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Terrain and raster analysis

Outcome: download a known DEM, create terrain derivatives in QGIS and use them as evidence rather than decorative relief.

This lesson uses one defined source so the steps can be repeated: New Zealand LiDAR 1m DEM from Toitū Te Whenua LINZ.

If you have not downloaded a LINZ DEM before, complete LiDAR and terrain in QGIS first. That guide shows the LDS crop/export steps and the exact QGIS menus.

1. Start with a small DEM extract​

Do not begin by processing the national 1 m raster.

  1. Open New Zealand LiDAR 1m DEM.
  2. Add it to the LDS map using Map + if needed.
  3. Zoom to the study area.
  4. Click Crop → Crop by rectangle and draw a small area around the kaupapa.
  5. Click Export.
  6. Choose GeoTIFF and EPSG:2193 where those options are offered.
  7. Download and extract the file into 01_source/elevation/.
  8. In QGIS choose Layer → Add Layer → Add Raster Layer… and add the .tif.

Right-click the layer → Properties → Information and record the source, resolution, horizontal CRS and vertical datum. The current national LINZ product uses NZTM2000 horizontally and NZVD2016 vertically.

2. Clip to the working boundary​

If you have a whenua, catchment or project-area polygon:

  1. Open Processing → Toolbox.
  2. Search Clip raster by mask layer.
  3. Open the GDAL algorithm.
  4. Set Input layer to the LINZ DEM.
  5. Set Mask layer to the study boundary.
  6. Save the output as 02_working/dem_study_area.tif.
  7. Click Run.

Use this clipped raster for the remaining steps. Keep the source download unchanged.

3. Create hillshade​

  1. Open Processing → Toolbox.
  2. Search Hillshade.
  3. Open the GDAL hillshade algorithm.
  4. Set Input layer to dem_study_area.tif.
  5. Save the output as 02_working/hillshade.tif.
  6. Run the tool.
  7. Put the result below boundaries, sites, awa and other vector layers.

If an important ridge or terrace is hard to see, run the tool again with a different azimuth and compare the result. Do not edit the DEM to make the landform more visible.

4. Create slope in degrees​

  1. Search Slope in Processing Toolbox.
  2. Open the GDAL slope algorithm.
  3. Set Input layer to dem_study_area.tif.
  4. Set the output to degrees for this lesson.
  5. Save as 02_working/slope_degrees.tif.
  6. Click Run.

Rename the layer so the unit remains obvious. slope is ambiguous; slope_degrees is not.

5. Create a simple raster condition​

For a teaching example, identify cells with slope of 10° or less.

  1. Choose Raster → Raster Calculator….
  2. In Raster bands, double-click the slope raster band so QGIS inserts its exact layer/band name.
  3. Complete an expression equivalent to:
"slope_degrees@1" <= 10
  1. Set the output file to 02_working/slope_le_10deg.tif.
  2. Click OK or Run.

The resulting raster contains the result of that condition. It does not mean those cells are automatically suitable for housing, restoration, access or any other kaupapa. The threshold must come from the actual question being investigated.

6. Inspect values rather than trusting the colours​

  1. Select Identify Features.
  2. Click several cells in the original DEM and record their elevation values.
  3. Click the corresponding cells in slope_degrees.
  4. Click the binary condition raster and confirm cells are represented as true/false values, commonly 1 and 0.

This check makes the relationship between the source raster and the derived rasters visible.

7. Save the processing evidence​

In the project's source or processing notes record:

source: New Zealand LiDAR 1m DEM
publisher: Toitū Te Whenua LINZ
dataset: https://data.linz.govt.nz/layer/121859-new-zealand-lidar-1m-dem/
horizontal CRS: EPSG:2193
vertical datum: NZVD2016
download date: YYYY-MM-DD
working clip: dem_study_area.tif
slope units: degrees
condition used: slope <= 10 degrees

If the source is an older regional DEM rather than the national product, replace the metadata above with the metadata from that dataset.

Check​

Before moving on, confirm that:

  • the original DEM is unchanged in 01_source
  • the analysis raster is a separate file in 02_working
  • slope units are stated explicitly
  • NoData areas have not been mistaken for zero elevation or zero slope
  • EPSG:2193 and the vertical datum are recorded
  • the Raster Calculator expression is written down

Next: Atlas layouts.

Related: Terrain derivatives, OpenTopography and LiDAR in QGIS.

Last reviewed: 27 August 2026

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