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Automated Techniques to Map Headwaters Stream Networks in the Piedmont Ecoregion of North Carolina

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Title: Automated Techniques to Map Headwaters Stream Networks in the Piedmont Ecoregion of North Carolina


1
Automated Techniques to Map Headwaters Stream
Networks in the Piedmont Ecoregion of North
Carolina
Valerie Garcia Forestry Department, North
Carolina State University Office of Research and
Development, Environmental Protection
Agency August 4, 2004
2
  • Why is the research important?
  • A large percentage of non-point source pollution
    is suspected to occur through headwaters streams
  • Current available USGS 124,000 Topographic maps
    lack accuracy in depicting the presence and
    location of headwaters streams
  • The lack of accurate maps for headwaters streams
    places an inordinate burden on both the
    regulatory agencies and the regulated communities
    in planning and implementing policy

3
  • Why is the research important?
  • Recent availability of LIDAR data for portions of
    North Carolina provide new opportunities for
    developing more accurate stream maps
  • This study focuses on the mapping of headwaters
    stream networks using Geographical Information
    System (GIS) approaches and LIDAR data
  • Study results limited to the Piedmont Ecoregion
    of North Carolina

4
  • Layout of Study
  • Phase I Extensive literature search to
    investigate state-of-science Geographical
    Information System (GIS) approaches relevant for
    using LIDAR data to map headwaters streams
  • Phase II Compare and evaluate approaches
    identified through the literature search by
    applying the techniques to a study site in the
    Piedmont Ecoregion of North Carolina

5
Phase I Literature Search
6
  • Some Definitions…
  • Triangulated Irregular Network (TIN)
  • Digital Elevation Model (DEM)
  • Hydro-enforcement

7
  • Triangulated Irregular Network (TIN)
  • Formulation of non-overlapping triangles from
    irregularly spaced x, y, and z points
    (vector-based)

8
Digital Elevation Model (DEM) As used in this
study…uniformly spaced, 3-dimensional
cartographic representation (x, y, z) in a grid
or raster format
9
Hydro-Enforcement
  • Both TINs and DEMs result in artifactsartificial
    disruptions of the natural drainage of water
  • Incorporation of known stream center-lines
    (breaklines) into the modeled terrain ensure the
    downstream drainage of water

10
  • Summary of Literature Search
  • Findings fell into two major categories
  • Production of accurate topographic maps
  • Extraction of stream networks
  • TINs produce more precise topographic maps
  • maintains LIDAR elevation points as triangle
    vertices
  • better retains linear structures (breaklines)
  • DEMs are better for automatically extracting
    headwaters streams
  • can automatically correct drainage problems and
    determine stream origin

11
  • Summary of Literature Search
  • The interpolation method and the resolution used
    to generate the topographic map can impact the
    accuracy of the map
  • more complex interpolation methods (e.g., Spline,
    Kriging) require more knowledge and are
    computationally demanding, but are expected to
    perform better in modeling terrain
  • resolution drives computation demands of the
    interpolation method and can impact the selection
    of which interpolation method can be used

12
  • Summary of Literature Search
  • Hydro-enforcement of the DEM enhances the
    accuracy of the extracted stream networks
  • derived from orthophotos (can be expensive)
  • typically not available for headwaters streams
  • Physical processes relevant at headwaters stream
    scales are different than watershed scales
    typically modeled
  • stream origin and hillslope (diffuse) flow are
    critical at headwaters stream scales

13
Phase II Application of Stream Mapping
Techniques to Study Site in Falls Lake, North
Carolina
14
  • Research Questions
  • What is the accuracy of the LIDAR surface
    elevation points?
  • When using densely spaced LIDAR data, does the
    interpolation method used to create a DEM make a
    difference in the accuracy of the DEM?
  • At what resolution do you begin to lose channel
    definition of headwaters streams, thereby
    affecting the production of headwaters stream
    maps?
  • Does using breakline data to hydro-enforce TINs
    and DEMs make a difference in the production of
    headwaters stream maps?
  • Do more complex stream flow algorithms and stream
    origin approaches produce better results than
    simpler methods in mapping headwaters streams?
    not covered in this presentation

