Estimation of Groundwater Bore Elevation Using a Digital Elevation Model

TR 2008/17
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Report: TR 2008/17
Author: Bevan Jenkins

Abstract

The elevation of groundwater bores within the Waikato region was required by Environment Waikato (EW) for entry into groundwater analysis software (Hydrogeo Analyst).  As of October 2007, there were 11600 groundwater bores in EW's database (LOCATED).  Of these only 1734 had an elevation assigned to their location, leaving 9866 bores with an unknown elevation. 

A Digital Elevation Model (DEM) was used to estimate the elevation for all bore locations.  These estimates were then verified against the 'known' elevations that existed in LOCATED.  A very strong relationship was found, with 96.8% or 1679 of 1734 estimated elevations within 20 metres of the measured elevations.  However, there existed individual cases where the DEM estimate was markedly different to the LOCATED value.

Further investigation was carried out on sites that had an absolute difference between the two methods of greater than 20 metres.  This was achieved by use of a Geographic Information System (GIS), which was used to overlay the sites in question and the relevant NZMS 260 topographic map. In all cases it was found that the topographic maps agreed with the DEM values rather than the LOCATED elevations.  Thus, an additional advantage of the methodology is the illustration of errors that exist in the current database.

The comparison analysis was repeated with the estimated values from the topographic maps being substituted for the outliers (absolute residual greater than 20 metres).  The replacement of these outliers led to an even stronger relationship, with 99.9% or 1733 of 1734 estimated elevations being within 20 metres of the measured elevations.

The methodology presented has application to be used for all locations that require an elevation within the Waikato region, as long as the uncertainties associated with the produced values are acceptable.  The methodology is not intended to replace more accurate methods such as direct surveying or Global Positioning System (GPS) when the situation and resources dictate.

Estimation of Groundwater Bore Elevation Using a Digital Elevation Model [PDF, 81 KB]