Marigold 2.8.0 introduces new hillshade and spatially variable alpha settings to fine-tune your layer visualization.
Hillshade visualizations can be a powerful tool to transform and interpret your data. While most commonly used with topography, it can also provide additional context and subtlety to geophysical datasets and even the results of mineral processing maps. Follow the tutorial below to familiarize yourself with the workflow for these new tools and settings.
Add a Raster Layer
To begin, we will start with a simple example using the Copernicus 30 meter Digital Elevation Model (DEM), available to all users through the Raster Catalog. However, hillshades are not restricted to DEM data and can be used to enhance many raster layer types.
Click the + symbol next to Raster layers on the lefthand panel, and select Catalog layer. Locate Copernicus Digital Elevation Model (DEM) GLO-30 under the topography section, select it, an click Add selected layers.
The resulting view will show a depiction of the elevation model with the default gist_earth colormap depicting variation in altitude. Modify the appearance of the layer as you prefer by changing the contrast stretch through either the settings menu or the magic wand button to automatically apply a 2% contrast stretch.
The Copernicus DEM over Mt. Rainier in Washington State shows a broad range in altitude but lacks subtle topographic details without the application of a hillshade layer.
Generate Hillshade
To generate a hillshade layer, expand Raster management in the processing toolbox, and select Generate Hillshade. In the resulting dialog, select your input Product (Copernicus DEM) and the input band for the hillshade calculations, which in this example is Altitude.
A greyscale preview layer will be generated and update as you modify the additional hillshade settings.
Sun Angle
0-90 degrees. This controls the height angle of the illumination source, with 90 degrees representing directly overhead. A lower sun angle will produce more shadows and potentially hide terrain features.
Sun Azimuth
0-360 degrees. This controls the cardinal directional of the illumination source, with 0 representing north, 90 east, 180 south, and 270 west. Modifying the sun azimuth can dramatically change the appearance of a hillshade based on the features present in the data.
Z factor
The Z factor provides an option for vertical exaggeration of the data, where higher values will produce more contrast and shadows between subtle features.
You will also find an opacity slider setting which allows you to temporarily control the transparency of the hillshade layer to help you fine-tune your preferred parameters. This setting can be adjusted later in the final hillshade layer's settings.
Once you are happy with the appearance of your result, optionally rename your output layer and click Create Hillshade.
Apply Hillshade
Hillshade layers that you generate can be applied to any other raster layer within the settings menu of that layer. In this example we will apply the Copernicus hillshade we created to the Copernicus DEM itself, however you can also apply it to the results of your spectral mapping, geophysics, or any other layer you have available.
To apply the layer, enter the settings of the Copernicus DEM and scroll to the bottom where you will find a new button for Advanced Settings. Check the Apply hillshade box and select your newly generated hillshade product.
Blending Modes
Four pixel blending modes are available to help you tune the visualization of your layer. This includes Multiply, Hard Light, Soft Light, and Linear Burn. Each of these modes represents a different algorithm applied to the RGB values of your input layer with the 0-1 shadow-illumination values of the applied hillshade. For more information on blending modes, there are many resources available on the web that describe these techniques.
We recommend a trial and error approach to get these settings just right for the layers you are working to highlight the features you are interested in and will depend on the the input layer's colormap, range of values, and opacity settings. Play around! You will also find slider settings for Brightness (-1 to +1), and Gamma (default 1) to adjust the final appearance.
Spatially Varying Alpha
Another option within the Advanced Settings menu is the ability to control the alpha, or the transparency of the layer based on data values from a raster layer. This tool can be applied in two ways, 1) to the Entire layer, or 2) only to the Hillshade layer that has been applied in the above settings.
Additional Examples
Generating and applying hillshade layers can be much more powerful than just working with topographic data. Below you will find a handful of examples highlighting some examples of hillshade application that may inspire your next Marigold workflow.
Alteration Map of Goldfield, Nevada
In the example below, a hillshade has been generated form the Copernicus DEM and we have applied it to the Alteration mineral index calculated from the Fused Bare Earth Composite over the Goldfield, Nevada area. In this example, we can see how the topographic hillshade adds important context to the alteration patterns in this area. In the hillshaded region, we see that some strong alteration regions for resistive ridges, while others are more recessive. Erosional features also become much more apparent when the topography hillshade is applied.
Magnetic Structures in Kalgoorlie Gold district
Applying hillshade settings to geophysical datasets like magnetic surveys can provide powerful visuals that can help with interpretation. In the example below, we have generated hillshade from the regional RTP magnetic data available from Geoscience Australia over an AOI in the Kalgoorlie gold district of Western Australia.
Next, the magnetic hillshade was applied to the RGB True Color Composite (TCC) of Sentinel-2 Bare Earth Composite imagery over the AOI. The visual provides a unique juxtaposition of the various magnetic features resulting from the complex structural regime in this area superimposed onto the satellite imagery.
Airborne Magnetic and Radiometric Surveys - Republic, Washington
Marigold has recently added a suite of new airborne surveys from the USGS's EarthMRI initiative to its catalog. Currently this includes magnetic and radiometric surveys in Nevada, New Mexico, and Washington state, with more to come as the projects progress and the data becomes available. Below we looks at a few ways to combine and enhance these datasets using the new hillshade capabilities, using the Republic, Washington survey as an example.
First, we have applied a topographic hillshade from the Copernicus DEM to the Sentinel-2 BEC. Next, we have added the airborne magnetic survey data (RTP) and generated a hillshade based on the RTP and applied it back to the magnetic data. This enhances the magnetic highs and lows. Finally, we have added the radiometric survey data and visualized it using a K-Th-U RGB ternary view. To combine these datasets, we've applied to hillshade data from the magnetics to the radiometric data to outline the magnetic high features in the area that are associated with local Cretaceous plutons.
The combinations and possibilities with the new hillshade and alpha tools in Marigold are limitless. We hope you enjoy these new features and look forward to seeing your results!