LAB 03: PROJECT! MAPPING THROUGH TRANSECTS > TUTORIAL

Transects and sectional drawings unveil the vertical dynamics of terrains. They are fundamental tools of geospatial representation that serve as both analytical and design devices. This exercise introduces a GIS-based transect drawing technique using global elevation data.

Premise and Objectives

Landscapes are three-dimensional. While planar views (maps) provide horizontal information, sections and transects reveal vertical relationships and dimensions, showcasing topographic changes, sub-surface features, and the relationships between different layers of a landscape. After completing this exercise, you will have:

• Familiarized yourself with the elevation (DEM) data
• Learned how to draw transect lines across a watershed/region of interest
• Extracted elevation data along transect lines
• Saved and exported elevation profiles for further processing in Illustrator
• Compiled and styled a layered drawing of multiple successive transects

You will then post your work-in-progress to Are.na.

Important Note: A free NASA Earthdata Login account is required to import DEM data to QGIS. You can register your free account here.

Prep

If you’re working in the SSDLAB virtual environment, copy the data repository for this exercise from the shared drive (G:) to your working location (U:). We will use the following datasets:

• NASA Shuttle Radar Topography Mission (1- or 3- arc seconds digital elevation model)
• USGS Watershed Boundary Dataset (WBD) (available in Lab_02/01_Data/Additional Data folder)

Set-Up

One of the Q’s most remarkable features is its extensible architecture, which allows developers and users to create and incorporate plugins. These plugins add additional functionalities that can be seamlessly integrated into QGIS, expanding its capabilities to cater to specific geospatial needs and operations. In this workflow, we will rely on the SRTM-Downloader and the QuickMapServices plugins.


Launch QGIS. From the top menu, select the Plugins option and then choose Manage and Install Plugins... In the QGIS Plugin Manager:

• Click on the All tab
• Type "SRTM-Downloader" in the search bar (the SRTM-Downloader plugin should appear in the search results)
• Select the SRTM-Downloader plugin from the list and click the Install plugin button
• Once installed, the plugin will be listed under the Installed tab

Close the Plugin Manager. You should now see an SRTM-Downloader icon on your QGIS toolbar or under the Plugins menu.

Repeat the plugin installation process for the QuickMapServices, which we will use to add basemaps to our project.


Next, set the project Coordinate Reference System to WGS_1984_Web_Mercator (ESRI: 102113) by double-clicking on the CSR code in the status bar, or by navigating to Project > Properties and selecting the Coordinate Reference System tab. There are two reasons for that choice:

1. While drawing section lines, we want to work in a projected CSR that uses meters as a linear unit.
2. We will download basemap and DEM data from web servers directly into Q and most global web maps use Web Mercator as a native CSR.

Click through Web > QuickMapServices > OSM > OSM Standard and navigate to your area of interest. In this tutorial, we are zooming into the Upper Mississippi/Lower Illinois (WBD07) watershed.

Add one of Watershed Boundary Dataset Level 8 feature classes (WBD_07…/WBDHU8.shp) to your map project. Select one or more contiguous sub-basins and save it as a new layer.




Importing and Downloading DEM Data

A digital elevation model (DEM) is a digital representation of the Earth’s topography. DEMs can be either raster or vector data, although raster DEMs are more common. In a raster DEM, the numerical value stored in each grid cell represents the average elevation of the terrain for that specific location. 

As the name of the plug-in we just installed suggests, we will be using SRTM (Shuttle Radar Topography Mission) data, which provides elevation values for a significant portion of the Earth's land surface. Although compiled in 2000, it remains one of the most popular global elevation datasets due to its wide coverage and consistent quality.

Zoom into the sub-basin you just extracted by right clicking on it selecting Zoom to Layer(s).

• Click on the SRTM-Downloader button on the toolbar or click through Plugins > SRTM-Downloader > SRTM Downloader (a new dialogue box will pop up)
• Click on the Set canvas extent
• Make sure that Load Images to QGIS is checked
• Click Download

The plugin will fetch the SRTM data for your defined area and temporarily download it to your system.