15
  • Method Steps
  • Step 1 Evaluate the accuracy of LIDAR surface
    elevation points
  • Step 2 Compare the accuracy of topographic maps
    generated using various interpolation methods
  • Step 3 Evaluate the effect of scaling on
    topographic and stream map accuracy
  • Step 4 Assess the impact of using
    hydro-enforcement to extract stream networks

16
  • Study Site
  • Forested headwater stream catchment near Falls
    Lake in the Piedmont Ecoregion of North Carolina
    was selected as the study site
  • Collected surveyed elevation points and mapping
    grade Global Positioning System (GPS) along
    headwater stream
  • All modeling techniques were run for the study
    site and compared against survey data and
    available ancillary data

17
  • GIS Applications Used in Study
  • ArcGIS used to generate topographic maps
  • Four interpolation methods compared Natural
    Neighbor, Inverse Distance Weighting (IDW),
    Spline and Kriging
  • Four resolutions compared 10 ft., 20 ft., 60
    ft., 90 ft.
  • ArcHydro used to extract stream networks
  • Networks extracted with and without
    hydro-enforcement
  • Networks extracted at four resolutions 10 ft.,
    20 ft., 60 ft., 90 ft.

18
  • Data Used in Study
  • Existing data used in modeling
  • LIDAR mass elevation points (NC State Floodplain
    Mapping Program, 2003)
  • Breakline data (stream centerlines and
    shorelines) (NC State Floodplain Mapping Program,
    2003)
  • Data used for comparisons
  • Field-collected data (survey, mapping-grade GPS)
  • Medium and high resolution National Hydrography
    Dataset (NHD) (2000)
  • USGS 124,000 Topographic Digital Raster Graph
    (DRG) (1994)
  • Wake County Hydrography Lines (2000) derived from
    112,000 aerial photography
  • 1999 Wake County Color Digital Orthophotography

19
  • Step 1 Evaluation of LIDAR Surface Elevation
    Points
  • Collection of high-accuracy ground truth data
  • Control Benchmarks established along ridge of
    study catchments (vertical /- 2cm)
  • Transect surveyed across study catchment
    (vertical /- 8cm)
  • GPS measurements taken along headwater stream
    (horizontal 1-2m)
  • Compared LIDAR data to field-collected survey
    points
  • TIN generated from LIDAR data (without
    breaklines) and used for comparison with survey
    data

20
  • Step 1 Evaluation of LIDAR Surface Elevation
    Points
  • RMSE was 1.32 or 40.1 cm (no points removed)
  • Recalculated using the 95 percentile
    methodology with adjusted RMSE of 28.7 cm
  • Published accuracy of LIDAR data is 25 cm (95
    percentile) -- but study did not limit survey to
    uniform slope and used only one landcover type

21
  • Step 2 Comparison of Topographic Maps
  • Generated DEMs from LIDAR data using four
    different interpolation techniques and compared
    to ground-truth data
  • 20 ft. resolution used as LIDAR data averaged at
    least one point every 20 sq. ft.
  • Four interpolation methods compared (Natural
    Neighbor, IDW, Spline and Kriging)
  • Results compared to survey data

22
Step 2 Comparison of Topographic Maps
IDW(brick), Kriging (majenta) vs. Survey Points
(blue)
23
Step 2 Comparison of Topographic Maps
Spline (red), Natural Neighbor (green) vs. Survey
Points (blue)
24
Step 2 Comparison of Topographic Maps
25
  • Step 3 Evaluate Impact of Scaling to Various
    Resolutions
  • Evaluate impact of scaling on accuracy of
    topographic map
  • Natural Neighbor, IDW, and Regularized Spline
    used to generate 10 ft., 20 ft., 60 ft. and 90
    ft. resolution DEMs
  • Results compared to survey data

26
  • Step 3 Evaluate Impact of Scaling to Various
    Resolutions
  • Evaluate impact of scaling on stream extraction
  • ArcHydro used to extract stream networks at
    various resolutions (10 ft., 20 ft., 60 ft. 90
    ft.)
  • Extracted stream networks compared to each other,
    GPS points and Wake County hydrography lines

