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Week 6 Hands-On — Raster Data in QGIS and an Elevation Profile

Hands-On Practice · Day 11 · Thursday of Week 6 — Working with Raster Data

Topics

  • Load a GeoTIFF in QGIS and read the Information and Source tabs (data type, rows and columns, cell size, units, projection)
  • Raster symbology: render types, singleband pseudocolor, color ramps, classification
  • Elevation surfaces and cross-section profiles (View > Elevation Profile, or the Profile Tool plugin)
  • Exam 1 review Kahoot in the last fifteen minutes

Feeds Lab 5 · ends with the Exam 1 Kahoot.

At a glance
Goal Students read a raster's properties, style a DEM three ways, pull an elevation profile across a valley, and run one local and one focal raster tool.
Why this week On Tuesday students did raster analysis by hand on engineering paper, then saw the same ideas named: cells, cell size, extent, no-data, and map algebra as same cell in, same cell out. Today the same ideas run on a real DEM. Lab 5 merges DEM tiles, styles them, reads elevations, and computes slope. Concepts Exam 1 is in the Testing Center next week, so the last 15 minutes are the review Kahoot.
Students bring QGIS 3.44 (lab machines or their own laptop) and UtahCountyDEM.tif from UtahCountyData.zip (they downloaded it in Week 2). A phone for Kahoot.
Graded item In Class Activity: DEM Profile (5 points). Upload a screenshot showing a pseudocolor DEM with an elevation profile.
Feeds Lab 5: Working with Raster Data. Due Saturday.

Practice run before class

Tip

Twenty minutes. The elevation profile is the part worth rehearsing; the rest is symbology.

What you need. QGIS 3.44 LTR and UtahCountyDEM.tif plus UtahCountyBoundary.shp from UtahCountyData.zip. Put the boundary on top with no fill, as an outline.

Do this, in order.

  1. Add the DEM. Right-click Properties > Information and check the numbers against the table in walkthrough step 1. Be able to say what each one means without notes; that reading is the first six minutes of class.
  2. Symbology > Render type: Singleband gray, then Singleband pseudocolor with a terrain ramp, Equal Interval, 8 classes, Classify. Then try Continuous.
  3. Duplicate the layer, set the lower copy to Hillshade with azimuth 315 and altitude 45, and drop the pseudocolor copy to about 60 percent opacity on top.
  4. Identify Features on the DEM to see a single elevation value in meters.
  5. Properties > Elevation, tick Represents Elevation Surface, Apply. This step is what makes the next one work.
  6. View > Elevation Profile, click Capture Curve, draw a line from Utah Lake east across Provo to the Wasatch, right-click to finish.
  7. Raster > Raster Calculator with "UtahCountyDEM@1" > 1500 for a 1/0 raster, then the same with 2000. Then Raster > Analysis > Slope on the DEM, ratio of vertical to horizontal units left at 1.

You are ready when one screenshot shows the pseudocolor DEM and the profile panel together, with the rise from the lake to the mountains clearly readable.

The one that bites. The Elevation Profile panel stays stubbornly empty until the layer is flagged as an elevation surface in step 5. If the panel misbehaves anyway, the Profile Tool plugin does the same job.

Before class

  • [ ] QGIS open with the DEM loaded and the county boundary on top as an outline.
  • [ ] The Exam 1 Kahoot from the Learning Suite Kahoot content page (Geomatics Exam 1 Prep) open in a second browser tab and started to the lobby screen.
  • [ ] Learning Suite open to the DEM Profile activity.
  • [ ] This page open on the projector or a second screen: the two figures in step 4 are worth showing.

Plan (50 minutes)

Time Segment
0:00 Mini-devotional
0:03 Layer Properties: Information and Source; what the numbers mean
0:09 Symbology: singleband gray, pseudocolor with classes, hillshade
0:17 Identify a cell; Elevation tab; View > Elevation Profile across Utah Lake to the Wasatch
0:25 One local tool (Raster Calculator, two thresholds) and one focal tool (Slope)
0:30 Students: pseudocolor plus a profile; screenshot; upload
0:35 Kahoot: Exam 1 prep
0:48 Lab 5 pointer; Exam 1 is in the Testing Center next week

Walkthrough

1. What is in the file

Right-click the DEM > Properties > Information. Read each line out loud and ask what it means. These are the values in the course copy of UtahCountyDEM.tif:

Line Value What to say
Dimensions 3874 columns × 2991 rows About 11.6 million numbers, and nothing else.
Pixel size 30, −30 Cell size in meters, because the CRS is in meters. The −30 is only the row direction: rows count downward.
CRS NAD83 / UTM zone 12N (EPSG:26912) Same CRS as the county boundary, so they line up with no reprojection.
Extent about 116 km east–west × 90 km north–south A rectangle a bit bigger than the county; the county outline sits inside it.
Data type Float32 Decimal elevations, in meters.
No-data value −3.4028235e+38 The smallest Float32 number, used as a flag. A no-data cell is not zero; zero is an elevation.

