Trail Flow: Analysis of Drainage Patterns Affecting a Mountain Bike Trail

Transcription

1 Andy Belcher BAE 590 Professor Huffman Location Trail Flow: Analysis of Drainage Patterns Affecting a Mountain Bike Trail The location of the project area rests in the Cary area of Wake County, North Carolina. This area contains a network of user-maintained trails that have become eroded due to poor design and subsequent lack of maintenance. Size/Scope The length of the existing trail used in this analysis is 1777 feet. This area of trail was chosen due to the location along a steep hillside and erosion due to drainage flow. The scope of the project is to identify water-flow problem areas along the existing route. GPS, GIS, and CAD data and tools will be used throughout the analysis. The findings of the analysis will provide guidance for altering the exiting trail to create a more sustainable route, and also provide data for 3D visualization purposes. The analysis results and data products will help to create a design along the existing path that will reduce erosion along the hillside, reduce possible sediment runoff into stream below, and establish a solid trail foundation that will require minimal maintenance (in comparison to current facility). *Note: The original project scope of designing a functional trail with scale features ultimately became out of reach due to the limited resolution of the GPS and GIS data in comparison to the needed resolution in a real world scenario. Even though Bare Earth Lidar was used to create a high resolution DEM (5ft) and subsequent surface, real-world topography and design require

2 more detailed and realistic surface than can be recreated digitally. Thus, this project has been adjusted to reflect areas of interest that can be used as a guide when working on the ground. Belcher 2 Design Objectives The objective of the project is to analyze the existing trail and the drainage features impacting this trail, and vice-versa. The creation of custom drainage lines in both the ArcGIS and AutoCad environment will target the areas along the trail where drainage is a problem and where the trail could possibly be relocated to alleviate the problem. GPS data collection and processing -Existing trail was captured using a Garmin Edge 305 recreational grade GPS unit. Garmin lists the horizontal positional accuracy of the unit at feet; however the accuracy has been much better when compared to aerial imagery. -The native file format of the unit is.gpx. In order to convert the file for use in ArcGIS, a software package created and published by the Minnesota Department of Natural Resources is used to convert directly to ArcGIS shapefile. Processing in the ArcGIS environment Lidar processing and DEM creation -Bare earth lidar was downloaded from NCFloodMaps for the project area. This data is downloaded in ASCII format and must be converted to spatial data within the ArcGIS environment. -A Geodatabase is created and given a coordinate system is North Carolina State Plane Feet (NCSP83ft) -Using the ASCII 3D to Feature Class tool, the file is read and converted to a multipoint feature class in the geodatabase with point spacing of 3. -The newly created point data set is used in the creation of a new Terrain. The terrain is a TIN surface that is used in the process of creating a DEM for use in ArcHydro and for creating a surface in AutoCad. -The Terrain is converted to a DEM raster using the Terrain to Raster tool found in 3d Analyst. A cell size of 5 feet is chosen to produce a DEM with 5x5 grid size/resolution. -In order to help processing speeds and data transport, a clipping boundary is created for the project area. This new polygon is used with the Extract by Mask tool to crop the DEM to the needed area. The new DEM is called dem_clipped. DEM export to text file -Using the Raster to ASCII tool, the dem_clipped file is exported to text file for use in AutoCad environment. The output text file is called dem_clipped.txt. Assigning elevation to Trail polyline -Using the Feature Vertices to Points tool, the polyline vertices are converted to points. The new file is titled trail_points.shp.

3 Belcher 3 -Using the Add Surface Information tool, elevation values are given to trail_points.shp using dem_clipped as the value source. The new file is titled trail_points_3d.shp 3D Trail export to text file -Using the Feature Class Z to ASCII tool, trail_points_3d.shp is exported to text file. The new file is titled trail_points_3d.txt. Creating ArcHydro drainage lines -The ArcHydro extension is used to create custom drainage polylines for the project area. - dem_clipped is used as the source for the geoprocessing. -A threshold of 50 is used to create the drainage lines. This number was determined after several higher and lower values were used, with 50 determined to be the most realistic. The value chosen closely mirrored Wake County hydrolines, but with additional tributary detail. -By default, the new file is a feature class titled DrainageLine in the ArcHydro file geodatabase. Clipping drainage lines to appropriate area -Using the Buffer tool, the trail polyline is buffered 50. The new file is titled trail_50ftbuffer.shp. -Using the Clip tool, the DrainageLine feature class is clipped to trail_50ftbuffer.shp. The new file is drainage_line_clipped.shp (intentionally saved as shapefile instead of geodatabase feature class). -The clipping of the drainage lines within 50 of the trail is to reduce the number of polylines needed to be drawn in autocad, and because this report is only concerned with drainage lines affecting the current alignment. Fig. 1 Assigning elevation to drainage polylines -Using the Feature Vertices to Points tool, the polyline vertices are converted to points. The new file is titled drainage_lines_points.shp. -Using the Add Surface Information tool, elevation values are given to drainage_lines_points.shp using dem_clipped as the value source. The new file is titled drainage_lines_points_3d.shp Processing in the AutoCAD environment A new AutoCAD drawing file is opened and the projected coordinate system is set to NC83f in the drawing settings. Surface creation -In the toolspace, create a new TIN surface as Trail Surface. -Under the Definition property under the new surface, choose to add a new DEM using the dem_clipped_.txt file (Esri ASCII Grid.txt option). Fig. 2 Add Trail point group -Under the Prospector tab, create a new Point Group titled Trail_Points