Important note: First time you try to execute the above commands, you will be prompted to enter your Earthdata credentials. Enter your login info, and check Save Credentials.


Two new temporary layers should appear in your Layers Panel:

• srtm_images: a collection of individual SRTM tiles that intersect with your region of interest (not visible in the map view)
• A mosaic of the same SRTM images stitched together, named by its most lower-left and upper-right tile coordinates (e.g. N39W092_N40W091)

Right-click on the stitched mosaic and select Export > Save As… and save the raster layer to your preferred working location. Remove or toggle off the temporary SRTM layers.





Introduction to Elevation Profile Tool

Once we've successfully imported our DEM data into QGIS, the next pivotal step in this exercise is to extract precise elevation values along transect lines. For this task, we'll employ the "Elevation Profile" tool in QGIS, which generates elevation profiles along a polyline, giving us a detailed cross-sectional view of the terrain's elevation changes over distance.

Click through View > Elevation Profile. The Elevation Profile Panel will appear docked on the bottom of your QGIS window. On the left-hand side of the panel, you should see the list of data layers. If not, click on the Show Layer Tree Tool (the first button on the Elevation Profile Toolbar).

It’s likely that you won’t find the DEM raster that you just exported on the list. That’s because Q doesn’t know which of the layers represent the elevation data and, by default, it assumes that your vector layers are elevation isocurves.

To add the DEM raster to the Layer Tree and enable proper elevation extraction:

• Open the DEM layer’s properties
• Navigate to the Elevation tab
• Check the Represents Elevation Surface and click OK
• Your DEM layer should now appear in the Layer Tree

We are finally able to draw actual elevation profiles!

The Elevation Profile tool allows you to draw lines or polylines interactively to cut sections. Make sure your DEM layer is selected and checked in the Layer Tree. Go ahead and click on the Capture Curve button on the Elevation Profile toolbar (second from the left):

• Your mouse cursor icon should change to a target.
• Click somewhere over your area of interest in the main map view to start adding vertices
• Right-click to finalize a (poly)line
• The profile view will appear rendered in the panel

You can interactively inspect elevation values by hovering over the profile line. In our case, both vertical and horizontal values represented on the graph are meters.

You will notice that the symbology properties assigned to the elevation profile are arbitrary. To change the Line style, open the DEM layer’s properties and navigate to the Elevation tab again. Under Profile Chart Appearance, set the desired Line style properties:





Cutting Sections With a Grid

In addition to generating your own line or polyline to cut a section, the Elevation Profile tool allows you to select existing linear features. To do so, simply select the Capture Curve from Feature tool immediately adjacent to the Capture Curve tool you just used. Why might we choose existing geometry rather than drawing a custom line? Drawing a cutting line manually may work well when you need only a single, one-off section, or when the line you want to cut along doesn’t follow any existing geometry. In many cases, however, you’ll want to be more systematic about selecting cutting lines, particularly when you need to cut multiple sections along a given axis or be precise about various kinds of spatial relationships (e.g., distance between sections, cutting angle, length of cutting line, etc). Recall the two key examples presented in class:

• Cross-sections that cut along a linear feature, e.g., the center line of a Great Lake, the St. Lawrence Seaway, or other engineered waterway
• Serial sections cut across a linear feature, e.g., the serial section by Mathur & da Cunha featured in Cartographic Grounds, Soak, and Mississippi Floods

In the first case, we can simply select the geometry from a linear water feature as our cutting plane, giving us a cross-section of the elevation profile that runs the whole length of that feature. This kind of feature will almost always be a polyline composed of multiple line segments. To experiment with this approach, try selecting a river centerline or other linear feature that traverses your DEM.

In the second case, we need to generate a regular set of linear features that can serve as cutting lines. This is the technique we’ll be exploring for the remainder of this tutorial. Like most of our workflows in QGIS, there are several ways to achieve this. You can, for instance, take some existing linear feature and create a series of parallel lines at regular intervals using the Array of offset (parallel) lines tool from the Processing Toolbox. You could also replicate the process used by canal engineers using function or expression-based Vector geometry tools, taking sections at either the vertices of the centerline (i.e., wherever the canal changes direction) or at regular intervals of distance where every section is perpendicular to the centerline of the waterway.

