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Simple topographical analysis in QGIS

A practical demonstration of how to carry out some basic topographic analyses with QGIS.

Our digital elevation model is useful on its own, but we can also use it to create completely new data that can give us even more information about our sites and the landscape in which they are situated.

Slope

The first new dataset we are going to create will show us how steep or flat different parts of our area of interest is.

  • 1) On the main menu navigate to ‘Raster’ > ‘Analysis’ > ‘Slope’.
  • 2) For ‘Input layer’ select your SRTM dataset.
  • 3) For ‘Ratio of vertical units to horizontal’ type 111,000.
This is like what we did with our hillshade – the elevation values are measured in metres, but the raster uses degrees of latitude and longitude. There are roughly 111km to one degree (111,000m), so we give QGIS this information to calculate the new values accurately.
  • 4) Click the browse button next to ‘Slope [Save temporary file]’ and select ‘Save to File’.
  • 5) Navigate to your course folder.
  • 6) Make sure ‘TIF files (*.tif)’ is selected for ‘Save as type’ (you can find it at the top of the list!).
  • 7) Give your new file a name, for example ‘SRTM_Slope’ and click Save.
  • 8) Click the Run button.
Slope tool screenshot Calculating slope.
A new raster will appear in your Map Window and your ‘Layers’ panel. Black areas are flat (near 0 degrees of slope), and white areas are steep (up to 90 degrees), with the different shades of grey somewhere in between.
Slope raster screenshot A default slope raster.
It doesn’t have to be black and white – using the steps you learnt in the last step, try changing the colours to something more attractive!
Coloured slope screenshot The colours make it easier to distinguish the flattest and steepest parts of the landscape.
Slope can be useful for archaeologists for all sorts of reasons. For prospection certain site types may be more or less likely to be discovered on very steep, or very flat, terrain. It can also help us to better understand the landscape, for example, how easy it is to access sites. In terms of heritage protection, sites in very flat areas are more likely to be at risk from agricultural or urban expansion.
Can you think of any examples of sites where slope data would help us understand or protect an archaeological site better?

Aspect

As well as mapping the gradient of the slope across the landscape, we can also calculate the direction of the slope. This is known as the aspect.
  • 1) On the main menu navigate to ‘Raster’ > ‘Analysis’ > ‘Aspect’.
  • 2) For ‘Input layer’ select your SRTM dataset (not your slope raster!).
  • 3) Click the browse button next to ‘Aspect [Save temporary file]’ and select ‘Save to File’.
  • 4) Navigate to your course folder.
  • 5) Make sure ‘TIF files (*.tif)’ is selected for ‘Save as type’ (you can find it at the top of the list!).
  • 6) Give your new file a name, for example ‘SRTM_Aspect’ and click ‘Save’.
  • 7) Click the ‘Run’ button.
Calculating aspect screenshot Calculating aspect.
A new raster should now have been added to your map window and ‘Layers’ panel.
Aspect raster screenshot A default aspect raster.
The minimum value is 0 degrees, which is due north. It may sound a bit strange, but the maximum value should be close to 360 degrees, which is also due north! This is because, unlike values for variables like elevation, which are linear, compass directions are circular.
It’s very hard to make sense of the data in black and white, so let’s change the colours. We need quite a specific set of colours that offer us a lot of choice, but which also look similar at the end and the beginning – as the minimum and maximum values are both due north we want these colours to be similar.
  • 8) Right-click your aspect data in the ‘Layers’ panel and select ‘Properties’.
  • 9) Click on the ‘Symbology’ tab.
  • 10) At the top, change ‘Render type’ to ‘Singleband pseudocolour’.
  • 11) Next to ‘Color ramp’ click the small black arrow and then go to ‘Create New Color Ramp’.
  • 12) Change the colour ramp type to ‘Catalog: cpt-city’.
  • 13) Scroll down until you find ‘Full_saturation_spectrum_CCW’, it looks like a rainbow!
  • 14) Select it and click OK.
Aspect colour ramp screenshot Selecting an appropriate colour ramp.
  • 15) Click ‘Apply’ and ‘OK’. You will now have a rainbow landscape with north-facing slopes in red, east in purple, south in blue, and west in green!
Final aspect raster screenshot A colourful aspect raster.
Aspect is most useful to archaeologists studying the landscape or looking for specific sites. Some kinds of crops will grow much better on a slope facing a particular direction, so aspect would be helpful finding archaeological sites related to this cultivation. Some areas might be more attractive for settlement as they are sheltered from the prevailing winds – aspect would be helpful for locating such areas.
Can you think of any other reasons why aspect might be useful for archaeologists?

