Cross-sections and digital elevation models

Detail

What a cross-section is

• A cross-section (also called a section or profile) is a side view of the ground. It shows what you would see if the land were cut straight down along a line.
• A contour map shows relief from above. A cross-section turns that into a side view. You can then see the actual shape of hills, valleys and slopes.
• The section is always drawn along a straight line (AB) between two chosen points on the map.

Steps for drawing a cross-section

1. Draw the line AB across the contours on the map.
2. Mark each crossing on a paper strip. Lay a strip of paper along AB. Mark every place where a contour crosses AB, and write its height value next to the mark.
3. Choose a vertical scale, e.g. ½ cm = 100 m. On graph paper, rule one horizontal line for each contour value (e.g. 100 m, 200 m, 300 m …).
4. Transfer the marks. Place the strip on the base line. From each mark, drop a perpendicular (a line at right angles) up to the horizontal line of the same height.
5. Join the points smoothly with a freehand curve, not with straight ruled segments, because real ground has no sharp corners.

• Horizontal scale = the map's own scale, because the strip is copied straight from the map.
• Tip: where two crossings next to each other have the same value (for example, two 300 m crossings with no higher contour between them), the ground goes a little above that value (a hilltop) or a little below it (a valley floor). Draw the curve slightly above or below the line. Do not draw it flat.

Vertical exaggeration

• Vertical exaggeration means the vertical scale is larger than the horizontal scale.
• Why it is used: heights are tiny next to distances. If both scales were the same, most relief would look almost flat. Stretching the heights makes slopes and landforms easy to see.
• How to work it out: vertical exaggeration = horizontal scale distance ÷ vertical scale distance (both for 1 cm on paper).
• Example: on a 1:50,000 sheet, 1 cm = 500 m along the ground. If the vertical scale is ½ cm = 100 m, then 1 cm = 200 m in height.

• Vertical exaggeration = 500 ÷ 200 = 2.5 times.

• Trap: a larger exaggeration makes slopes look steeper than they really are. The profile is a diagram, not a true picture.

Reading landforms from a profile

Profile shape	Landform	Contour pattern on the map
Rises to a single peak, falls on both sides	Conical hill	Closed, roughly circular contours, with the highest value in the centre
Sharp V-shaped dip	V-shaped valley (river in hills)	V-shaped contours pointing upstream (towards higher ground)
Broad U-shaped dip	U-shaped valley (glaciated)	Wide U-shaped contours
Rises steeply, then stays flat on top	Plateau	Contours packed closely at the edges, with a wide gap on top
Very steep drop on one side	Escarpment / cliff	Contours very close together, or merging
Steep top, gentle foot (curved inwards)	Concave slope	Contours close near the top, far apart near the bottom
Gentle top, steep foot (bulging outwards)	Convex slope	Contours far apart near the top, close near the bottom

Intervisibility

• Intervisibility means whether one point can be seen from another point.
• How to test it on a profile:
• Draw a straight line between the two points on the profile (the "line of sight").
• If any ground rises above this line, the view is blocked. The points are not intervisible.

• If the ground stays below the line all the way, they are intervisible.

• Rule of thumb:

• On a concave slope, the top and the foot can see each other.

• On a convex slope, the bulge hides the foot from the top.

• Uses: placing watch towers, radio and mobile towers, and military observation posts. This is why the Defence Series Maps of the Survey of India stay restricted.

Digital elevation model (DEM) — beyond NCERT

• A digital elevation model (DEM) is a grid of ground heights stored in digital form. Each square cell (a raster cell) holds one height value.
• Resolution is the size of one cell. "30 m" means one height value for every 30 m × 30 m square of ground.
• DEM vs related terms:

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

• The Class 12 chapter Spatial Information Technology lists the digital terrain model among GIS spatial analyses.
• What a GIS (Geographic Information System: software that stores and analyses map data in layers) works out from a DEM:
• Slope: how steep each cell is. It comes from the height difference between a cell and its neighbours.
• Aspect: the compass direction a slope faces (e.g. 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 that fill first when water rises.

• Viewshed: an automatic intervisibility test that shows every cell seen from a chosen point.

• Link: a contour map, a hand-drawn cross-section and a DEM all store the same thing, which is height. The DEM stores it as numbers that a computer can analyse.

Shuttle Radar Topography Mission (SRTM)

• SRTM is NASA's mission that made the first near-global, high-resolution DEM of Earth's land.
• Flight: on the Space Shuttle Endeavour, an 11-day mission from 11 to 22 February 2000 [2].
• How it measured height (radar interferometry):
• The shuttle carried two radar antennas. One was in the payload bay. The other was at the end of a 60 m mast that extended out in space [2].
• Each antenna recorded a radar image from a slightly different position.

