Creating a Block World Map (QGIS3)

Contributed by: Steven Kim

In the previous tutorial Basic Filtering and Styling with Expressions (QGIS3) we looked at how simple expressions can be used to filter and style data in QGIS. We expand on this by creating a block world out of a hex grid with elevation values.

Overview of the task

We will take a hex grid containing elevation values and use a mathematical function to set a height variable for the 2.5d renderer and a conditional statement to apply a color ramp.

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Other skills you will learn

  • Discovering and downloading elevation datasets.

  • Creating a hexagonal grid and extracting raster statistics for each grid cell.

  • Adding a new color ramp to QGIS.

Get the data

OpenTopography provides access to many openly licensed elevation datasets. We will extract and download a DEM for your region of interest from a global DEM and Ocean Bathymetry dataset hosted on OpenTopography.

  1. Visit the Global Bathymetry and Topography at 15 Arc Sec: SRTM15+ V2.5.5 data page on OpenTopography website. This is a 500m resolution global dataset derived from SRTM DEM and various ocean bathymetry dataset.

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  1. Scroll down to the 1. Select area of data to process section. Click on the SELECT A REGION button and draw a rectangle covering your region of interest.

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  1. Once the region is set, scroll down to the 2. Data Outputs Formats section and select GeoTiff. Leave all other options to default and scroll down to the Job Description section. Enter your email address to be notified when the download is ready. Click SUBMIT button.

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  1. The download may take a few minutes to get processed. Once ready, you will receive a link to download the selected subset. Download the file to your computer. The downloaded file is a tar compressed file that needs to be uncompressed to be opened in QGIS. On Windows, you may use an external program like 7-Zip to uncompress and extract the output_SRTM15Plus.tif file.

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  1. In QGIS, locate the output_SRTM15Plus.tif file in the Browser. Drag and drop it to the QGIS Canvas. A new layer output_SRTM15Plus will be added to the Layers panel.

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  1. Open the Processing Toolbox from Processing ‣ Toolbox. Search and locate the Vector creation ‣ Create grid algorithm. Double-click to open it.

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  1. In the Create Grid dialog, choose Hexagon as the Grid type. We will create a grid spanning the entire DEM. So for the Grid extent, click the down arrow and select Calculate form Layer ‣ output_SRTM15Plus.

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  1. Our selected region is pretty big, so we can create a 1 degree grid. Select 1 for both Horizontal spacing and Vertical spacing. If you are working in a smaller region, you may choose a smaller grid spacing. Leave other options to their default values and click the Run button to create the grid.

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  1. A new layer Grid will be added to the Layers panel. We will now extract the minimum, maximum and average elevation values in each grid polygon. Select the Grid layer and locate the Raster analysis ‣ Zonal statistics algorithm from the Processing Toolbox. Double-click to open it.

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  1. Select Grid as the Input layer and output_SRTM15Plus as the Raster layer. Enter dem_ as the Output column prefix. Next we want to configure what statistics we want to extract from the raster. Click the … button next to Statistics to calculate.

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  1. Check the options for Mean, Maximum and Minimum values and click OK. We will save the results to a file. Click the … button next to Zonal Statistics and select Save to File….

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  1. Browse to a folder on your computer and enter the file name as hex_grid_with_elevation. Make sure the file type is selected as GeoPackage files (*.gpkg). Click Save. Once the Zonal Statistics dialog is configured, click Run.

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  1. The processing may take a few minutes depending on the size of your grid. Once the calculations are complete, a new layer hex_grid_with_elevation will be added to the Layers panel. This is a vector layer having attributes dem_mean, dem_min and dem_max containing elevation statistics extracted from the DEM.

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For convenience, you may directly download a copy of the prepared grid from below:

hex_grid_with_elevation.gpkg

Data Source [OPENTOPOGRAPHY]

Procedure

  1. Locate the hex_grid_with_elevation.gpkg file in the QGIS Browser and expand it. Select the hex_grid_with_elevation layer and drag it to the canvas.

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  1. A new layer hex_grid_with_elevation will now be loaded in QGIS and you should see a hex grid.

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  1. Before continuing, we must first add a color ramp suitable for coloring DEMs. Click on Settings in the menubar and select Style Manager.

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  1. Click on the plus sign Add item at the bottom and select Catalog: cpt-city….

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  1. Scroll down the color ramps until you find wiki-1.02. There may be several with the same name, just click on one and click OK.

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  1. At the Save New Color Ramp window, enter the value wiki-1.02 as the Name.

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  1. Click Save to exit the window, then click Close to exit Style Manager. Now the wiki-1.02 color ramp is available to use in your projects going forward.

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  1. Click on Open Layer Styling Panel and switch from Single Symbol to 2.5 D. Click the ε button next to Height.

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  1. In the Expression Builder, enter the following expression. This expression uses scale_exp() function to change the range of the attribute values dem_mean from 0-10000 to 0-4 and to use an exponent of 2. The exponential scaling looks pretty in block world. The +8000 is to make the bottom of the ocean equal zero.

scale_exp(("dem_mean"+8000),0,10000,0,4,2)
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  1. Change the Angle to 135.

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  1. We will be applying our own colors to the grids. Click off Shade walls based on aspect and Shadow.

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  1. You should see the topography of region begin to take shape. Click on Open Layer Styling Panel and switch from 2.5D back to Single Symbol.

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  1. In the Symbol window, notice there are two Geometry Generators, each with a Simple Fill layer type. The first Simple Fill is the top face of each block, whereas the second Simple Fill is the side face.

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  1. Click on the first Simple Fill to change the color of the top face. Click on Data define override button for the Fill color and select Edit… on the menu.

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  1. This brings up the Expression Builder dialog for Fill color. Enter the following expression. This expression has a CASE conditional statement to use wiki-1.02 colors when value of the attribute dem_mean is greater than or equal to zero and green-blue colors when dem_mean is less than zero.

CASE
WHEN "dem_mean" >= 0 THEN ramp_color('wiki-1.02',scale_linear("dem_min",0,1500,0.4,1))
ELSE ramp_color('GnBu',scale_linear("dem_mean",-6000,0,1,0))
END
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  1. The hex grid is now rendered in colors determined by our expression. Click on Data define override button for the Fill color and select Copy on the menu.

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  1. Click on Data define override button for the Stroke color and select Paste on the menu.

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  1. You should see the colors take effect and our map is finished. Feel free to experiment with the expressions in this tutorial, like changing parameters in the height function or adding and trying new color ramps.

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If you want to give feedback or share your experience with this tutorial, please comment below. (requires GitHub account)