DTM

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DIGITAL TERRAIN MODELLING

SEMINAR

GAGANDEEP SINGHROLL NO.- 131861M.TECH- RS & GIS(2013-2015)NIT-Warangal

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INTRODUCTION

•Digital representation of 3-D surfaces. Surfaces- continuous phenomena•Surface modeling- an infinite points = infinite data storage.•DTM- bare ground surface only.

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PURPOSE•DTM- powerful tool- GIS analysis & visualization.•DTM- digital representation- part of the earth's surface.•DTM- required for- flood or drainage modelling, land-use studies, geological and land mgt applications.•DTM- stored in a GIS :• set of contour vectors;• regular grid of spot heights (DEM);• an irregularly spaced set of points connected as triangles (TIN)

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GRID, TIN, CONTOURS

GRID• Regular raster grid • Irregular grid (denser where needed)• Easy structure, easy to interpolate, fast.

TIN (TRIANGULATED IRREGULAR NETWORK)• Point storage, but not in regular raster• Approximates the surface better with fewer points, but • more difficult to store and interpolate.

CONTOUR LINES• - Vector data with isolines (each line has a Z-value)• Difficult to interpolate. Sometimes masked by buildings.

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GRID DTM TERRAIN RELIEF REPRESENTATION

TIN TERRAIN RELIEF REPRESENTATION

CONTOUR LINES SUPERIMPOSED ON GRID

DTM

SET OF CONTOUR VECTORS

RECT. GRID OF ELEVATIONS

TRIANGULATED IRREGULAR NETWORK

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DTM vs DEM vs DSM

DSM = (earth) surface including objects on it

DTM = (earth) surface without any objects

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DEMDSM

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SEQUENCE OF TASKS TO PREPARE DEM

PROCESSING

ANALYSIS

VISUALIZINGAPPLICATION

DTMDATA

ACQUISITION

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DIGITAL TERRAIN DATA SAMPLING• IMPOSSIBLE- record each and every point (Earth’s surface).

• Two approaches:

• SYSTEMATIC- elevation measured at FIXED intervals

MATRIX of elev. values (DEM)• ADAPTIVE- elevation measured at SELECTED points;

Irregularly distributed elev. values, TIN (need to be structured)

DIGITAL TERRAIN

DATA SAMPLING

SYSTEMATIC APPROACH

ADAPTIVE APPRAOCH

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MORE on DEM & TIN• DEMs- square grids - arranged in rows and columns • grid point represents the elevation at that location. • Square grids - contain superfluous data in flat areas (unable to handle

abrupt changes in elevation easily)  • TIN networks - triangular elements - vertices at the sample points. • TINs can easily model sharp features such as peaks and ridges, and

they can also incorporate discontinuities.• TINs are more efficient from the point of view that the number of

sample points and triangles can be varied to match the surface roughness. • Computer storage space is less using TINs compared to regular grids.

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SOURCES OF DIGITAL TERRAIN DATA• National Oceanic and Atmospheric Administration (NOAA)• A free DEM -  GTOPO30 (30 arcsecond resolution, approx. 1 km) • A much higher quality DEM from - Advanced Spaceborne

Thermal Emission and Reflection Radiometer (ASTER) instrument of the Terra satellite - freely available for 99% of the globe - elevation at 30 meter resolution.• A similarly high resolution was previously only available for the 

United States territory under the Shuttle Radar Topography Mission (SRTM) data• In 2014, acquisitions from radar satellites TerraSAR-X and TanDEM-X will

be available in the form of a uniform global coverage with a resolution of 12 meters• United States Geological Survey (USGS) produces the 

National Elevation Dataset, a seamless DEM for the United States, Hawaii and Puerto Rico based on 7.5' topographic mapping.• National Imagery and Mapping Agency (NIMA) formerly the Defence Mapping Agency

(DMA)

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ELEVATION DATA CAPTUREPHOTOGRAMMETRIC MAPPING IN STEREO IMAGES• Can produce both DEM, DTM and DSM data• Manual 3D mapping – tedious, but high quality• Automatic matching – fast, but low quality, needs editing.

LASER SCANNING (LIDAR)• Faster than photogrammetric mapping• Expensive, but good quality • Can produce both DEM, DTM and DSM data• Need to reduce data

GROUND SURVEY• Tedious and expensive but high quality• Difficult to get DSM data

DIGITIZE FROM EXISTING MAP DATA (CONTOURS ETC.)• Quality depends on original data

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DATA ACQUISITION (FIELD SURVEY)• Data acquisition by an Electronic Tacheometer- TIN is

generated• Data acquisition by GPS

(PHOTOGRAMMETRY)• Data acquisition using analog stereo plotter• Data acquisition using analytical plotters• Data acquisition using digital photogrammetry

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DATA ACQUISITION cont.DIGITIZING EXISTING MAPS• Less time consuming• Economical• Accuracy of contours is only one third of that of the

spot heights even when both are obtained from the same photograph.