4 Belcher 4 -Using the Create Points tool, import the points by importing trail_points_3d.txt. The ENZ (comma delimited) option with be chosen upon opening. Choose to add the points to the Trail Points point group. Create 3D trail polyline -Create a new layer in the Layer Manager named Trail with an appropriate line weight and style. -Using the 3D Polyline tool with snapping turned on, begin at the first point in the Northwest and move from point to point creating the 3D trail. Use another screen with ArcMap open or a paper copy map of the trail to serve as a guide. Add Drainage Line point group -Under the Prospector tab, create a new Point Group titled Drainage_Line -Using the Create Points tool, import the points by importing drainage_line_points_3d.txt. The ENZ (comma delimited) option with be chosen upon opening. Choose to add the points to the Drainage_Line point group. Create 3D Drainage polyline -Create a new layer in the Layer Manager named Drainage Line with an appropriate line weight and style. -Using the 3D Polyline tool with snapping turned on, begin at the first point in the Northwest and move from point to point creating the 3D drainage line. Use another screen with ArcMap open or a paper copy map of the trail to serve as a guide. Import Wake County Hydrolines shapfile for comparison -Using the MAPIMPORT command, navigate to the directory containing the needed GIS data. Fig. 3 Viewing and analyzing in the AutoCAD environment Drape aerial image over surface -Using the DRAPEIMAGE command, select the aerial image to drape over Surface1. -Select SE Isometric view, and Realistic rendering style. -Using the Orbit panning tool, pan and zoom to needed viewing angles. Create water drop paths -Using the Water Drop tool in the Analyze tab, choose an appropriate layer to create the paths (default C-TOPO-WDRP will work, but changed to yellow for visibility); choose to place a marker at start location. -Choose a location near the trail and along a surface draw/drainage design object to place the begin point. -Compare ArcHydro created drainage lines and Wake County hydrolines with the AutoCad water drop path. Fig. 5

5 Export 3D Trail, Drainage Line Feature, and AutoCad Water Drop Lines to GIS Belcher 5 Create object tables -Using the ADEDEFDATA command, create a new table titled 3D_Trail -Add a new field titled Name to identify where the drawing object originated. Assign the field a Character data type. -Repeat for a new table titled 3D_Drainage, assigning the field name Origin. -Repeat for a new table titled acad_waterdroplines, assigning the field name Origin. Attach object tables -Using the ADEATTACHDATA command, attach the 3D_Trail object table to 3D_Trail design object by selecting the polyline drawing object when prompted. -Repeat for the new table titled 3D_Drainage -Repeat for the new table titled acad_waterdroplines Edit object tables -Using the ADEEDITDATA command, edit the Origin field to list Regency for the 3D_Trail. -Repeat for the new table titled 3D_Drainage and list Arc Hydro as the origin. -Repeat for the new table titled acad_waterdroplines and list acad Water Drop Tool as the origin. Export drawing objects to GIS shapefile -Using the MAPEXPORT command, choose the proper working directory, file name, and data type (ESRI Shapefile) for the Trail. -Choose the select objects manually option, and select the appropriate drawing objects. -Choose the appropriate export option, Line in this case. -Choose Length under Properties. -Carry over the respective object table. -Make sure the output coordinate system is NCSP83ft. -Repeat for above steps for Drainage_Lines (3D_Drainage.shp) and acad_waterdroplines (acad_waterdroplines.shp). Note, acad_waterdroplines.shp does not contain elevation values. View exported GIS data in ArcScene GIS environment -Open a new ArcScene project and add the 3D_Drainage and 3D_Trail to the table of contents. -Add the dem_clipped DEM file to serve as base height for the project. -Add the Aerial raster image and set the base height to dem_clipped. -Pan and zoom to observe the data in the ArcScene environment. Fig. 6

6 Belcher 6 Observations The interoperability of GIS and CAD data is very helpful when analyzing spatial features. I am typically against the exchanging of data back and forth between the two environments due to opportunity for data loss across platforms and possibility of additional human error, but I have become a believer after completing this project. Drawing 3-dimensional features in the CAD environment is quite efficient compared to the GIS environment. Additionally, I am impressed with the AutoCad Water Drop analysis tool. The resolution and detail of the polyline appears much higher than the ArcHydro created drainage line. It would be worthwhile in the future to process the exported AutoCad created drainage feature in the manner of the ArcHydro drainage features to apply elevation values for visualization purposes.

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