DEM Interpretation Geology

Center for Remote Sensing (CRS) Faculty of Earth Sciences and Technology Bandung Institute of Technology (ITB) Gdg. Labtex IX-C, Lt. 3 Jln. Ganesha, No. 10, Bandung, Indonesia

PT. KARVAK NUSA GEOMATIKA Ruko Jambore Park, Jl. Jambore Raya No. 89A, Cibubur - Jakarta Timur. Indonesia. 13720.

Asep Saepuloh, Dr. Eng.





What is a Digital Elevation Model (DEM)?

Digital representation of topography model based on scale of original data

Commonly a cell based data where a cell has a single elevation which represents the entire area covered by the cell





Why use elevation data?

Importance of terrain in hydrology and environmental modeling

Visualization of landscapes

Geomorphologic

Surface Geology

Structures Geology





Creation of DEMs

Conversion of paper maps Scanned, digitized contour lines

USGS produces 10 and 30 meter DEMs

From original photogrammetry (Lidar)

From Space Shuttle topography mission 30 - 90 meter DEM data.





Santa Barbara, California

http://srtm.usgs.gov/srtmimagegallery/index.html






San Andreas Fault, California






Salt Lake City, Utah






Mt Kilimanjaro, Tanzania





Shuttle Radar Topography Mission (SRTM)

1 arc-second elevation data for the United States, 3 arc-second data for the globe

Produced by radar measurements from a Shuttle mission, Feb 11-22, 2000

http://srtm.usgs.gov/data/obtainingdata.html









Interferometry used by SRTM In interferometry, two

images are taken from

different vantage

points of the same

area.

The slight difference in

the two images allows

scientists to determine

the height of the

surface.





SRTM Coverage 56°S to 60°N





Cell Size 30m 100m





Basic storage of data

340 335 330 340 345

337 332 330 335 340

330 328 320 330 335

328 326 310 320 328

320 318 305 312 315

DEM as matrix of elevations with a uniform cell size





Adding geography to data

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337 332 330 335 340

330 328 320 330 335

328 326 310 320 328

320 318 305 312 315

Xmin, Ymin – XY are in projected units

Xmax, Ymax

Cell index number x cell size defines position relative to Xmin, Ymin and Xmax, Ymax and infers an exact location





Scale in DEMs

Scale determines resolution (cell size)

Depends on source data

Resolution determines use of DEM

what spatial features are visible





Errors in DEMs

Typos occur frequently in DEMs

Most common variety are “sinks” and “spires”.

Sinks occur when a very low elevation, relative to surrounding cells, is entered.

Spires occur when a very high elevation, relative to surrounding cells, is entered

Both appear as tightly packed contours





A natural sink?

340 335 330 340 345

337 332 330 335 340

330 228 320 330 335

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320 318 305 312 315

By default, this “sink” is removed, whether or not it is real.





Correcting sinks and spires

Most GIS have a “Fill” function

which looks for sinks and fills

them or looks for spires and

removes them





Estimating slopes in a DEM Slopes are calculated locally using a

neighborhood function, based on a moving 3×3 window

Distances are different in horizontal and vertical directions vs diagonal

Only steepest slopes are used

1.41… 1.00 1.41…

1.00 0.00 1.00

1.41… 1.00 1.41…





Slopes

340 335 330

337 332 330

330 328 320

(elevations)

8/42.47 3/30 2/42.47

5/30 0 -2/30

-2/42.47 -2/30 -12/42.47

(difference/distance)





Flow direction in a DEM

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337 332 325 335 340

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328 326 310 320 328

320 318 305 312 315

Flow directions for individual cells





32 64 128

16 Source

Cell 1

8 4 2





Finding watersheds …

Begin at a source cell of a flow direction database, derived from a DEM (not from the DEM itself

1. Find all cells that flow into the source cell

2. Find all cells that flow into those cells.

3. Repeat …

All of these cells comprises the watershed

The watershed is generalized based on the cell size of the DEM





Watersheds …

Contour lines (brown) Drainage (blue) Watershed boundary (red)

Once done manually …





Flow accumulation

1. The number of cells, or area, which

contribute to runoff of a given cell

2. Accumulation, once it reaches a

threshold appropriate to an region,

forms a drainage channel

3. Accumulation is the area of a

watershed that contributes runoff to a

given cell.





Flow accumulation in a DEM

0 0 0 0 0

0 1 3 1 0

0 1 8 1 0

0 1 13 1 0

0 2 24 2 0

Flow accumulation for individual cells Errors may occur at the edges of DEMs.





Flow accumulation as drainage network

Drainage network as defined by cells above threshold value for region.





Tugas

Menghitung gradien slop dengan data

DEM (spreed-sheet)

Tentukan/Delineasi daerah akumulasi air

(lembahan)

Tentukan/Delineasi daerah tangkapan

air (punggungan)





SHADED RELIEF MAP

Applicability for:

1. Geological structure identification

2. Geological hazard assessment





30 km

1. Shaded relief map for active faults identification



10-m

ete

r DE

M fro

m c

on

tou

rs





12-ft D

EM

from

LID

AR





From the Newport and

Withamsville quadrangle

by Gibbons (1973):

Landslide types and geology

2. Shaded relief map for land

slides identification





Kenton and Campbell Co.

landslides





Example: LiDAR derived

hillshade DEM with contours

Mapped landslide






hillshade DEM without contours

Mapped landslide






hillshade DEM with contours

Mapped landslide





Another example of subsequent failure





Another example of subsequent failure





Shaded Relief Creation Cos (incidence angle) =

[Sin(elevation)*Cos(slope)]+

[Cos(elevation)*Sin(slope)*Cos(azimuth - aspect)]





TUGAS 1. Gunakan ReliefShader untuk

membuat peta shaded Lidar dengan

arah penyinaran azimuth/altitude sbb:

0/50

90/50

180/50

270/50

2. Identifikasi struktur untuk setiap peta

shading di atas:

Linear (faults/fractures)

Circular (crater/land sliding)

Documents

DEM Interpretation Geology