Scale of mapping

Hazards, risk and associated information should be mapped at a scale appropriate for the end-use, in this case enabling planners to make decisions about land use on or close to hazardous areas. Local authorities should map hazard information to an appropriate planning-level scale of approximately 1:10,000, instead relying on existing smaller-scale maps showing areas of unstable land (1:250,000 or 1:50,000 scale). While such maps are appropriate for regional studies, they are indicative only and do not provide adequate detail for many planning purposes which require detail to at least property-boundary level.

Scale for mapping natural hazards and risks

  • National (1:1,000,000)
  • Regional (1:100,000 to 1:500,000) – QMAP Geological Map series
  • Medium (1:25,000 to 1:50,000) – typically municipal or small metropolitan areas
  • Large (1:5,000 to 1:15,000) – typically site or property level

Examples of the different scales are provided below for Moenui, Marlborough Sounds. The examples show landslide information provided at different scales, and how these scales affect the information provided:

Figure 1. Landslide Inventory map. A snapshot of the QMAP 1:250,000 (Begg & Johnston, 2000) regional level geological map showing main features of Mahau Sound (Marlborough Sounds). The speckled yellow areas show the location of large landslides. This is a most basic landslide inventory map. Moenui is located in the inset.

Figure 1. Landslide Inventory map. A snapshot of the QMAP 1:250,000 (Begg & Johnston, 2000) regional level geological map showing main features of Mahau Sound (Marlborough Sounds). The speckled yellow areas show the location of large landslides. This is a most basic landslide inventory map. Moenui is located in the inset.

Figure 2. A snapshot of the 1:30,000 Marlborough District Council Wairau/Awatere Resource Management Plan Area, Mahau Sound (Marlborough District Council, 2003). This simplistic landslide hazard zonation map highlights existing landslides (thick red line – an inventory map) and potentially unstable areas (red hatching).

Figure 2. A snapshot of the 1:30,000 Marlborough District Council Wairau/Awatere Resource Management Plan Area, Mahau Sound (Marlborough District Council, 2003). This simplistic landslide hazard zonation map highlights existing landslides (thick red line – an inventory map) and potentially unstable areas (red hatching).

Figure 3. A snapshot of the 1:10,000 Marlborough District Council Wairau/Awatere Resource Management Plan Area, Mahau Sound, showing cadastral boundaries at Moenui (Marlborough District Council, 2003) and landslides (thick red lines) or potentially unstable areas (red hatching).

Figure 3 A snapshot of the 1:10,000 Marlborough District Council Wairau/Awatere Resource Management Plan Area, Mahau Sound, showing cadastral boundaries at Moenui (Marlborough District Council, 2003) and landslides (thick red lines) or potentially unstable areas (red hatching).

When undertaking any hazard and/or risk modelling, planners should take the opportunity to discuss their scale needs with the modeller to ensure a practical map is produced that suits both the planners and modellers.

For example, typically tsunami modellers present their inundation maps with a scale based on grid spacing (e.g. 20m), while planners require a ratio scale (e.g. 1:20,000). There are two primary issues that control the modelling outputs:

1. Having a scale that is fine enough so that the inundation maps are not pixelated when viewing and;
2. Computing restrictions, especially: the amount of data in the modelling; the computational complexity; and the run time of the model (which can take from hours to weeks for an individual model, and a probabilistic study may require running tens to hundreds of models). A process of “line smoothing” is often required when raw map data is ambiguous i.e. when no clear pattern of tsunami risk/inundation emerges from the modelling.