is not an affected boundary.  However in terms of the Act, the boundary will have moved either upwards or downwards with the ground movement and any pre-earthquake RL defining the stratum boundary will no longer be correct.  Rule 20.10 requires it to be corrected (refer to ‘Understanding the term ‘affected boundary - Rule 20.1’).

When considering the application of rule 20.10:

  • because the stratum boundary is not an affected boundary, it is not required to be defined by survey in terms of rule 20.3 and therefore the principal rules apply.  In some cases the boundary will be able to be adopted or accepted (with a corrected RL value),
  • a stratum boundary may remain unaffected even where it has moved in a vertical direction that is different to vertical movement affecting related witness marks or bench marks.
  • where a stratum boundary was defined by a RL in terms of an assumed datum, the RL will not need to be corrected if it has maintained the same vertical relationship to its related witness marks or bench marks that defined the datum.
Last Updated: 1 March 2017
Authority: Surveyor-General - Section 7(1)(ga) of the Cadastral Survey Act 2002
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