When analyzing terrain, understanding the slope and aspect of a landscape is crucial for a wide range of applications. Slope analysis quantifies the steepness or incline of the terrain, while aspect analysis identifies the direction a slope faces. Together, these analyses offer insights into the physical characteristics of the landscape, aiding in decision-making across various fields.
What is Slope Analysis?
Slope analysis measures the gradient or steepness of a surface, typically expressed in degrees or as a percentage. It is particularly useful in identifying areas prone to natural hazards such as landslides or avalanches. For example, steep slopes combined with certain soil types or vegetation cover can signal potential landslide zones. In urban planning, slope analysis helps determine suitable locations for construction, guiding decisions on building sites, road design, and drainage systems.
What is Aspect Analysis?
Aspect analysis, on the other hand, determines the compass direction that a slope faces, which influences environmental conditions like sunlight exposure, wind patterns, and temperature. For instance, in agricultural planning, south-facing slopes in the Northern Hemisphere generally receive more sunlight, making them ideal for crops that require more warmth. Similarly, aspect analysis can inform decisions in forestry management, solar energy site selection, and ecological research by predicting how sunlight and wind affect vegetation growth and wildlife habitats.
Slope and aspect analysis can also be combined to add additional insights. For example, avalanches may be more common on aspects facing the sun during spring.
Applications of Slope and Aspect Analysis
The insights gained from slope and aspect analysis are invaluable in numerous scenarios:
Natural Hazard Assessment: Identifying areas at risk for avalanches, landslides, or rockfalls by analyzing slope steepness and orientation.
Agricultural Planning: Optimizing crop placement based on sunlight exposure and soil moisture retention.
Urban and Infrastructure Planning: Guiding the development of roads, buildings, and drainage systems by assessing terrain suitability.
Environmental Conservation: Managing forest ecosystems, planning reforestation efforts, and predicting fire behavior based on terrain characteristics.
Outdoor Recreation: Designing hiking trails, ski slopes, and other recreational facilities that maximize safety and user experience.
Leveraging Terrain Data from the Living Atlas
ArcGIS Pro offers a seamless way to perform slope and aspect analysis by integrating terrain data from the Living Atlas, a rich resource that provides global coverage without the need for manual data downloads. This data includes high-resolution digital elevation models (DEMs) that can be directly accessed and used within ArcGIS Pro, saving time and simplifying the workflow. The Living Atlas' terrain data ensures that you always have access to up-to-date, accurate information, making your analysis fast, reliable, and easy to integrate with other datasets.
In this blog, we’ll demonstrate how to perform a slope and aspect analysis using ArcGIS Pro, with the Grampians in Victoria as a case study. By following these steps, you’ll be able to quickly apply these powerful tools to your own projects, whether you’re assessing natural hazards, planning infrastructure, or enhancing recreational areas.
Case Study: The Grampians, Victoria
Setup
First, we identify our subject site. It’s often a good idea to bookmark your site extent so any clipping is done at a consistent extent.
Topographic basemap showing the Grampians, Victoria.
Then we go to Add Data > Portal > Living Atlas and search for “Terrain”.
How to add the Terrain DEM from the Living Atlas
We add the Terrain DEM that is managed by Esri.
Note, that depending on the terrain of your study site, you may get a black screen. To remedy this, you need to enable Dynamic Range Adjustment (DRA) to see the detail.
Export DEM (optional)
In our case study, we are going to export a local copy of the data to use in our analysis. However this part is optional as the following workflows will still work if you use the Living Atlas layer directly.
Export raster to Grampians extent.
Note, it is important to keep the raster size to less than 5000 pixels width and height as that is the maximum allowable export size from the Living Atlas Terrain layer. The Clipping Geometry was set to ‘Current Display Extent’, meaning it will clip to the extent of what is in our map frame.
Terrain DEM from Living Atlas.
Next we go to the ribbon, Analysis tab and select “Raster Functions”. Scroll down to the “Surface” section where you will find a selection of useful functions.
Surface Raster Functions
Hillshade (optional)
Then we will generate a hillshade layer. This process is definitely not required, but helps us produce a good quality, engaging final map. We will leave all settings default, except ensuring we select our DEM as the input raster. This will produce a new layer.
The hillshade layer
We will change the basemap to Imagery, and apply an Overlay blend mode to the hillshade layer as shown below.
Add Imagery basemap
Overlay blend mode added to hillshade layer
The result gives a terrain boost to the imagery of the study site.
Imagery with hillshade applied with overlay blend mode.
Slope
The next part is critical. We will return to the raster function dialogue box and select Slope from the Surface section. Ensure the input is your DEM layer. Now we need to decide if we want the result in degrees or represented as a percentage. We will leave it as default (degrees) in this example.
The result will be a black to white stretch where white represents the areas with steepest slope.
We will resymbolise to something a little more intuitive; a red to green gradient.
Symbology settings of slope layer
Symbology of slope as it appears in the map frame.
Identifying areas where the slope is greater than a certain threshold
This is a great map and we could leave it there for the users to assess. However, we’ll take it to the next step and symbolise areas with a slope greater than 30 degrees.
Start by duplicating the layer (so you have the original green to red version still). Then in the symbology settings of the newly duplicated layer, change the primary symbology from ‘Stretch’, to ‘Classify’, and the Method to ‘Manual Interval’ with 2 classes. In the first class you want to make the upper value <30 with all other values in the other class. For the first class (<30), make the symbology entirely transparent, and make the other class red (to indicate DANGER).
Symbology settings of duplicated layer showing steep slopes in red
Ensure you turn off the original green to red layer, to have an imagery basemap with slopes greater than 30 degrees displaying in red.
Steep slopes shown over an imagery basemap.
As an optional extra, you can increase the transparency of the imagery layer to ensure the focus of your map is the analysis layer, while maintaining imagery to some extent.
Increasing imagery transparency.
Steep slopes displayed red with imagery at 30% transparency
And to assist with interpretation of the data, you can convert to a local scene to view in 3d (Insert Tab, New Local Scene).
So in this post, we’ve explored how to create a map displaying locations where the slope is steep. By using Living Atlas layers, raster functions and symbology settings to produce the map, it has made it very rapid to produce.
In our next blog post, we will add aspect to the analysis.