• The difference between the two images gives the surface height [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 surface, between 60°N and 56°S [2][3].
• It covers all of India, which lies between about 8°N and 37°N.

• Areas near the poles, such as Greenland and Antarctica, are left out.

• Resolution: one height point every 1 arc-second (about 30 m) [3].

• Accuracy: 16 m vertical and 20 m horizontal, at 90% confidence [3].
• Why radar: radar sees through clouds and works at night. This allowed mapping of cloudy tropical areas such as the Western Ghats and the Northeast.

CartoDEM (India)

• CartoDEM is ISRO-NRSC's DEM of India's land surface. It is a surface model (heights of the top surface) [4].
• Satellite: 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].
• Stereo imaging:
• Cartosat-1 has two cameras. One looks almost straight down. The other looks forward.
• Both photograph the same ground from two angles.

• Just as our two eyes judge depth, the two images give height [4].

• Products:

• CartoDEM 1 arc-second (about 30 m) is free to download on Bhuvan, ISRO's geoportal [4][5].
• A national DEM at 1/3 arc-second (about 10 m) was also made for the whole country, using ISRO's own Augmented Stereo Strip Triangulation (ASST) software [5].

• A 2.5 m CartoDEM has cells of 1/12 arc-second, with 8 m vertical accuracy and 15 m horizontal accuracy at 90% confidence [6].

• Uses in India: watershed planning, landslide zoning in the Himalaya, flood mapping in the Ganga and Brahmaputra plains, and road and rail alignment.

Feature	SRTM	CartoDEM
Agency	NASA (USA)	ISRO-NRSC (India)
Platform	Space Shuttle Endeavour (Feb 2000)	Cartosat-1 satellite (launched May 2005)
Method	Radar interferometry (C-band SAR)	Optical stereo images
Coverage	60°N–56°S (about 80% of land)	India
Free resolution	about 30 m (1 arc-second)	about 30 m (1 arc-second) on Bhuvan
Works through clouds?	Yes (radar)	No (optical camera)

Bathymetric charts

• A bathymetric chart is a map of water depth. It is the underwater version of a contour map.
• Isobaths are lines joining points of equal depth below sea level. They are read like contours: close isobaths show a steep sea floor, and wide gaps show a gentle slope.
• Seen together, isobaths show the continental shelf, the continental slope, submarine canyons and ocean trenches.
• The National Hydrographic Office (NHO), Dehradun (Uttarakhand), under the Indian Navy, surveys and charts Indian waters. It produces the nautical charts that ships use for safe navigation.
• Trap: Dehradun is the headquarters of both the Survey of India (land maps) and the National Hydrographic Office (sea charts).

Prelims Hooks

• Cross-section = side view of the ground along a straight line. The horizontal scale is the map scale, and the vertical scale is chosen by you.
• Vertical exaggeration = vertical scale larger than horizontal scale. On a 1:50,000 sheet with ½ cm = 100 m, it is 2.5 times.
• Intervisibility: a convex slope hides the foot from the top, while a concave slope does not.
• SRTM: Space Shuttle Endeavour, 11–22 February 2000, C-band radar interferometry, 60°N–56°S, about 30 m [2][3].
• CartoDEM: made from Cartosat-1 (launched 5 May 2005) stereo images, free at about 30 m on Bhuvan [4][5].
• Trap: SRTM used radar, while CartoDEM used optical stereo images. Only radar works through clouds.
• DSM vs DTM: a DSM includes trees and buildings. A DTM is bare ground only, and it is the one listed in NCERT Class 12 GIS analyses.
• Isobath = line of equal depth (bathymetric chart). Isohypse/contour = line of equal height (topographic map).
• National Hydrographic Office = Dehradun, the same city as the Survey of India headquarters.
• Aspect = the direction a slope faces. Slope = how steep it is. Both are worked out from a DEM.

Mains Points

• DEMs in disaster management (GS-III): slope and aspect layers from CartoDEM/SRTM help with:
• landslide zoning in the Himalaya and Western Ghats;
• flood mapping in low-lying plains (Bihar, Assam);

• storm-surge zones along low coasts (Odisha, West Bengal).

• From paper to digital (GS-I): hand-drawn cross-sections and intervisibility checks are now automated as 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 angle (GS-III, security): intervisibility and terrain data are sensitive. This is why India keeps separate Defence Series Maps and Open Series Maps. A home-built DEM (CartoDEM) also reduces dependence on foreign data such as SRTM.
• Blue economy (GS-III): bathymetric charts from NHO Dehradun support safe navigation, port development, offshore oil and gas exploration, and claims to the outer continental shelf beyond India's EEZ.