Here, we’ll only cover the one we consider to be most flexible for a variety of end-uses: generating a grid of horizontal and vertical lines over our specified Area of Interest. There’s a dedicated tool in the Processing Toolbox for this operation, which you can locate by searching (control-k/command-k) or navigating to the Processing Toolbox > Vector creation > Create grid. Go ahead and open it up.

You should see a dialogue box that looks like this:


Note the description on the right that details exactly what the algorithm does: “This algorithm creates a vector layer with a grid covering a given extent...” and so on. There are a few variables for which we need to carefully choose values.

First, what kind of grid are we creating? For a section, you need a cutting line, so select line from the Grid type menu.

Second, what should your extent be? In this case, let’s choose the sub-basin you’ve already selected as your Area of Interest. Click the button next to the Grid Extent entry dialogue and select Calculate from Layer. The grid will cover the full X-Y extents of the sub-basin geometry– in other words, it will create a grid in an imaginary rectangle that represents the greatest dimension in longitude and latitude of the sub-basin. It will not be clipped by the outlines of the sub-basin; if you want to do this, it will involve an extra step (for clipping functions, see previous labs).

Next, let’s move on to the spacing. By default, your units should be set to meters– these are the units for Web Mercator. You can double check that you’re in Web Mercator in the Grid CRS selection dialogue. If you’re in a Geographic Coordinate System like WGS84 on the other hand, these units will be in degrees. This makes a huge difference! The minimal unit in a Geographic Coordinate System is the single degree– about 111,139 meters. Depending on the DEM you’re using, 1 degree in each direction might be the bounding box of the entire dataset, which would create a grid that doesn’t cross the DEM at all. Instead, we’re using a metric projection so that we can scale our grid appropriately. Meters, however, are too small a unit for this scale. We can work in kilometers instead. Try setting the Horizontal spacing and Vertical spacing at 10 kilometers each.

You can decide whether to save this file imediately or to create a temporary file only by clicking the button next to the Grid dialogue entry box. You can also save your grid later by using the typical right-click > Export>...  option.

Finally, hit Run. The algorithm should generate a 10km-spaced set of grid lines over the area of interest. It should look something like this:


What now? These grid lines can be used for cutting transects (across) or longitudinal (lengthwise) sections, so go ahead and navigate back to your docked Elevation Profile window and try selecting different line segments with the Capture Curve from Feature tool (third button from the left). As you do so, consider turning your OSM layer back on– what do you notice about information displayed on the OSM layer and changes in elevation? Are there particular sections that are more interesting than others?


Exporting

The last step in QGIS is to export the Elevation Profiles as vector geometries that can be further manipulated in Adobe Illustrator. The most straightforward way to do this is with the Export to PDF function (featuring the same icon as in Print Layout). There are also ways to export the geometry for use in 3D modeling and architectural drawing software, but we won’t cover those here.

After clicking the Export to PDF button, you’ll be prompted to specify a location to save your file– choose a convenient location. Next, you’ll need to make a few compositional decisions in the PDF Export Options dialogue.

• In Page Size > Size, choose a custom size, 1920 x 300 – since this will remain a digital drawing, it’s best to work with pixel-based sizes rather than standard print page sizes (e.g., A4, Letter, etc)
• Under Chart Ranges, let’s use consistent values for every section we export so the vertical scaling remains the same. If we look at the range of elevation values from the DEM (Layer Properties > Information), we find a minimum value of 120, and a maximum of 293. Input these as our min and max chart range values.
• You may want to adjust values for the other options– Distance Axis Settings, and Elevation Axis Settings – but the default settings are fine too.

Go ahead and save your file. Open it to double check it looks correct. If it looks like part of your section is cut off, don’t worry! The linework is all still there, even if it runs off the page. When you open the file in Illustrator, you can adjust the page size (artboard) or resize the artwork.