Hydrology

Digital elevation models can be used to create hydrological maps – models that show where rivers and streams are located, and how large they are. We could create our own hydrological model in QGIS, but there are also freely available datasets where the hard work has been done for you! We are going to download and load one of these instead.
HydroRIVERS website screenshot The HydroRIVERS website.
  • 2) Scroll down to the bottom of the page and find the ‘Shapefile’ version of the region relevant to your area of interest.
  • 3) Click the ‘Download’ link for this dataset.
Data download website screenshot Downloading the right data.
  • 4) Once it has downloaded, move the ‘HydroRIVERS’ zip file to your course folder.
  • 5) Extract the zip file.
Hydrorivers folder screenshot The extracted folder.
  • 6) In QGIS, on the main menu navigate to ‘Layer’ > ‘Add Layer’ > ‘Add Vector Layer’.
  • 7) Click the browse button and navigate to the HydroRIVERS directory in your course folder.
  • 8) Double-click on the second HydroRIVERS folder inside the first (if there is one). You will see six files that all share the same name but have different endings. These six files are actually one part of a single shapefile!
Adding river shapefile screenshot Adding the shapefile.
  • 9) Find the file that ends ‘.shp’, select it and click ‘Open’.
  • 10) Click ‘Add’ and then ‘Close’. You should now have a new line layer in your ‘Layers’ panel and a river network in your map window.
River data in QGIS screenshot HydroRIVERS data in QGIS.
The colours of the river data are chosen by QGIS at random – let’s choose something more suitable.
  • 11) Right-click the rivers layer in the ‘Layers’ panel and select ‘Properties’.
  • 12) Go to the ‘Symbology’ tab and select the ‘simple blue line’ option.
  • 13) Click ‘Apply’ and ‘OK’.
We now have rivers that look a bit more realistic! However, the HydroRIVERS dataset includes information about the size of each river or stream, so let’s use that in our visualisation.
Blue river screenshot Blue rivers.
  • 14) Right-click the rivers layer in the ‘Layers’ panel and select ‘Properties’.
  • 15) At the top change ‘Single Symbol’ to ‘Graduated’.
  • 16) For ‘Value’ select ‘UPLAND_SKM’.
‘UPLAND_SKM’ is short for ‘upland square kilometres’ – it refers to the surface area of the drainage basin upstream of every channel.
  • 17) Click the little black arrow next to ‘Color ramp’ and select ‘Blues’.
  • 18) Click the ‘Classify’ button near the bottom of the window.
  • 19) Click ‘Apply’ and ‘OK’.

Graudate symbology screenshot Graduated river symbology.

The rivers will now have a different colour depending on how big they are! Larger rivers, draining big areas, will be dark blue, and small streams will be white or pale blue.

Final topographic map screenshot A finished topographic map with elevation, relief and rivers.

Hydrology is hugely important in site prospection and landscape archaeology as all humans need water to survive – in the present and the past! Settlement sites will need a good water source to sustain them, and rivers and streams are one obvious option. The larger the settlement the more water it would have needed which is why such sites are so often found near sizeable rivers. Can you think of why a hydrological map may also be relevant to heritage protection?

Well done for completing Week 4! We have learnt a lot about radar and topographic data. Next, we will be looking at how we can use historic maps and imagery to learn more about archaeological sites.

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Advanced Archaeological Remote Sensing: Site Prospection, Landscape Archaeology and Heritage Protection in the Middle East and North Africa

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