Digital elevation model

Also called: DEM, Digital terrain model, DTM · Topic: Topographical Maps: Relief Representation and Interpretation · NCERT: Class 12, Ch 4 "Spatial Information Technology"

Meaning

A digital elevation model (DEM) is a grid of ground heights stored as numbers on a computer. Each small square cell (a raster cell) holds one height value.

It is the digital form of what contours and cross-sections show on paper. A GIS can use it to work out slope, drainage, watersheds, flood zones and lines of sight automatically. India uses DEMs for disaster planning, watershed work and road and rail planning.

Explanation

How a DEM works

• The ground is split into a grid. Each square cell gets one number: the height of the ground in that square.
• Resolution is the size of one cell.
• "30 m" means one height value for every 30 m × 30 m square of ground.

• Smaller cells mean finer detail.

• Same information, new form. A contour map, a hand-drawn cross-section and a DEM all store height. The DEM stores it as numbers, so a computer can analyse it.

How DEMs are made

• Radar interferometry (two radar images compared):
• Two radar antennas record the same ground from slightly different positions.
• The difference between the two images gives the surface height [2].
• Radar sees through clouds and works at night.

• Example: SRTM (Shuttle Radar Topography Mission).

• Optical stereo images (two photos from two angles):

• One camera looks almost straight down and another looks forward. Both photograph the same ground.
• Our two eyes judge depth in the same way, so the two images give height [4].
• An optical camera cannot see through clouds.

• Example: India's CartoDEM, made from Cartosat-1 images.

• From contours: heights read from existing topographic maps can also be turned into a grid.

Types: DSM, DTM and DEM

Term	What it records
Digital Surface Model (DSM)	The top surface: the ground plus trees and buildings
Digital Terrain Model (DTM)	Bare ground only, with trees and buildings removed
DEM	The general term for any height grid. It is often used to mean a DTM

• NCERT Class 12, Spatial Information Technology, lists the digital terrain model among GIS spatial analyses.
• CartoDEM is a surface model, so it records heights of the top surface [4].

What a GIS works out from a DEM

A GIS (Geographic Information System) is software that stores and analyses map data in layers. From a DEM it can work out:

• Slope: how steep each cell is. It uses the height difference between a cell and its neighbours.
• Aspect: the compass direction a slope faces. For example, south-facing slopes in the Himalaya get more sun.
• Drainage:
• Water is sent from each cell to its lowest neighbour.

• Joining these paths traces the stream network.

• Watersheds: the area that drains to one outlet, marked off along the ridgelines.

• Profiles: a cross-section drawn automatically along any line, without a paper strip.
• Flood zones: low cells next to rivers, which fill first when water rises.
• Viewshed: an automatic intervisibility test (can one point be seen from another?). It shows every cell visible from a chosen point.

Where it occurs

• SRTM (world, NASA, USA):
• Flown on the Space Shuttle Endeavour on an 11-day mission from 11 to 22 February 2000 [2].
• Two radar antennas: one sat in the payload bay and the other at the end of a 60 m mast [2].
• It used C-band Synthetic Aperture Radar (SAR), a radar that builds a detailed image as it moves. It collected about 8.6 terabytes of data [2].
• Coverage: about 80% of Earth's land, between 60°N and 56°S [2][3]. It covers all of India (about 8°N–37°N) but leaves out Greenland and Antarctica.
• Resolution: 1 arc-second (about 30 m) [3].
• Accuracy: 16 m vertical and 20 m horizontal, at 90% confidence [3].

• Because it used radar, it could map cloudy areas such as the Western Ghats and the Northeast.

• CartoDEM (India, ISRO-NRSC):

• Made from Cartosat-1 images. Cartosat-1 was launched on 5 May 2005. It was ISRO's first stereo mission, with 2.5 m resolution [4][5].
• The 1 arc-second (about 30 m) version is free to download on Bhuvan, ISRO's geoportal [4][5].
• A national DEM at 1/3 arc-second (about 10 m) was made using ISRO's own Augmented Stereo Strip Triangulation (ASST) software [5].

• A 2.5 m CartoDEM has 8 m vertical and 15 m horizontal accuracy at 90% confidence [6].

• Uses in India:

• Landslide zoning in the Himalaya and Western Ghats.
• Flood mapping in the Ganga and Brahmaputra plains (Bihar, Assam).
• Storm-surge zones along the low coasts of Odisha and West Bengal.
• Watershed planning, and road and rail alignment in hilly states.

Don't confuse with

• Digital Surface Model (DSM) includes trees and buildings. A DTM shows bare ground only. CartoDEM is a surface model [4].
• SRTM vs CartoDEM: SRTM used radar (it works through clouds). CartoDEM used optical stereo images (it does not work through clouds). Both are free at about 30 m [3][4][5].
• Contour map / cross-section: these show height on paper (a top view and a side view). A DEM stores the same height as numbers in grid cells for computer analysis.
• Slope vs aspect: slope is how steep a cell is. Aspect is which direction it faces. Both are worked out from a DEM.

Prelims Hooks

• DEM = a grid of height values, one per raster cell. "30 m resolution" means one value per 30 m × 30 m square.
• DTM = bare ground. DSM = ground plus trees and buildings. NCERT Class 12 lists the digital terrain model among GIS analyses.
• SRTM: Space Shuttle Endeavour, 11–22 February 2000, C-band radar interferometry, 60°N–56°S, about 30 m [2][3].
• CartoDEM: Cartosat-1 (launched 5 May 2005) stereo images, free at about 30 m on Bhuvan [4][5].
• Trap: "CartoDEM can map through clouds". This is wrong. Only radar (SRTM) works through clouds.
• Viewshed = automatic intervisibility test from a DEM. Watershed = area draining to one outlet, bounded by ridgelines.

Mains Points

• Disaster management (GS-III): slope and aspect layers from CartoDEM and SRTM support:
• landslide zoning in the Himalaya and Western Ghats;
• flood mapping in the low plains of Bihar and Assam;

• storm-surge zones along the coasts of Odisha and West Bengal.

• From paper to digital (GS-I):

• Hand-drawn cross-sections and intervisibility checks are now automatic DEM profiles and viewsheds.

• The free 30 m CartoDEM on Bhuvan [4][5] supports panchayat-level watershed planning and road and rail alignment in hilly states.

• Strategic self-reliance (GS-III, security):

• Terrain and intervisibility data are sensitive. This is why India keeps separate Defence Series Maps and Open Series Maps.
• A home-built DEM (CartoDEM) reduces dependence on foreign data such as SRTM.

Related concepts

• Cross-section
• Shuttle Radar Topography Mission
• CartoDEM
• Bathymetric chart

Read more

• Cross-sections and digital elevation models