How to calculate the area between contour lines on a DEM with QGIS 1.8?

Decoding Terrain: Calculating Area Between Contour Lines in QGIS 1.8 (and Beyond!)

Ever looked at a topographical map and wondered how much land sits between those squiggly contour lines? Turns out, figuring that out can be super useful, whether you’re a hydrologist trying to estimate water storage, a land manager divvying up parcels, or even an ecologist studying habitat ranges. We’re going to dive into how to calculate that area using QGIS 1.8. Now, QGIS has come a long way since then, but the core ideas? They’re still rock solid. So, think of this as your foundation for tackling this task, even if you’re rocking a newer version.

Why Bother with Contour Intervals Anyway?

Contour lines, those lines connecting points of equal elevation, are like a visual shorthand for understanding elevation changes. The area nestled between them? That’s the spatial extent of the terrain within a specific elevation range. Knowing this area unlocks a ton of possibilities. Imagine being able to easily estimate water storage in different elevation zones – pretty cool, right? Or picture quickly assessing how much land falls within certain slope categories. This kind of analysis is surprisingly versatile.

Let’s Get Our Hands Dirty: The Step-by-Step Guide

Alright, time to get practical. Here’s how you can calculate the area between contour lines using QGIS 1.8. Don’t worry, it’s not as scary as it sounds!

1. Load Up Your DEM:

• Fire up QGIS 1.8.
• Drag and drop your Digital Elevation Model (DEM) into the QGIS window, or use the “Add Raster Layer” button. Think of your DEM as the raw data – the digital representation of the terrain’s height.

2. Conjure Up Some Contour Lines:

• Navigate to Raster > Extraction > Contour.
• A dialog box will pop up. Here’s where the magic happens:
  • Tell QGIS which DEM layer to use as your input.
  • Decide on the “Interval” between your contour lines. This is key! Too big, and you lose detail. Too small, and your map gets cluttered. Pick an interval that makes sense for your terrain.
  • Give your contour lines an output file name and location.
  • Hit “OK” and watch QGIS do its thing.

3. Zero In On Your Target:

• Figure out the specific elevation range you’re interested in. Let’s say you want the area between 3400m and 3500m. Got it? Good.

4. Mask It Like a Pro: Raster Calculator to the Rescue!

• This step is all about isolating the area we care about. We’re going to use the Raster Calculator to create a mask.
• Go to Raster > Raster calculator.
• Now, for the expression. This might look a little intimidating, but bear with me. We’re telling QGIS to select only the cells within our elevation range. Something like this should do the trick (replace “DEM@1” with the actual name of your DEM layer):
  
  
  (“DEM@1” >= 3400) AND (“DEM@1” <= 3500)
• This creates a new raster where the area between 3400m and 3500m is highlighted (usually with a value of 1), and everything else fades into the background (usually with a value of 0).
• Save this new raster.

5. From Raster to Vector: Polygonizing the Mask

• Time to turn that raster mask into a vector polygon. This gives us something we can actually measure.
• Go to Raster > Conversion > Polygonize (Raster to Vector).
• In the dialog:
  • Select your reclassified raster as the input.
  • Give the field that will store the raster values a name (like “DN”).
  • Specify an output file.
  • Click “OK.”

6. Area, Area, Everywhere! Calculating Polygon Sizes

• Right-click on your new polygon layer in the table of contents and choose “Open Attribute Table”. This is where all the data about your polygons lives.
• Click the “Toggle editing” button (it looks like a pencil) to start editing the table.
• Open the “Field Calculator” (the abacus icon).
• Create a new field called “Area” (or whatever you like). Make sure it’s a “Decimal number (real)” type.
• In the expression box, type $area. This tells QGIS to calculate the area of each polygon.
• Click “OK.” Boom! You’ve got area values.
• Turn off editing mode and save your changes.

7. Pick Your Prize: Selecting the Right Area

• Back in the attribute table, find the polygon(s) that represent your elevation range (look for the value of 1 in the “DN” field, if you named it that).
• Select those polygons. These are the areas between your contour lines!

8. Add ‘Em Up (If Needed): Summing Areas

• If the area between your contour lines is split into multiple polygons, you’ll need to add them together.
• Use the Statistics tool in the attribute table to sum the “Area” field for your selected polygons.

Pro Tips and Gotchas

• Zonal Statistics: If you’re using a newer QGIS version, explore the “Zonal Statistics” tools. They can make this whole process even easier.
• CRS is Key: Make sure your DEM and contour lines are in a projected Coordinate Reference System (CRS) suitable for measuring area. Geographic CRSs are a no-go for area calculations. Trust me, I learned this the hard way!
• Units Matter: The area will be in the units of your CRS. So, if your CRS is in meters, you’ll get square meters. Convert as needed.
• Accuracy Counts: The more accurate your DEM and contour lines, the more accurate your area calculation will be. Garbage in, garbage out, as they say.
• QGIS Evolving: Remember, QGIS is constantly improving. While this guide is based on 1.8, the core concepts apply to newer versions. Just be prepared for some menu changes.

Final Thoughts

Calculating the area between contour lines in QGIS might seem a bit involved at first, but once you get the hang of it, it’s a powerful technique. Whether you’re sticking with QGIS 1.8 or embracing the latest version, understanding these steps will give you a solid foundation for analyzing terrain and extracting valuable information. So go forth, explore your DEMs, and unlock the secrets hidden between those contour lines!