Memorandum
To:
Brett McKay, Manager District Plan Team, Wellington City Council
CC:
Glenn May, Planner, District Plan Team, Wellington City Council
From: Shona McCahon, Boffa Miskel Limited
Date: 04/12/2003
Re:
Mapping Overlay Methodology, Wellington’s Ridgetops and Hilltops
This memo sets out the methodology that was adopted to map the ridgetop and hilltop areas
identified in the 2001 Boffa Miskell study,
Wellington’s Ridgetops and Hil tops, the Natural
and Amenity Values.
Background
The overall objective of the 2001 study was:
To identify the intrinsic natural values and the amenity values of Wellington’s
ridgetops and hil tops in order to enable those ridgetops and hilltops to be
appropriately protected, maintained and /or enhanced.
The study was confined to ‘undeveloped’ ridgetops and hilltops because that was where
competing demands for ridgetop and hilltop land was an issue. The important undeveloped
ridgetops and hilltops of the district were identified and described in the published report but
were not mapped in detail, as detailed mapping was to be carried out as a separate process.
Four types of value were identified as the basis for the assessment, based upon issues raised in
public submissions:
• natural values;
• visual values;
• heritage values;
• recreation values.
Mapping Overlay Method
The mapping was carried out according to a range of exclusions or inclusions aimed at
ensuring the areas of particular ridgetop / hilltop value and/or risk of potential adverse effects
on ridgetop / hilltop values were included and that the continuity of ridgetops was recognised.
1.
Identify important ridgetops and hilltops.
This step was carried out in the 2001 study and mapped indicatively (by broad dotted
lines along the apex of the identified ridgetops and hilltops). The 2001 study described
the natural and amenity values associated with each identified ridgetop and hilltop.
2. Exclude developed residential areas
The study was confined to ‘undeveloped’ ridgetops and hilltops and this was defined as
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all ridgetop and hilltop areas that were not dominated by residential development.
1
3. Include relatively high visibility areas
Higher visibility areas were included as these areas are where the effects of changed
activities are potentially most noticeable.
Intervisiblity mapping had been carried out as part of the 2001 study, at a district-wide
scale and also at the scale of defined communities of interest. Areas that were visible
from 15% or more of the district and from 30% or more within each community of
interest were included within the mapped area. At the district-wide scale visibility of even
the most visible landform is comparatively low (Mt Kaukau at just over 30% visibility)
because of the hilly terrain; hence the lower threshold. Within the communities of interest
the surrounding landforms tend to be far more visible, so the visibility threshold is
correspondingly higher.
4. Include steep and moderately steep slopes
Areas of steeper slopes on the summits or upper slopes of ridges and hills were included
because these are areas where particular care in the siting and design of built features
(including roads) and the associated earthworks is needed.
District-wide slope analysis maps were also prepared during the 2001 study, and these
maps were used as the basis for mapping the steeper areas. Areas showing slopes of 210
(moderately steep) or more were included in the mapping overlay.
5. Include areas or places of specific ridgetop / hilltop value
Various places or areas were identified in the 2001 study as having particular value.
These included geological features, vegetation of ecological significance, historic features
or recreation routes and destinations.
6. Include saddles
Saddles tend to attract road and residential development because of their lower elevation
but development can potentially affect the continuity of values associated with the ridge
as a whole, such as continuity of open space for a recreational route.
7.
Extrapolate the mapping line to link identified areas
Continuity of landform is also an important factor in ridgetop and hilltop values, as
evidenced in the public submissions on the draft district plan provisions, where
undeveloped ridges and hills were clearly valued because they provided a “natural
backdrop to Wellington”.
2 In some instances, continuity is also important for other
linkages such as recreation routes. Therefore, the various areas identified in steps 3 – 6
above were linked where necessary to be consistent with the overall importance of the
ridge or hil described in the 2001 study. The mapping line was generally extrapolated by
interpreting contours between identified areas to draw a line sympathetic to the landform.
8.
Exclude contextual areas that are likely to be managed separately
In identifying the important ridgetop and hilltop areas, adjacent areas were also
recognized as providing important context or being an intrinsic part of the entire
landform. For instance, the coastal escarpments around a headland or the fault escarpment
below a hil top. Where it is intended that these adjacent features are to be provided for
separately in the district plan (such as coastal escarpments), or would be better provided
for separately, these areas have been excluded.
1 P.13, 2001 study
2 See P.11, 2001 study
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Drawing the line
A particular challenge in a mapping exercise such as this is to decide the location of the
boundary line because, although the values ascribed to the ridgetop and hil top areas can be
readily described, they are not generally associated with definite ‘lines on the ground’. The
overall intention, in applying these steps, was to achieve a mapping line that adequately
covered the areas of identified value and was responsive to the underlying landform: - that is,
not a formulaic line but one that was site specific.
The 2001 study defined ‘ridgetop’ as “the upper slopes of a ridge, including the summit” and
illustrated the idea of a ridge or hill being composed of lower, middle and upper slopes in
roughly equal proportions.
3 However, it was intended as an indicative definition; if
interpreted strictly, it could result in a line being drawn to take in the upper 1/3 of a slope, but
this would be both arbitrary and present difficulty in determining from where to take the
measurements. In reality, the mapping line has rarely followed the 1/3 proportion – at times
extending further down the landform (where the middle and/or lower slopes provide
important context) or at times extending less than 1/3 down where, for instance a change of
slope had indicated a landform boundary. Such site-specific cues were used as much as
possible as the basis for drawing the line. The following representative cross sections
illustrate some typical positions of the boundary line on landforms.
3 See P. 16 , 2001 study
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