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GROUND SURVEYING

These techniques require that observations for elevation

models be made directly in the field.

The approaches normally considered for achieving this are:

1. Conventional total station or spirit levelling surveys

2. Global Positioning Systems (GPS)

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Merits1. Extremely accurate.2. Total stations and spirit levelling can acquire elevations

under forest canopy and other vegetated areas.3. Provide ground control for almost all airborne and

spaceborne sensors.4. Acquisition of quality control information.5. Provides measurements for filling in data voids in DEMs.

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Limitations:1. Expensive and time consuming.2. GPS systems do not provide reliable results in heavily

vegetated areas and urban canyons (receivers need line of sight to satellites).

3. Access is required to measure points.4. Safety issues - area of interest may include a dangerous

or hostile environment.5. Line of sight is required for total station surveys.6. GPS requires clear view to at least 4 satellites at all times

.

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DIGITAL AREAL PHOTOGRAMMETRY• Art and science of making accurate measurements using

photographs or images.• The steps for generating a DEM using digital

photogrammetric techniques are as follows:-1. Acquisition and pre-processing of aerial photos2. Interior orientation3. exterior orientation involving aerial triangulation or

relative 4. Absolute orientations5. DEM generation and DEM editing.

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MERITS:-It is a proven and well understood approach.The photos can be used for other purposes - Provides an

optical image of the landscape for interpretation and measurement.

Relatively economical for surveys of large areas.Aerial photographs can provide a good historical record of

actual inundation extents.Potential for high accuracies in plan and height

measurement.

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LIMITATIONS:-In manual measurements

− Observers ability to see stereoscopically.

− Skill in measuring the 3D stereo model created from pairs of photographs.Scale of photographs/images.Identification of ground control points used to provide the

relationship between the imagery mapping coordinate system.

Difficult to generate bare earth models in densely vegetated areas.

Image quality . resolution.

Delays in obtaining photographs/ images - restricted by weather

conditions and environmental conditions.

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MAP DIGITIZATIONdigitizing or scanning - contour lines and spot heights on

an available topographic map.Once this data has been converted to a digital format it

can be interpolated.Since these contour lines and spot heights normally

represent bare earth surfaces, the product generated is a DTM.

This approach is usually relatively economical accuracy depends on - quality of the map , skill of the

operator.

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Merits1. Normally based on widely available topographic maps.2. Easy to create if the digital contours are available.3. Only areas of interest need to be digitized.Main Sources of Errors and Limitations4. Errors in the topographic map (shrinkage, positional,

generalization).5. Digitizing errors.6. Interpolation errors.7. Highly dependent on the quality and scale of the base map.

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(InSAR)• collect sufficient data to generate DEM - tens of kilometers - with

10m res.• A powerful technique for generating digital elevation models.two

passes of a radar satellite (such asRADARSAT-1 or TerraSAR-X ), or a single pass if the satellite is equipped with two antennas (like the SRTM instrumentation)

Merits: • operate in almost any weather condition.• Generates its own illumination • Has shown to be capable of measuring deformations (changes in

height) of the land surface to a high accuracy.

Limitations: • Volume scattering in vegetated areas leading to poor coherence.• Confusing height data in water body regions.

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LiDARMerits: • high accuracy when compared to other airborne DEM techniques.• Can generate DEM for a surface with little or no texture.• Could measure vegetation heights when set to record first and last pulse.• Works both day and night making it a flexible acquisition system.

Limitations:-• Atmospheric delay on laser pulse.• Range errors due to inaccurate measurement of pulse.• Physical offset between recording centres of Laser, GPS and IMU• GPS associated errors (e.g. geometry of satellite constellation; multipath;

troposphere; distance to base station).• Transformation from WGS84 to local map projection and geoid measurements.• Often a narrow swath width, so many flight lines are required for extensive areas.• Cannot work in all weather conditions (e.g. strong winds, fog, clouds).• May require complementary data, such as aerial photo, if interpretation of points

is necessary.

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QUALITY OF DIGITAL TERRAN DATAFACTORS GOVERNING DATA QUALITY• Method of data acquisition • Types of source data (satellite & sensor used)• Methods of interpolation (different algorithms used)

QUALITY CONTROL CRITERIA & PROCEDURES• Statistical accuracy test• Data editing• Visual inspection

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CORE ACTIVITIES OF DTM• Processing terrain data- to ensure its optimized for

storage & application.• Performing analysis • Presenting the terrain information

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SOFTWARES FOR DTM PROCESSINGTerrain model construction software:• Used to generate DEM & TIN• ANUDEM-Best known & most widely used DTM building package