On your own: Grab and export 5-10 successive elevation profile using the grid lines.

You now have sections, but we need to export some other features from the map in a new print layout (use a 1920x1920 layout to allow enough space for multiple sections). These additional features will fulfill the two primary purposes:

• aligning artwork in Illustrator
• creating geographic context for the section

In the first case, we need to know exactly where our sections have been cut. Fortunately we already created a grid for just this purpose!

In the second case, you’ll need to make your own compositional decisions. What additional spatial information will make your section meaningful? What other vector geometries and raster images will add appropriate context to your drawing? For the purposes of this lab, let’s use what we’ve already been working on here and in previous labs: our watershed boundary, lakes and rivers, and our DEM. If you want to get adventurous, try exploring land use data from the National Land Cover Database in the Additional Data folder. Depending on the scale you’re working on, you could also explore data from the Global Human Settlement Layer dataset.

After deciding what features to include in your map, export your raster and vector layers separately. If you plan to use more than one raster, export each layer separately. For your vector layers, you can export everything together– just make sure you maintain your layer structure as usual. You don’t need to worry too much about styling the linework precisely– we’ll save that for Illustrator.


Now let’s move on to Illustrator.

Compiling and Arranging Sections in Illustrator

Open Illustrator and create a new file with a 1920x1920 artboard (recall that “artboard” is just Illustrator’s term for your page, canvas, or working area) and give it an appropriate title. Now let’s bring in our artwork.

First, let’s load up our raster “basemap.” You can open your image directly in Illustrator and simply copy and paste it in, but we’re going to use a “cleaner” technique called “Placing.” Placing effectively links an external file into your Illustrator file not unlike data in QGIS. The advantage here is that we can make modifications to the external file in a program designed for raster editing (for instance, Photoshop) and it will update in Illustrator (you may need to relink the file on occasion if it’s not updating by itself). You can Place several different file types– standard image formats like .png, Photoshop files (.psd), PDFs, etc.

To place your basemap image, select File > Place (or use the keyboard shortcut, Command or Control-Shift-P). After you select your file, your cursor will turn into a small arrow with a thumbnail of the image next to it. It should look like this:





Navigate to the upper left corner of the artboard and click to place the image. Now we need to resize it to fill the page. We can do this four ways:

• Click-drag: Click, hold, and drag any control point on the corners or sides of the object’s bounding box. Hold down shift to maintain the proper proportions, then move the image manually.
• Properties > Transform: Locate the “Properties” panel and open it. (If you can’t find it, select Window > Properties to enable it.) You can use the Transform options to input the exact size and position of the object– in this case, 1920x1920. Keep the little chain-link icon enabled to maintain proportions– if you input 1920 for either height or width, it will automatically update the other to match (given the original proportions of the image). You can also choose where the “anchor” for rescaling the object should be by selecting one of the small squares in the grid next to the size options. Choose the upper left square, and make sure the X and Y positions are both set to 0.
• Scale tool: Type “S” to activate the Scale tool. Choose an anchor point– upper left corner is always a good choice– and move the object so that your anchor point is in exactly the right position. Next, click the anchor point– this will anchor your object as you scale. Finally, find another control point and click-drag to the desired size.
• Object > Transform > Scale: If you want to rescale something based on percentage increase or decrease, use this tool. It’s less useful here since our target is a specific size, not a specific percentage increase.

Now let’s bring in our vector linework. Simply open your exported map and copy-paste everything over to the working file, and resize as needed using the techniques above.

Note that in order to do this properly, you’ll likely need to deal with some linework inconsistencies from the QGIS > SVG export. Some linework may extend beyond the intended export size. So how can you align your linework properly with the correctly-sized raster image?

The easiest way to do this involves a bit of a trick– using the background “page” from the exported map as the correct frame. Unless you’ve specified otherwise, the background will be white and on a “Page” layer. Select the Page layer from your Layers panel. For purposes of keeping things tidy, move the whole group to a new layer by clicking the plus symbol at the bottom of the panel and dragging the group over. Call this layer “Page” or something similar.

