Digital Terrain Models by M. Varshosaz 1

DTM tasks: generation Buy global or national data set Collect data

Digital Terrain Models by M. Varshosaz 2

Buy global or national data set

Digital Terrain Models by M. Varshosaz 3

Examples Topographic Data

(*) completed[Eidenbenz et al, 1997]

Examples: 3 ENMAs

Current products Production method

Current activity

OS (GB)

Paper maps 1:1250, 1:2500 2D database ( 1:1250 , 1:2500 ) National DTM ( 1:10k,1:50k) Road centreline DB

Photogrammetry Map digitizing (*) Map digitizing (*)

DB updating by A.Ph. & monoplot. Contours updating by A.Photogr.

IGN (F)

Paper maps 1:25,50,100,250 k 2D database (acc. 10m) 2.5D database (acc. 2m) National DTM Road DB

Map digitizing (*) Photogrammetry Map digitizing (*)

Data collection by A.Ph. & D.Ph. Develop prod. line for image matching Develop prod.line for auto road extrac.

SFOT (CH)

Paper maps 25, 50, 100k 2D database (acc. 5m) ( 1:25k) National DTM ( 1:25,000 )

Photogr. (*) Map digitizing (*) Map digitizing (*)

Map revision Prepare for upgrading to 2.5D Develop prod. line for auto AT Develop prod. line for digital OP from auto AT and national DTM

NCC Paper maps (1/10,000 , 1/25,000 ) DTM of some states (1/25,000 )

Photogrammetry Production/Revision

4

DTM tasks: generation Main steps

Data capture Data Sampling Choice of data source Data acquisition techniques

Model construction Establishment of topological relations Defining a suitable interpolation method

5

Data Data for a DTM should consist of:

Elevation Data:Observations about terrain elevations.

Morphological Information: Information about phenomena that significantly influence the

shape of the terrain surface (i.e. structural features such as drainage channels, ridges and other surface discontinuities).

Key issue:The selection of a particular data acquisition technique for

any given application considering the available/required efficiency, cost, and technological maturity.

6

Sampling techniques Choice of sampling technique

Terrain shape Available instrumentation Required accuracy

Techniques: Random/Selective Systematic/grid based Progressive Composite

7

Selective Sampling Capture topographic break lines.Advantage:

Capture all the morphological information associated with the surface.

Elevation data are collected whenever needed.Disadvantage:

Requires experienced human operator.Automation is very difficult.

8

Random/Selective sampling Selection of significant points by the

operator Usually results in less points More thought should be given to the

structuring and management of the measured data

Can not be automated

9

Systematic/grid based Systematic pattern of spot

heights Can be squares, rectangles,

triangles, or hexagonal Sampling patterns are

arranged as profiles or regular geometric shapes.

Fixed sampling distance is used: Need to determine the optimal

sampling interval.

10

Grid Sampling: Discussion Location of the required grid node is preprogrammed and

driven under computer control. Advantage:

Can easily be programmed May be applied in a semi-automated or automatic mode.

Disadvantages: Too many points are sampled in low relief regions. Too few points are captured in rugged terrain.

11

Progressive Sampling: Procedure The sampling process is initiated by measuring

a low-density grid. The accuracy of the sampled data is then

analysedWherever necessary, the sampling grid is

recursively densified until the required accuracy level is reached.

12

Progressive Sampling: DiscussionAdvantage:

Fewer points are needed to accurately represent the DTMs.

Disadvantage:Details may still be disregarded in the first runStill more points than necessaryToo many points in terrain breaksMay fail in areas with sharp discontinuities

13

Composite Sampling Composite Sampling = Progressive sampling /

Systematic sampling + selective sampling:Selective sampling is used to capture abrupt surface

changes.Progressive sampling yields the data for the rest of the

terrain. Advantage:

Recursive refinement of progressive sampling is kept to a minimum and terrain discontinuities are represented accurately.

Disadvantage: Requires human intervention (partial automation).

14

Composite Combines grid based and selective

techniques Can only partially be automated

15

Sampling Methods: Summary

16

Data capture The choice of data source

Size of the area to be modelled Required accuracy Type of the data to be extracted Cost and technological maturity

Data capture techniques Data capture techniques

Ground surveyingPhotogrammetryDigitising cartographic data sourcesRADAR, LIDAR (or Laser Scanning), and sonar.

17

18

Choice of Data Acquisition Techniques