Functional extensions to GIS software:• In GIS packages DTM is offered as an optional software extension rather

than generic function• ArcTIN, Geoterrain, 3D Analyst for ArcVIEW GIS

Terrain data visualization software:• OpenGL, VRML Support• SHORTCOMINGS: (a) lack of ability in using georeferenced data

(b)Lack of database management capability

Contouring Software• Developed within working environment of existing GIS & CAD packages

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SURVEYING & MAPPING APPLICATIONS• DEM & TIN models are standard data structures in

digital cartography.• DTM enables production of high quality contour maps

more quickly and economically• Errors in data acquisition and map production

processes may also be detected more easily when the data are examined in 3D

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HYDROLOGICAL & GEOMORPHOLOGICAL APPLICATIONS• Digital terrain data & RS catchment characteristics are

now considered as crucial data to generate hydologic and water quality models• DEM, TIN, Contour-based used in different models

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ENGINEERING APPLICATIONS• DESIGN- DTM used to identify geometric skeleton of

the engineering design.• ANALYSIS- area, volume, slope, aspect calculation;

triangulation of surfaces & contouring, delineation of watersheds• VISUALIZATION- evaluation of design before its

finalised

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APPLICATIONSCommon uses of digital terrain models include:

• Extracting terrain parameters• Modelling water flow or mass movement (for example avalanches and

landslides)• Creation of relief maps• Rendering of 3D visualizations• Rectification of aerial photography or satellite imagery• Terrain analyses in geomorphology and physical geography• Geographical Information Systems• Engineering and infrastructure design• Global positioning systems (GPS)• Base mapping, flight simulation or surface analysis• Precision farming and forestry• Intelligent transportation systems (ITS)• Auto safety / Advanced Driver Assistance Systems (ADAS)

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Case study : Assessment Of DEMs From Different Spatial Data Sources

• 3 different sources – DEM generation (Shuttle Radar Topography Mission SRTM 30, Digitized Topographical map and Google Earth Pro.) • The main campus of The Ahmadu Bello University, Nigeria.• compared with field measured data from Total Station, i.e.

DEMs from 495 radial points over the test site.• The accuracy assessment - statistically by comparing (1)

estimates of some topographic attributes(slope and aspect), (2)overall spot height estimation performance

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Methodology• Total station instrument was utilized in the ground

survey exercise. • Sufficient number of scattered points (495 points)

were observed at the site to define the topography of the site.• Elevation from SRTM imagery: DEM of 80% of

globe wrt WGS-84Downloadable • Elevation from digitised topographic map:Maps are georefrenced. Contours are digitised.• Elevation from Google Earth Image :Generated online by conerting planimetric coor

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Generating DEM• The three dimensional coordinates from the various spatial data sources were plotted by gridding using Kriging method in ArcGIS 9.2 to produce the DEM, Figures (5)-(8) shows the DEMs with colours representing different ranges of elevation values.

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RESULTS & DISCUSSIONcalculated standard error, standard deviation and sample variance from the topographic map and Google earth imagery are closer to those of the total station(ground survey) than those of the SRTM

The descriptive statistics for the spot heights as presented in Table (2) clearly show the poor relationship of the SRTM data source when compared to other data sources under investigation.

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VISUAL INSPECTION

Figure.10 represents the terrain of the test site better when compared with reality.

Figure.11 gives a poor surface representation of the test site

Figure.12 also performs well but the differences between Figure(12) and Figure(10) may be as a result of erosion and human activities that have taken place from when the topographic map was produced.

Figure(13) also performs well

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Topographic attributes• the slope map statistics derived from the SRTM 30 have the lowest minimum and maximum slope value which indicates that the SRTM derived terrain is flatter• while the topographic map with the highest minimum and maximum slope values shows that the terrain is steeper and is also closer to the reference terrain

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• Aspect identifies the steepest down slope direction at a point on the earth surface. • Table 4, shows the Aspect map statistics from the various aspect maps.The mean values from the reference indicate that the steepest down slope is in the direction of South-East, • while the SRTM 30, Google Earth and topographical map are all in the south direction.• Figures.18 -21 shows that the aspect value

-1 indicates flat slope and flat slope have no direction. • While the red colours in aspect map ranging

from (0-22.5) and(337.5-360) shows direction due north.

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REFERENCES• P.A.Burrough & R.A. McDonnell, Principles of Geographical Information Systems, Oxford: Oxford University Press, 1998• C.P. Lo & Albert K.W. Yeung, Concepts & Techniques of Geographical Information Systems, India: Prentice Hall of India, 2006• TS05I - Spatial Information Processing I, FIG Working Week 2011, Marrakech, Morocco, 18-22 May 2011  • en.wikipedia.org/wiki/Digital_elevation_model• http://www.technion.ac.il/~dalyot/docs/Intro-DTM• http://geog.hkbu.edu.hk/geog3600• http:// http://www.fig.net/pub/fig2011

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THANK YOU !