27
  • Step 3 Evaluate Impact of Scaling to Various
    Resolutions - DEM
  • Comparison of DEMs at different resolutions
  • Very little difference exists between the 10 ft.
    and 20 ft. resolution DEMs

28
  • Step 3 Evaluate Impact of Scaling to Various
    Resolutions Stream Extraction
  • Comparison of DEMs at different resolutions
  • At 60 ft. resolution, 2 ft. headwater stream
    channel becomes a 120 ft. depression
  • At 90 ft. resolution, the entire drainage is lost

29
  • Impact of Scaling to Various Resolutions
  • Comparison of drainages extracted at different
    resolutions
  • Similar to the topographic map results, very
    little difference exists between the extracted
    stream drainages generated from the 10 ft. and 20
    ft. resolution DEMs

30
  • Impact of Scaling to Various Resolutions
  • Comparison of drainages extracted at different
    resolutions
  • At 60 ft. resolution, drainage lines become much
    more unnaturally linear and have more occurrences
    of drainage interruptions
  • At 90 ft. resolution, this problem is more
    extreme

31
  • Step 4 Assess the effect of Hydro-enforcement
    on the Extraction of Stream Networks
  • Accuracy of source breakline data
  • Overlain on NHD, Wake County hydrography lines,
    Wake County digital orthophotography and USGS
    124,000 Topographic DRG
  • Evaluate the impact of hydro-enforcement on
    extracting headwaters stream networks
  • ArcHydro used to extract stream networks for each
    DEM, with and without hydro-enforcement
  • Extracted networks compared to stream GPS points,
    NHD, Wake County hydrography lines, Wake County
    digital orthophotography and USGS 124,000
    Topographic DRG

32
Step 4 Impact of Hydro-enforcement Accuracy
of Breakline Data
  • NC Floodplain Program breakline data aligns well
    with the 1999 Wake County digital orthophotography

33
  • Step 4 Impact of Hydro-enforcement Accuracy
    of Breakline Data
  • NHD (high resolution (blue) and medium resolution
    (red)) aligns well with USGS 124,000 Topographic
    DRG

34
  • Step 4 Impact of Hydro-enforcement Accuracy
    of Breakline Data
  • NHD (high resolution) does NOT align with 1999
    orthophotography (beige polygons emphasize
    alignment problems)

35
  • Step 4 Impact of Hydro-Enforcement
  • Most of the differences were in the higher-order
    streams
  • The headwaters stream network was substantially
    the same regardless of whether the grid was
    hydro-enforced or not
  • Hydro-enforced Natural Neighbor DEM produced the
    cleanest drainage lines in the breakline areas

Drainage created from hydro-enforced TIN
(converted to grid) produces a confused drainage
in the lake area
36
Step 4 Impact of Hydro-Enforcement
Best drainage generated from Natural Neighbor
DEM with hydro-enforcement (blue) overlain on
Wake County hydrological lines (green)
37
  • Conclusions
  • Simpler interpolation methods (e.g., Nearest
    Neighbor, IDW) did as well or better than the
    more complex interpolation methods (e.g.,
    Kriging) for generating the base DEMs that are
    used for extracting headwaters stream networks
  • Hydro-enforcement did not improve the results in
    extracting the headwaters stream networks
  • Hydro-enforcement did generate more direct
    drainages in the lake area, indicating that
    flatter areas or any area prone to flooding, will
    be aided by breakline data

38
  • Conclusions
  • Breakline data available through the NC State
    Floodplain Program are better aligned than
    currently available NHD or USGS 124,000
    Topographic Map
  • Because misalignments are carried throughout the
    stream network, only the highest quality
    breakline data should be used for
    hydro-enforcement alternatively, no
    hydro-enforcement should be used

39
Bottomline… For the study catchmentLIDAR data
and GIS modeling approaches did a better job than
the best stream data currently available for the
area
Zooming in shows that the model does better
than the Wake County hydrography lines as
compared to the GPS stream points
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