|
Mapping India in 3D
(Digital Elevation Modeling)
Background Note:
A drainage basin or watershed is a natural unit draining run-off water to a
common point. Watershed forms a convenient, clearly defined and an
unambiguous topographic unit available on the basis of stream network. The
concept of watershed as a planning unit for development of land and water
resources is widely accepted. In India, since 1974, various developmental
projects such as Drought Prone Area Programme (DPAP) Desert Development
Programme (DDP), Hill Area Development Programme (HADP), etc., adopt
watershed as basic unit for development. Therefore, it is necessary to
delineate watersheds at various levels of hierarchy based on topography.
The government is giving high priority to
holistic and sustainable development of rain-fed areas based on the
waterehed approach through its Integrated Watershed Development
Project Scheme (IWDP). Watershed development programmes are implemented
by the Zilla Panchayats through watershed associations. A Project
Implementing Agency (PIA), which may be a Government Department or an NGO,
is assigned about 10 micro watersheds, each of which covers an area of about
500 hectares.
The PIA constitutes a watershed development team
that interacts with the watershed associations and provides technical
assistance to the watershed association in planning and implementation of
the watershed programme. Participation of residents of the area organized
into self-help or user groups is encouraged, since their involvement is
considered crucial to the success of the programme as they are the ultimate
beneficiaries. In fact, these user groups are the beginning point as well as
the end point for Watershed Development programmes.
The availability of computer techniques for dynamic viewing of digital
landscapes offers a new way of examining land resource data.More
information:
http://dolr.nic.in/iwdp1.htm.
3D Mapping Applications:
Viewing the earth’s surface in 3D gives new perspectives about land systems,
resources and their management. A digital elevation model or DEM is a
quantitative, three dimensional digital representation of a topographic
surface. It is a representation of the continuous variation of relief over
space. Relief can be any variable that depends on geographic coordinates,
for example:
·
Continuous soil characteristics (pH, depth etc.)
·
Climate characteristics (rainfall, evaporation etc.)
·
Vegetation characteristics (biomass etc.)
The term DEM is also used to refer to a raster or regular grid of spot
heights.
Digital elevation models can be used to derive
a wealth of information about the morphology of land surface.
Elevation data available as contour maps was hitherto used to achieve
topographic expression by means of 3D imagination.
Traditional raster analysis methods were used
to derive slope, aspect, and shaded relief information. This
required a high degree of skill on the part of the interpreter. However,
the first decade of 21st Centaury witnessed a tremendous surge in
acquisition of elevation data of the earth’s surface from space borne
sensors in digital form using radar interferometry and stereo imaging, as
also the availability of high ended computer applications for reconstructing
the earth’s surface in 3D using this data. Viewing data in three dimensions
gives new perspectives about earth systems and processes. It adds insights
that would not be readily apparent from a planimetric view. Having multiple
viewpoints of the data guarantees to have a first raw exploration of their
quality.
By varying sun azimuths and elevation, together with different height
exaggerations of the terrain, shaded relief images can be produced for
visualizing features from different angles and different directions of
illumination. Vector and raster data describing landcover and land use can
be overlaid on the DEMs in order to obtained a 3D combined models called
Digital Terrain Models (DTMs).
Free Data Availability:
Digital elevation data may be created from many different sources – contour
maps, spot heights, stereo photographs of satellite imagery. The result of
digital elevation modeling is always expressed as gridded data, the quality
of which is determined by the quality of original data and the grid size
assigned for interpolation process. However, a free and readily available
source of digital elevation models is the SRTM data of which a brief
introduction follows.
On
February 11, 2000 a unique mission – the interferometric Shuttle Radar
Topography Mission (SRTM) – dedicated to acquiring earth’s surface elevation
data from space was launched in the form of the space Shuttle Endeavour.
SRTM was spearhead by the National Geospatial-Intelligence Agency (NGA) and
the National Aeronautics and Space Administration (NASA). The purpose of
this mission was to create a global, high-resolution, homogenous and
continuous digital elevation model of the area between 60o north
and 54o south latitudes. The spacecraft carried two radar
systems onboard, and orbited the earth 182 times within a short period of 11
days, and successfully obtained highly accurate data of the earth’s surface
elevation with a spatial resolution of 30 m.
The SRTM
data is distributed free of charge and is available as 3 arc second (approx.
