Transcript digital elevation model

DEM
Definition
In scientific literature there is no universal
agreement about the usage of the terms:
• digital elevation model (DEM)
• digital terrain model (DTM)
• digital surface model (DSM)
DEM is often used as synonimous of
DTM, but in many cases it used as a
generic term for both DTMs and DSMs.
The most common representation of a
DEM is a raster, where the DNs
correspond to the average elevation value
of the area framed by the cell.
DEM
Uses
• Terrain analysis in geomorphology
and physical geography
• Modelling water flow for hydrology
• Modelling soil erosion or mass
movement
• Creation of relief maps
• Rendering of 3D visualizations
• Rectification of aerial photography or
satellite imagery
• Climatology
• Urban planning
• Logistics and communications
• Mining
• ...and many more other GIS
applications
DEM
Sources of elevation data
• Stereo photogrammetry from aerial surveys
– traditional: manually by a trained photogrammetrist
• contours of topographic maps, to be interpolated
– modern: by automatic stereo-correlation
• Stereo-correlation from optical satellite imagery
– SPOT (off-track stereoscopy)
– Aster DEM (along-track stereoscopy)
• Interferometry from radar data
– European Remote Sensing Satellite ERS (multi-pass)
– Shuttle Radar Topography Mission SRTM (single-pass)
• Lidar
– interpolation of a matrix of xyz points obtained using a laser
beam
DEM
Aspect
• The rate of change in the x direction for cell 'e' is :
[dz/dx] = ((c + 2f + i) - (a + 2d + g)) / 8
• The rate of change in the y direction for cell 'e' is :
[dz/dy] = ((g + 2h + i) - (a + 2b + c)) / 8
a
b
c
d
e
f
g
h
i
• Taking the rate of change in both the x and y
direction for cell 'e', aspect is calculated using:
aspect = 57.29578 * atan2 ([dz/dy], -[dz/dx])
• The aspect value is converted to compass direction
values (0–360 degrees, 0 and 360 indicate North)
• The aspect image is generaly classified into 8
classes, i.e. N, NE, E, SE, S, SW, W, NW
• NoData value is assigned to flat areas
atan2
DEM
Slope
a
b
c
d
e
f
g
h
i
• Can be computed in different ways: like the aspect, it is usually
calculated using a 3 x 3 matrix around each pixel
• Horn’s algorithm:
slope 
a  2d  g  c  2f  i2  a  2b  c   g  2h  i2
8  pixelsize
DEM
When deriving aspect and slope information from a DEM, if the quality is
poor, artefacts become visible specially in level or flat areas.
DEM
aspect
slope
DEM
Hillshading
Each pixel of the DEM is “artificially illuminated”, with
the virtual sun positioned at a given azimuth and
elevation (90° - zenith angle).
The max illumination is when the pixel slope is
perpendicular to the sun direction, and the aspect
coincides with the azimuth.
Pixel in shadow (not reachable by direct illumination)
are set to 0. Usually algorithms calculate only direct
shadows, and do not determine whether a surface is
shadowed by another one.
As most software procedures assume that planar
coordinates and elevation data are expressed with
the same units (e.g. both easting, northing, and
elevation are in meters), special care should be paid
when using DEM in geographic coordinates.
DEM
Direction of Steepest Descent
30
Slope:
30
80
74
63
80
74
63
69
67
56
69
67
56
60
52
48
60
52
48
67  48
 0.45
30 2
67  52
 0.50
30
DEM
Flow direction grid
DEM
The problem of the sink
A pixel is surrounded by points having a higher elevation.
If it is a DEM artifact, increase elevation of the pixel until the pit drains to a
neighbor, or “carve” one of the neighbors.
DEM
Flow Accumulation Grid.
0
0
3
0
0
0
0
0
0
0
0
0
0
0
0
0
2
0
0
3
2
2
0
0
1
0
0
11
0
1
0
0
0
1
15
0
1
0
2
5
24
1
0
2
11
1
2
15
5
24
DEM
Threshold Drainage Area
0
0
3
0
0
Example:
5 cells
0
0
0
2
0
0
2
0
0
0
11
1
0
1
0
15
2
5
1
24
DEM
Stream segments
0
0
3
0
0
0
0
0
2
0
0
2
0
0
0
11
1
0
1
0
15
2
5
1
24
DEM
Convert stream segments into vectors
Drainage Lines are drawn through the centers of cells on the stream segment.
Drainage Points are located at the centers of the outlet cells of the catchments.
DEM
Catchments
For every stream segment, there is a corresponding catchment.
Also the catchments can be converted to vectors (polygons).
DEM
Catchments
Subwatersheds
Catchments
Watershed
Catchments can be hyerarchically grouped into watersheds.
Watersheds are defined by their outlet points.
DEM
Sources of global DEM data
• Aster Global Digital Elevation Model (GDEM)
– Version 1 (2009)
• 30m
– Version 2 (2011)
• reduction of spikes and wells
• improved elevation accuracy
• Improved definition of water bodies and coastlines
• Shuttle Radar Topographic Mission
– Version 1 (2000 ->)
• 90m (3 arc-seconds), 30m (1 arc-second) for the USA
– Version 2 (2005)
• well-defined water bodies and coastlines
• absence of spikes and wells
• missing data ('voids') are still present
– Version 3 (“SRTM Plus”, 2014)
• void-filled (mostly using GDEM)
• 1 arc-second already released for most of Africa, other areas in the future
• http://gdex.cr.usgs.gov/gdex/