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Dem analaysis and catchment delineation using GIS

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This presentation describes how a GIS can be used to derive DEM derivatives and delineate catchments and streams

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Dem analaysis and catchment delineation using GIS

  1. 1. DEM analysis and catchment delineation Dr. Hans van der Kwast
  2. 2. 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 2
  3. 3. 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 3
  4. 4. DEM acquisition • Ground surveying • DGPS measurements • Stereo photogrammetry • Digitizing contour lines • LIDAR • Radar interferometry 4 INS laser- scanner
  5. 5. Use of DEMs • Determining the catchment area • Delineate drainage networks • Slope • Aspect • Identify geological structures 5 • Viewshed analysis • Orthorectification • 3D simulations • Change analysis • Creating contour maps
  6. 6. Use of DEMs: Example French Alpes 6
  7. 7. Use of DEMs: Example French Alpes 7
  8. 8. Use of DEMs: Raster map 8
  9. 9. Use of DEMs: Hillshade 9
  10. 10. Use of DEMs: DEM 10
  11. 11. Use of DEMs: DEM + hillshade 11
  12. 12. Use of DEMs: Contour lines 12
  13. 13. Use of DEMs: 2.5D 13
  14. 14. Use of DEMs: DEM + orthophoto 14
  15. 15. Use of DEMs: Slope 15
  16. 16. Use of DEMs: Aspect 16
  17. 17. Use of DEMs: Plan and profile convexity • http://courses.soil.ncsu.edu/resources/soil_classifi cation_genesis/soil_formation/hill_shapes.swf 17
  18. 18. Use of DEMs: Topographic Wetness Index 18
  19. 19. Catchments (terminology) UK US Catchment or Watershed drainage basin Watershed 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 19
  20. 20. Catchments 20
  21. 21. Stream and catchment delineation 21 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
  22. 22. 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 22
  23. 23. Mosaic DEM tiles 23
  24. 24. 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 24
  25. 25. Calculation of slope 25 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
  26. 26. 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 26
  27. 27. 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
  28. 28. 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 28 Source: GITTA (2006) Water trapped in a pit
  29. 29. Fill sinks • Pits can be removed by: • Cutting through • Filling up 29 Source: GITTA (2006)
  30. 30. 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 30 Source: Brad Hudgens (1999)
  31. 31. 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 31 D8 Dinf
  32. 32. Calculate flow directions: D8 33 80 74 63 69 67 56 60 52 48 30 4 5 6 3 7 2 1 8 45.0 230 4867   50.0 30 5267   Slope = Drop/Distance Steepest down slope direction
  33. 33. Calculate flow direction D8 34 D8 for each cell Stream link
  34. 34. Derive streams: Flow accumulation 35 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 22 20 15 The area draining each grid cell includes the grid cell itself.
  35. 35. Derive streams: Flow accumulation 36 1 1 11 1 1 1 2 1 1 1 1 1 1 3 3 3 11 2 1 25 15 202 Flow Accumulation > 10 Cell Threshold Stream Network for 10 cell Threshold Drainage Area
  36. 36. 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 • … 37
  37. 37. Derive catchment 38
  38. 38. Effect of different stream threshold values 39
  39. 39. Strahler order 40
  40. 40. Stream and catchment delineation 41 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
  41. 41. Boundary condititions • This GIS workflow for stream and catchment delineation does not work when applied to: • Flat areas • Human controlled environments 42
  42. 42. Input for modelling 43

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