You’ll notice everything in “Page” is grouped, and that several groups exist but contain no objects– an annoying quirk of the export process. Ungroup the objects by clicking the button that appears, or use the Object > Ungroup menu option (or shortcut: Control-Shift-G). Then locate the “Path” object (in this case, the page outline) and delete anything else (e.g., the empty groups). Do this same clean-up process for the other map layers so that you have a well-organized working file with no extraneous paths or empty groups.

One key point here: Illustrator is full of keyboard shortcuts to select different tools. Most often you’ll use “V” (Selection) and “A” (Direct Selection). These two can help with the otherwise tedious clean-up process described above. If there’s a path you’re having trouble locating the layers panel (hidden in subgroups), try using the Direct Selection tool. This will directly select an object even if it’s grouped. You can then delete it without needing to ungroup it.

Once you’ve cleaned up your linework, select it all (this may involve multiple layers). This can be very slow if you’re manually selecting each layer in the panel; try locking or turning off layers you’re not using (e.g., the basemap image). Anything that’s hidden or locked won’t be selected, and you can simply Select All using the standard keyboard shortcut (Control-A).

Using the Page path as your guide, move everything so that the upper left corner (or another anchor point you’re using) aligns precisely with the upper left corner of the artboard. Then use the S keyboard shortcut to Scale everything into place.

If you’re having trouble aligning everything, you can use the Page path to make a Clipping Mask that hides all the linework it contains without deleting any of it. To do this, make sure your Page layer is on top of all your linework layers. Then Select All and use the Object > Clipping Mask > Make (Control-7 keyboard shortcut). The Clipping Mask automatically makes everything it contains into a group, and you can move/scale it easily into place. The drawback here is that to work on each set of linework (rivers, the grid, etc), you need to undo this step: Object > Clipping Mask > Release. Unfortunately you then need to reorganize your linework back into separate layers.

After you’ve rescaled your linework to match the basemap image, start exploring the various styling options using the “Stroke” and “Color” panels. You can also use the “Appearance” panel for a summary of the styles applied to the selected paths and objects. Try dropping down the “Stroke” menu and choosing “Opacity”– use transparency and blending modes to create layered effects to your drawing. There are too many options to review comprehensively here– the best way to get a sense of what these tools can do is play with them!



 

Last up, let’s bring in our sections. There are again multiple ways to do this, including the copy > paste > clean-up workflow detailed above. Let’s try a slightly different, streamlined approach that uses the Direct Selection tool.

• First, prep your sections by opening each file and renaming the Layer to reflect the section number (e.g., Section 1).
• Second, use the Direct Selection tool (A) to select the elevation profile. Simply click the filled polygon.
• Third, copy the object and paste it into your working file. No messy “ghost groups” or background paths!
• Repeat the process with each section. Don’t worry about placing them into the right position just yet, just copy them over.

Now we’re ready to position, align, and scale all our sections at once.

• Find the corresponding grid line for each section. Move the section anywhere along the line so that its base and the line are aligned along the same Y position and it doesn’t go beyond the line’s edge on the X axis. (Recall you can do this manually or using the techniques detailed earlier.)
• Next, select all your sections and any horizontal grid line. Open the Align panel and find Align > Horizontal align left. All your sections should align with the object that is furthest to the left, which should be your grid line.
• Last, scale your sections so they run the full extent of the grid line. You can also play with the vertical scaling of your section as long as you make the same modifications to every one. This is where the Object > Transform > Scale or Object > Transform > Transform each tools come in handy; you can adjust vertical and horizontal scales independently. If you want to vertically rescale all the sections at once, use the Transform each tool to preserve the position of your sections; the standard Transform > Scale tool will rescale the objects by their collective bounding box, which will result in a changed position.

You’re done! Well, at least with the basics. Spend some additional time styling your sections– experiment with scale, change their fill color (or use a gradient), use transparency/blending effects, etc. See if you can replicate the overlapping serial sections by Mathur & da Cunha, or the Paola Viganó’s sections in Cartographic Grounds. 

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