DEM analysis and catchment delineationDr. Hans van der Kwast

OpenCourseWareocw.unesco-ihe.org

Learning objectives

After this lecture you are able to:• define DEM, DTM, DSM• describe different methods of DEM acquisition• give examples how DEMs can be used• describe what data can be derived from DEMs• explain the GIS procedure for delineating streams and catchments

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Digital Elevation Models

• Digital Terrain Model (DTM):a quantitative model of a part of the Earth’s surface in digital form (Burrough & McDonnel, 1998)

• Digital Surface Model (DSM):DTM + all natural or human-made features

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DEM acquisition

• Ground surveying• DGPS measurements• Stereo photogrammetry• Digitizing contour lines• LIDAR• Radar interferometry

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INS

laser- scanner

Use of DEMs

Determining the catchment areaDelineate drainage networksSlopeAspect• Identify geological structures

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Viewshed analysisOrthorectification3D simulationsChange analysis• Creating contour maps

Use of DEMs: Example French Alpes

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Use of DEMs: Example French Alpes

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Use of DEMs: Raster map

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Use of DEMs: Hillshade

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Use of DEMs: DEM

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Use of DEMs: DEM + hillshade

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Use of DEMs: Contour lines

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Use of DEMs: 2.5D

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Use of DEMs: DEM + orthophoto

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Use of DEMs: Slope

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Use of DEMs: Aspect

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Use of DEMs: Plan and profile convexity

• http://courses.soil.ncsu.edu/resources/soil_classification_genesis/soil_formation/hill_shapes.swf

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Use of DEMs: Topographic Wetness Index

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Catchments (terminology)

UK USCatchment or Watersheddrainage basinWatershed Drainage divide

Drainage basin:An extent or an area of land where surface water from rain, melting snow, or ice converges to a single point at a lower elevation, usually the exit of the basin, where the waters join another waterbody , such as a river, lake, reservoir, estuary, wetland, sea, or ocean

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Catchments

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Stream and catchment delineation

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Download DEM tiles

Mosaic DEM tiles Reproject DEM Subset DEM

Interpolate voids

Fill sinks / remove spikes

Burn-in the stream

network

Calculate the flow direction

map

Derive streams Define outflow point

Derive catchment

Convert dataset to

model format

Download DEM tiles

• Open access data:• SRTM 1 Arc-Second Global (~30 m)• SRTM Void Filled

−~30 m for USA−~90 m global

• ASTER Global DEM (GDEM) (~30 m)

• Resolution ≠ accuracy!• Download at http://earthexplorer.usgs.gov

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Mosaic DEM tiles

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Reproject DEM

• Global datasets are usually in EPSG:4326(Geographic Coordinate System, Lat/Lon)

• For correct calculation of DEM derivatives, the DEM should be reprojected to a Coordinate Reference System

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Calculation of slope

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Moving Window or Kernel (3 x 3)

x

slope = arctan(Δz/ Δx)z

In a grid slope is calculated as a focal operation. The steepest slope in the window is assigned to the cell

Subset DEM

• DEM too large: calculation times for the following steps can become too large or computer runs out of memory

• DEM too small: catchment boundaries are cut off

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Interpolate voids

• Voids are pixels with NODATA in your DEM as a result of the acquisition procedure

• Voids can be interpolated using the values of surrounding cells

27Source: Markus Neteler

Fill sinks

• DEM creation results in artificial pits in the landscape

• A pit is a set of one or more cells which has no downstream cells around it

• Pits are removed using the fill sinks function in GIS software

• If landscape contains real sinks (e.g. lakes), these need to be added after pit removal

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Source: GITTA (2006)

Water trapped in a pit

Fill sinks

• Pits can be removed by:• Cutting through• Filling up

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Source: GITTA (2006)

Burn-in the stream network

• When a river network layer exists it can be used to force the flow direction algorithm to follow the river network

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Source: Brad Hudgens (1999)

Burn-in streams: method 1

1. Create a standardized DEM with standardized values in the range [0,1]

2. For the cells in the river network subtract the value 1, this will shift those cells to the range [-1,0]

• This procedure will maintain the lowest neighbour relation between cells except for those cells where one neighbour is in the river network and the other one is not.

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Burn-in streams: method 2

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Source: ILWIS

Calculate flow direction

• D8 algorithm: uses 8 discrete directions to calculate flow direction (0, 45, 90, 135, 180, 225, 270, 315 degrees) to steepest cells downwards

• Dinf algorithm: uses continuous directions

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D8 Dinf

Calculate flow directions: D8

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80 74 63

69 67 56

60 52 48

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4

5

6

3

7

2

1

8

45.02304867

50.0305267

Slope = Drop/Distance

Steepest down slope direction

Calculate flow direction D8

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D8 for each cell Stream link

Derive streams: Flow accumulation

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1 1 11 1

1

1

2

1

1

1

1

1

1

3 3 3

11 2

1

25

15

202

1 1 111

1

1

2

1

1

1

1

1

13 3 3

11 2

1

5 2220

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The area draining each grid cell includes the grid cell itself.

Derive streams: Flow accumulation

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1 1 11 1

1

1

2

1

1

1

1

1

1

3 3 3

11 2

125

15

202

Flow Accumulation > 10 Cell Threshold

Stream Network for 10 cell Threshold Drainage

Area

Define outflow point

• Outlet needs to be defined in a delineated river that corresponds with the flow directions that have been calculated

• Outlets can be:• Location in river with discharge measurement• Outlet of a tributary• …

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Derive catchment

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Effect of different stream threshold values

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Stream and catchment delineation

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Download DEM tiles

Mosaic DEM tiles Reproject DEM Subset DEM

Interpolate voids

Fill sinks / remove spikes

Burn-in the stream

network

Calculate the flow direction

map

Derive streams Define outflow point

Derive catchment

Convert dataset to

model format

Boundary condititions

• This GIS workflow for stream and catchment delineation does not work when applied to:• Flat areas• Human controlled environments

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Input for modelling

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