Digital Elevation Model and Catchment Areas

 

What is a DEM?

 

A Digital Elevation Model (DEM) is a representation of the topographic bare ground (bare earth) of the Earth, free of any surface objects like trees and structures. 

DEMs are usually produced from remotely sensed data tracked and collected by satellites, drones, and airplanes. In the past, topographic maps were the primary source of USGS DEMs. DEMs created from high-resolution lidar and IfSAR (Alaska only) data are gradually replacing those ones. Any digital depiction of a topographic surface is often referred to as a DEM. The most basic type of digital topographic representation is a DEM. To calculate topographical characteristics like height at any point, slope, and aspect, DEMs are employed. Today, DEMs are mostly used in GIS applications. DEM data is an integral part of urban planning.

 

What are the fundamentals of DEM analysis?

 

Four key elements make up a DEM analysis, namely:

• Data acquisition includes taking pictures of the terrain or scanning the earth's surface
• Data modelling includes multidisciplinary techniques including photogrammetry, interferometry, and image processing.
• Data management includes computer graphics, geographical database technology, data coding, and data structure.
• Application development includes danger detection and monitoring, urban planning, mine management, surveying, geomorphology analysis, facility management, civil engineering, resource management, geological engineering, landscape design, and even computer games and missile/aircraft navigation.

 

Steps to Find Catchment Area using Dem data and QGIS

 

Objective:

The calculating catchment area of a river from its basin using the Digital Elevation Model.

 

Tools Used:

• DEM data
• QGIS software

 

Procedure:

 

Step 1:

Add DEM in QGIS

Preferably DEM with high resolution

 

 

 

Step 2:

In the processing toolbar, search for fill using (wang and Lui method)

Save the output file and run.

 

 

The output opens up into the QGIS table of contents (Layers)

 

 

Step 3:

Search for Strahler order in the processing toolbar

Input filled DEM and output file.

 

 

 

 

Step 4:

Open Raster calculator to distinguish streams of higher order

Type in the condition based on the results of your steam order i.e, in this case, it is 11 so >=6 is taken.

Save the file to the respective location to run the process.

 

 

Step 5:

Take the output file properties and set symbology to unique value and transparency with zero.

 

 

 

Step 6:

For better visualization, we can convert the created stream into a vector. 

For more accuracy DEM of high resolution is used.

 

 

In the processing, toolbar, search for channel network and drainage basin and click on it.

Enter the necessary input, untick all other outputs except channels and save the output.

The output file is a vector.

 

 

 

 

Step 7:

Select the main river channel which you want to consider at the start of the river section.

Create a point feature, start editing, and add attributes of x and y coordinates using the field calculator.

 

 

 

 

Step 8:

Search for the upslope area in the processing toolbar, and enter the x and y coordinates with fille DEM input on elevation.

Choose the deterministic 8 methods for accurate calculation.

Save the output file name and location.

 

 

 

Step 9:

The result is raster so go to raster conversion raster to polygon and select the input file.

 

 

You can also calculate the area similarly in the attribute table with the help of the field calculator.

 

 

The basic unit system of the QGIS area is Sq.m we can convert it to Sq.km by dividing by 1000000.

 

Step 10:

Visualization with a background of web map services

 

 

Applications using DEM data

By giving you a visual representation of the environment in an issue along with elevation data, you may determine which places are most susceptible to sea level rise, identify vegetation encroachment, and steer clear of unsightly areas while developing an urban area.

The common approaches to represent elevation are:

• Digital Elevation Model (DEM)
• Digital Surface Model (DSM)
• Digital Terrain Model (DTM)

The above article was a stepwise instruction on one of the many uses of DEM data.