90m resolution) DEMs for all of Indian territory. The vertical error of the
DEM's is reported to be less than 16m. The data currently being distributed
by NASA and USGS contains ‘data holes’ where water or heavy shadow prevented
the quantification of elevation. This data, in mosaiced 1o x 1o
tiles is obtainable from:
http://www2.jpl.nasa.gov/srtm/
http://glcf.umiacs.umd.edu/data/srtm/
ftp://e0srp01u.ecs.nasa.gov/
The
Consultative Group on International Agricultural Research (CGIAR) Consortium
for Spatial Information (CSI) provides SRTM data that does not contain
‘no-data’ holes (Version 4). This data is available in mosaiced 5 o
x 5 o tiles for easy download and use from:
http://srtm.csi.cgiar.org/
SRTM data
is also available from the NASA funded Global Land Cover Facility (GLCF) of
the University of Maryland, Institute of Advanced Computer Studies (UMIACS)
through their website:
http://glcfapp.umiacs.umd.edu:8080/esdi/index.jsp
Free Software Availability:
Global Mapper:
Global Mapper is a software package for viewing
and analyzing digital elevation models. It is designed and distributed by
Global
Mapper Software LLC. The application is also distributed by
the USGS (United State Geological Survey) under
the name dlgv32 Pro.
Global
Mapper is more than just a utility;
it has built in functionality for distance and area calculations,
raster
blending, feathering, spectral analysis and contrast adjustment,
elevation querying, line of sight calculations, cut-and-fill
volume
calculations, as well as advanced
capabilities like
image
rectification, contour generation
from surface data,
view shed
analysis (including Fresnel) from
surface data, terrain layer comparison (including differencing), and
triangulation and gridding of 3D point data. Repetitive tasks can be
accomplished using the built in scripting language or comprehensive
batch
conversion functionality.
Global
Mapper also includes the ability to
directly access multiple online sources of imagery, topographic maps, and
gridded terrain data. This includes access to
worldwide
high resolution color imagery from DigitalGlobe
(watermarked access for free) and access to the entire
TerraServer-USA database of USGS
satellite imagery and topographic maps free-of-charge. Global Mapper also
has the ability to easily access WMS data sources, including
built-in
access to elevation data and color imagery for the entire world,
and to view elevation data in
true 3D
with any loaded raster imagery and vector data draped on top of it. The
free version of the software (with limited functionality) may be downloaded
from:
http://www.globalmapper.com/product/download.htm#download1
Microdem: MICRODEM
is an extraordinary freeware microcomputer mapping program geared to
geoscience teaching with a simple user interface written by Professor Peter
Guth of the Oceanography Department, U.S. Naval Academy. It can display and
merge raster, grid and vector data including digital elevation models,
satellite imagery, scanned maps, vector map data and GIS databases.
The program handles input
from a huge variety of file formats, and
does everything from anaglyphs to variograms.
MICRODEM is excellent for generating
slope and aspect maps, contour maps and stereo pair images from digital
elevation data. It also has a number of geologic functions and can be
downloaded for free from:
http://www.usna.edu/Users/oceano/pguth/website/microdemdown.htm
To get an overview of the
full functionality of MICRODEM visit
http://www.forestpal.com/microdem.html
3 DEM:
3DEM produces three dimensional terrain scenes and flyby animations from a
wide variety of freely available data sources including SRTM data. Terrain
data files can be saved in many different formats for use by other GIS
programs. 3DEM can merge multiple DEMs to provide high-resolution overhead
maps and 3D projections of large surface areas, limited only by the
computer's memory. Geographic coordinates are shown on all map displays.
allowing display and measurement of position to high accuracy. Global
Positioning System (GPS) receiver waypoints, routes, and tracks can be read
via serial interface and displayed on 3D images and flybys of the terrain,
allowing visualization of the path of a trek through the wilderness.
3DEM will render 24 bit color three dimensional
projections or red-blue projections requiring red-blue 3D glasses for
viewing. 3DEM allows low resolution flyby of DEM landscapes. The path
through space is recorded in memory during flight, allowing subsequent
creation of a full resolution mpeg animation along the flight path. 3DEM is
a freeware program and can be downloaded at no cost from
http://www.visualizationsoftware.com/3dem/downloads.html
Applications of
Digital Terrain Modeling
DEMs may be used for many purposes, for example, delineation of flood and
landslide risk zones, road alignment, cut and fill estimation, locating
signal transmission towers, sentry outposts, planning supply routes etc.
Gridded elevation data can be represented as shaded relief images for
geological interpretation, particularly in terms of identification of rock
formations, major structures, and drainage basin morphometry.
Identification of geological formations in turn allows visualization of
foundation characteristics of an area, which is very essential for designing
mega engineering projects.
Watershed Delineation:
A watershed (or catchment) is an extent of land
in which water from rain or snow melt drains to a common point. The
watershed includes both the streams and rivers that convey the water as well
as the land surfaces from which water drains into those channels, separated
from adjacent basins by a highland or drainage divide. This makes a
watershed an ideal planning unit for conservation of soil and water, as also
for agricultural and infrastructure development. SRTM data may be used for
rapid, automated watershed delineation.