Elevations run from about 1,313 m to 3,636 m. Ask what is at each end before you say it. The highest cell is Mount Nebo, at the south end of the county. The lowest is not Utah Lake: it is on the north edge of the DEM, where the Jordan River leaves the valley, because the river keeps dropping after it drains the lake.

The Source tab shows the file path and lets you rename the layer. Renaming changes nothing on disk, but it does change the name the Raster Calculator uses in step 4, so leave it as UtahCountyDEM today.

2. Three ways to see one grid

  1. Symbology > Render type: Singleband gray. Min and max stretch. Dark is low.
  2. Singleband pseudocolor. Pick a terrain color ramp, Mode: Equal Interval, 8 classes, Classify, Apply. Then try Continuous. Say what "pseudo" means: the cells are elevations, not colors; the ramp is a lookup table you chose. This is the right half of Tuesday's elevation-grid slide: change the classes or the colors and you get a new picture of the same numbers.
  3. Hillshade. Azimuth 315, altitude 45. Duplicate the layer, keep one as pseudocolor with 60 percent opacity on top of the hillshade. That pair is how most published relief maps are built.

3. Reading elevations

  1. Identify Features on the DEM: one band, one value, in meters.
  2. Properties > Elevation: tick Represents Elevation Surface. Apply.
  3. View > Elevation Profile. Click the Capture Curve tool, draw a line from Utah Lake east across Provo to the Wasatch, right-click to finish. The profile appears below the map. Ask where campus is on it, and what the vertical exaggeration is doing.
  4. If the profile panel misbehaves on someone's laptop, the Profile Tool plugin (Plugins > Manage and Install) does the same job.

4. One local tool, one focal tool

Tuesday's map algebra was local: every output cell comes from the same cell in each input, and nothing else is consulted. That is the rule students saw on the slide:

Layer A plus Layer B equals A + B, cell by cell; the highlighted cell is 2 + 3 = 5, and a NoData cell in B stays NoData in the answer

  1. Raster > Raster Calculator. Double-click UtahCountyDEM@1 in the band list so the quotes are right, then type > 1500. The expression is "UtahCountyDEM@1" > 1500. Run it: a 1/0 raster of everything above 1,500 m. Tuesday's engineering-paper reclass was this, done by hand.

  2. Change the number once. Run it again with 2000. The threshold is a choice, and a raster tool makes it cheap to test. Measured on the course DEM:

    Threshold Area above it Share of the DEM's cells
    1,500 m about 8,460 km² 82 %
    2,000 m about 5,790 km² 56 %

    Ask what each one would mean as a building rule, and whose decision the number is.

Then show a tool that is not local. Slope cannot be computed from one cell: it needs the neighbors. That makes it a focal tool. A 3 × 3 window steps across the grid and writes one number at every stop:

A 3 by 3 window over an input grid: nine cells in, one cell out. The mean of the nine values is 4.1, so the spike of 9 is smoothed to 4.1. At the grid edge the window hangs off the data and border cells are NoData.

The figure shows a mean, because the arithmetic fits on one line. Slope uses the same window with a different formula: how fast elevation changes across those nine cells.

Now run it: Raster > Analysis > Slope. Input the DEM; leave Ratio of vertical units to horizontal at 1, because both the elevations and the cell size are in meters. Run. Style the result pseudocolor. Lab 5 asks for exactly this. Point at the outermost ring of cells: the window hangs off the edge there, as in the figure.

Student activity

Students style the DEM as singleband pseudocolor with at least five classes, draw an elevation profile across the county, and take one screenshot that shows the map and the profile panel together. Upload it to In Class Activity: DEM Profile on Learning Suite. Full credit for pseudocolor plus a profile.

Kahoot (15 minutes)

Start the Geomatics Exam 1 Prep Kahoot from the Learning Suite Kahoot page. Students join with the PIN on their phones. Read the questions that get the most wrong answers twice; those are the ones to review before the exam. Remind them the Testing Center closes early on some days and the exam is closed book.

Common snags

  • The DEM draws as a flat gray square. Min and max are not set. Symbology > Min/Max Value Settings > Cumulative count cut, Apply.
  • "Represents Elevation Surface" is missing. They are on an older QGIS. The Profile Tool plugin covers it.
  • Raster Calculator says the expression is invalid. The layer has been renamed, or the name was typed by hand. Double-click the layer in the band list instead of typing, so the name and quotes are exactly what the dialog expects.
  • Slope output is nearly all 90° or nearly all 0°. The DEM is not the course copy: someone downloaded a DEM in a geographic CRS (EPSG:4326), whose cell size is in degrees while the elevations are in meters. Reproject it to EPSG:26912 first with Raster > Projections > Warp (Reproject). The course DEM is already in meters and does not have this problem.
  • Kahoot PIN will not join. Refresh the lobby; the BYU network sometimes blocks the first attempt.