Drainage Mapping: Watersheds
are natural hydrologic entities that cover specific areal extent of land
from which rain water runs to defined gully stream or river at any
particular point. The size of watershed is dependent on the size of the
stream, river, the point of interception of stream or river, the drainage
density and its distribution.
A hierarchical approach is essential in delineating watersheds in a river
system. An appropriate terminology to clearly indicate each sub-division is
also equally important. The Central Groundwater Board has demarcated
watershed boundaries based on the drainage and elevation contour maps on
1:250, 000 scale from SOI and have divided the Country into 34 basins. This
work could be expedited by using DEM data and inexpensive software to
achieve very accurate results.
Hydrological Modeling: Water
shortages and the degradation of water supplies threaten the food security
and health of people in many parts of the world. This is particularly true
in developing countries that are experiencing rapid population growth but
have limited means to manage water resources. Steeply sloping regions of the
Himalayan region where small-scale farming is the predominant production
activity and source of food are examples of these threatened areas.
Digital elevation models are
extensively used in hydrological analysis to obtain the direction of flow on
a topographic surface. Knowing the direction of flow helps in determining
channel networks, and in obtaining the distributed specific catchments,
which are important hydrological attributes in a DEM based analysis.
Slope
Mapping and Analysis for delineating Landslide Hazards:
Slope maps distinguish areas of different steepness or classes of slopes
such as flat areas and steep slopes. The availability of digital
elevation model data have simplified the process of producing highly
accurate and detailed slope maps. One can classify the slope into any
number of desired classes, measure the relationship of elevation and
horizontal scale, and combine this data with other datasets in the GIS
environment to produce landslide hazard zonation maps.
Slope maps are not only handy in landslide zonation, but also find
applications in road and canal alignment, assessing surface runoff and rate
of erosion etc.
Floodplain Mapping: Accurate and
current floodplain maps can be the most valuable tools for avoiding severe
social and economic losses from floods. Accurately updated floodplain maps
also improve public safety. Early identification of flood-prone properties
during emergencies allows public safety organizations to establish warning
and evacuation priorities. Armed with definitive information, government
agencies can initiate corrective and remedial efforts before disaster
strikes.
Floodplain management studies have hitherto been expensive and awkward, with
much of the analysis performed “by hand” using paper maps. The availability
of DEMs and new technologies, such as GIS, GPS, and remote sensing are
helping floodplain managers to create accurate and current floodplain maps
with improved efficiency and speed at a reasonable cost. Accurate
floodplain maps are the key to better floodplain management.
Visualization: Views of shaded
relief and drapes of coverages over digital elevation models highlight
relationships with landforms. They provide a three-dimensional perspective
difficult to attain using other methods. Multispectral or other forms of
land cover data are more readily interpreted when draped over a high
resolution digital elevation model. The availability of computer
techniques for dynamic viewing of digital landscapes offers a completely
new way of examining land resource data.
Signal Coverage Area:
Cellular Phone Coverage (CPC) is dependent on optimal placement of
cellular towers. In mountainous terrains particularly, the coverage is
restricted due to topographic highe interposing between the transmission
towers and populated areas. Through the use of digital elevation data, it
is possible to assess cellular coverage areas using viewshed analysis,
available through 3D analysis functionality of most applications. This
functionality is used to predict the
Line-of-Sight (LOS) of radio-wave propagation.
Visibility algorithms, both LOS and
viewshed, applied to high-resolution digital elevation data can be used to
predict high-frequency radio signal coverage. Although the DEM resolution
affects the results of this prediction, the availability of high resolution
stereo image data can give excellent results.
Lineament Mapping:
Topographic lineaments are commonly interpreted as surface
expressions of rock fractures which may provide pathways for transport of
groundwater from/to the subsurface. Lineaments also represent
different geological features, such as shear zones/faults, rift valleys,
truncation of outcrops, fold axial traces, joint and fracture traces,
topographic, vegetation, soil tonal changes alignment, etc. Mapping
and study of lineaments is important from the point of view of groundwater
studies, hydrothermal systems, lithological characterization, geotechnical
projects viz., tunnel, aqueduct and road alignment, landslide hazard
zonation and mining and quarrying.
DEM Products:
How to view the stereo images
Stereo Photos of
Kalagarh Reservoir
Stereo Photos of Nanda
Devi Biosphere Reserve
3D Animation
of an area in Eastern Kumaon Himalaya
3D Animation
of the area around Srinagar (Garhwal)
|
